Measurement of neutron spectra generated from bombardment of 4 to 24 MeV protons on a thick 9Be target and estimation of neutron yields Sabyasachi Paul, G. S. Sahoo, S. P. Tripathy, S. C. Sharma, Ramjilal, N. G. Ninawe, C. Sunil, A. K. Gupta, and T. Bandyopadhyay Citation: Review of Scientific Instruments 85, 063501 (2014); doi: 10.1063/1.4880202 View online: http://dx.doi.org/10.1063/1.4880202 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Generation of high-energy (>15 MeV) neutrons using short pulse high intensity lasers Phys. Plasmas 19, 093106 (2012); 10.1063/1.4751460 Measurement of Differential Thick Target Neutron Yields (TTY) from Fe, Cu(p,n) Reactions at 35, 50, and 70 MeV AIP Conf. Proc. 769, 1568 (2005); 10.1063/1.1945305 Measurements of Neutron Spectra Produced from a Thick Iron Target Bombarded with 1.5GeV Protons AIP Conf. Proc. 769, 1513 (2005); 10.1063/1.1945292 Experimental Studies on Particle and Radionuclide Production Cross Sections for Tens of MeV Neutrons and Protons AIP Conf. Proc. 769, 884 (2005); 10.1063/1.1945147 Measurements of Leakage Neutron Spectra from Thick Spherical Shells of Vanadium and Lead with 14MeV Neutrons and Validation of their Nuclear Data AIP Conf. Proc. 769, 490 (2005); 10.1063/1.1945054
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 063501 (2014)
Measurement of neutron spectra generated from bombardment of 4 to 24 MeV protons on a thick 9 Be target and estimation of neutron yields Sabyasachi Paul,1 G. S. Sahoo,1 S. P. Tripathy,1,a) S. C. Sharma,2 Ramjilal,2 N. G. Ninawe,2 C. Sunil,1 A. K. Gupta,2 and T. Bandyopadhyay1 1 2
Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
(Received 22 April 2014; accepted 15 May 2014; published online 4 June 2014) A systematic study on the measurement of neutron spectra emitted from the interaction of protons of various energies with a thick beryllium target has been carried out. The measurements were carried out in the forward direction (at 0◦ with respect to the direction of protons) using CR-39 detectors. The doses were estimated using the in-house image analyzing program autoTRAK_n, which works on the principle of luminosity variation in and around the track boundaries. A total of six different proton energies starting from 4 MeV to 24 MeV with an energy gap of 4 MeV were chosen for the study of the neutron yields and the estimation of doses. Nearly, 92% of the recoil tracks developed after chemical etching were circular in nature, but the size distributions of the recoil tracks were not found to be linearly dependent on the projectile energy. The neutron yield and dose values were found to be increasing linearly with increasing projectile energies. The response of CR-39 detector was also investigated at different beam currents at two different proton energies. A linear increase of neutron yield with beam current was observed. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4880202] I. INTRODUCTION
Intense fast neutron sources are produced primarily by charged particle induced nuclear reactions using protons as projectile with low Z thick target elements.1–3 The emitted fast neutrons from these reactions are important for different medical applications like accelerator based boron neutron capture therapy (BNCT) for cancer treatment3–5 and for studies on shielding6 and dosimetric7, 8 applications. For this purpose, a thick 9 Be target has been bombarded with proton energies ranging from a few MeV to few tens of MeV to produce neutrons.9–13 In this energy range of protons, a set of different reaction channels open up and produce neutrons having broad spectrum of energy with a peak at lower energy followed by a tail with decreasing neutron yield at higher energies. Among these, the low energy neutrons play an important role for the cancer treatment, especially for BNCT, as the relative biological effectiveness (RBE) is significantly high due to a better interaction probability and preferential absorption of low energy neutrons in the organs and tissues.14 So, to study the neutron spectral yield generated from the reaction of protons of various energies with a thick 9 Be target, a systematic investigation has been carried out at the BARCTIFR Pelletron Accelerator facility, Mumbai, India. The set of discrete reactions for the system under study, i.e., 9 Be + 1 H reaction, is well known and is listed in Table I. The neutron yield and dose values produced by these reactions depend primarily on the projectile energy. For the measurement of the neutrons generated from this reaction, several methods using active and passive techniques such as time of flight, Bonner Sphere Spectrometer, activation analysis, etc., have a) Author to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected]
0034-6748/2014/85(6)/063501/7/$30.00
been established which often involve bulky and expensive instrumentation. In addition, these measurements (except time of flight) involve complex spectrum unfolding procedures using codes like MAXED,15 ANDI-03,16 FRUIT,17 BUNKI,18 GAMCD,19 LOUHI,20 etc., to generate the neutron spectrum which further require a well-defined response matrix and initial guess spectrum.21–23 Considering this difficult instrumentation, time consuming analysis, and complex unfolding procedures, the passive technique using CR-39 detectors and image analysis programs as an alternative for neutron spectrometry has gained popularity in the recent past.24 Researchers have developed response matrices for Bonner Sphere Spectrometry using boron coated CR-39 detectors25 with experimental validation. A few works have also been reported regarding the generation of neutron spectrum using CR-39 detectors.21, 22, 24, 26, 27 Studies have also been carried out for estimation of dose responses of CR-39 detector in neutron fields by measuring the linear energy transfer (LET) spectra28 and also by correlating with the optical properties.29 A comprehensive literature survey on the 9 Be (p,n) reaction reveals that the first quantitative estimation of neutron yields from this projectile target combination has been reported in the early 1950s30 using a thin beryllium target with projectile energies from threshold to 4 MeV. Since then several experiments were carried out with protons to study the neutron yield and angular distributions with different target thickness at various projectile energies using mainly time of flight measurements.10, 31, 32 But reports on spectrum generation using passive detectors like CR-39 for various proton energies from 4 MeV to 24 MeV are rare and hence an effort has been made in this work to systematically study the emitted particle spectra using CR-39 detectors. For the present study, the CR-39 detectors were chosen mainly due to the following reasons: (i) passive technique and no
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TABLE I. Neutron producing reactions for 24 MeV projectile energy.
Reaction 9 Be + p
→ n + p + 2α → n + p + 8B 9 Be + p → n + 9 B 9 Be + p → α + p + 6 Li∗ → α + n+ p 9 Be + p → p + 9 Be∗ → α + 5 He → n + α 9 Be + p → α + 6 Li∗ → p + 5 He → n + α 9 Be + p → α + 6 Li∗ → n + 5 Li 9 Be + p → α + 6 Li∗ → n + 5 Li 9 Be + p → n + d + 7 Be 9 Be + p → n + 3 He + 6 Li 9 Be + p → n + 2p + 7 Li 9 Be + p → 2n + 8 B 9 Be + p → 2n + p + 7 Be 9 Be + p
Rev. Sci. Instrum. 85, 063501 (2014) 9 Be
(p,n) reactions up to
Q-value (MeV)
Threshold (MeV)
− 1.573 − 1.665 − 1.850 − 2.185 − 2.46 − 3.245 − 3.525 − 3.54 − 18.339 − 18.452 − 18.918 − 20.427 − 20.563
1.749 1.851 2.057 2.312 2.74 3.608 3.920 3.93 20.389 20.516 21.034 22.711 22.863
requirement for associated instrumentation, (ii) customizable size, easy and convenient irradiation procedure, (iii) insensitivity to low LET radiations, (iv) permanent mode of track registration, (v) easy track development procedure and imaging. The difficulty in measuring neutron spectrum by CR-39 detectors was due to the lack of information on track length and angle of neutron induced recoil tracks from the twodimensional (2D) track images. This is now made possible by the recently developed image analysing program autoTRAK_n.24, 33, 34 Therefore, CR-39 detectors were used along with this image analysing program for obtaining the neutron spectra in this study. This code estimates the neutron spectrum from the track parameters in an easy and effective way using two simple energy dependent folding parameters, viz., track to neutron response function35 and ICRP-74 dose conversion coefficients (DCC).36 The detailed algorithm of spectrum generation from the track parameters was explained earlier.24, 33 In the present study, neutron spectral yield for 9 Be (p, n) reaction at 0◦ with respect to the beam direction with different proton energies was determined. The dose related parameters and detector responses at different beam currents were also investigated. II. MATERIALS AND METHODS A. Irradiations
The CR-39 detectors (12 × 12 × 1.5 mm3 , Intercast, Parma, Italy) were irradiated at the 6 m irradiation port above the analyzing magnet of BARC-TIFR Pelletron Accelerator facility where maximum proton current can be extracted. A new setup was designed for this irradiation experiment and the schematic of the setting is given in Fig. 1. As described in the figure, the proton beam of fixed energy passes through a 4 mm Ta collimator and then interacts with the 6 mm 9 Be target. The neutrons emitted from this reaction irradiated the CR-39 detector kept in a sample holder of dimensions 2.0 cm (diameter) × 2.2 cm (depth). The irradiations were performed at a distance of 1.75 cm from the 9 Be target along the beam direction. This distance factor has been taken into
FIG. 1. Schematic illustration of the irradiation setup.
account for all the further calculations of the neutron yield and the dose values at the source (9 Be target). This whole assembly is a detachable unit enabling easy and quick loading and removal of detectors without disturbing the vacuum in the beamline. This feature helps in performing several experiments during the same run (beam time) by varying the type and energy of ions and to study their reactions with different types of targets. This was not so far achievable without this new assembly. This system has been used in this experiment where the samples were exposed at different projectile (proton) energies ranging from 4 to 24 MeV with an energy interval of 4 MeV. The detectors were exposed at 0◦ with respect to beam direction for 1 min duration at 70 nA beam current which was found to be sufficient to produce statistically significant number of tracks. In this condition, a total of (2.70 ± 0.05) × 1014 protons interacted with the 9 Be target as measured by a current integrator attached to the Faraday cup. Another experiment was performed to study the linearity of detector response and neutron yield with beam current where two different energies, viz., 12 and 22 MeV were chosen for irradiation. Different proton beam currents ranging from 25 to 250 nA were applied, but the total proton fluence in each irradiation was kept constant (∼1015 protons) by adjusting the irradiation time accordingly.
B. Track development and imaging
After the irradiation, the neutron-induced recoil tracks in CR-39 detectors were developed using the conventional chemical etching (CE) technique, i.e., 6.25 N NaOH at 70 ◦ C for 6 h. The developed tracks were then captured using a 5 MP camera, attached to an optical microscope (Carl-Zeiss) at a magnification of 200×. The light intensity was set in a
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way that the centre part of each track receives a higher luminescence compared to the boundary part. For the background correction, a pristine detector was also etched and imaged using the same procedure.
C. Image analysis
The in-house image analysis program autoTRAK_n has been used for the analysis of the 2D track parameters like major and minor axes, eccentricity, track density and also the 3D parameters like track depth, and the particle incidence angle. The program works on the principle of variation in the grey values in and around each track.33 Once the recoil track depth and incidence angles were obtained by the program, the recoil particle energy and hence the incident neutron energy was calculated using Eq. (1): ER = En
4A cos2 θ, (A + 1)2
(1)
where ER and En are the recoil particle and incident neutron energies, respectively, A is the recoil particle mass, and θ is the recoil angle. Finally, this neutron energy distribution was folded with the energy dependent neutron to track response function35 of CR-39 detectors to estimate the total number of neutrons, as discussed in our earlier work.24 Furthermore, the neutron ambient dose equivalent, H∗ (10) was estimated for the neutron spectra at different projectile energies by multiplying the spectrum with the ICRP-74 energy dependent DCC using Eq. (2): Dose =
n
(φi × DCCi × 10−9 ) mSv,
(2)
i=1
where i is the energy bin index, φ i and DCCi are the corresponding neutron fluence (cm−2 ), and ICRP-74 dose conversion coefficients (pSv cm2 ) at ith energy bin. After the complete analysis of neutron spectra and total neutron dose, the quantities such as neutron yield and neutron dose per projectile (proton) were determined for different proton energies.
FIG. 2. Size distribution of the tracks at different proton energies ranging from 4 to 24 MeV.
jectile incident energy. The distribution of major axes for all projectile energies is shown in Fig. 2. As can be seen from these distributions, the median track diameter is about (3.5 ± 0.8) μm. The inset of Fig. 2 shows the variation in the mode value of track diameters with various proton energies. This corroborates the fact that these track parameters are not suitable to correlate with the incident projectile or neutron energy. Similar observations were reported by Phillips et al.38 Therefore, the track depth and angle are determined to generate the neutron spectra, as discussed in Sec. III B. However, most of the tracks, as expected, were found to be circular in nature. A typical distribution pattern of major to minor axes for the neutron-induced recoil tracks due to 24 MeV protons is presented in Fig. 3. The median value for the track size is found to be about (3.46 ± 0.81) μm for major axis and (3.10 ± 0.71) μm for the minor axis. Considering a Gaussian distribution of the track sizes at 95% confidence interval, it has been found that about 92% of the tracks were circular in nature, which are close to the line at 45◦ (Fig. 3).
III. RESULTS AND DISCUSSION
The neutron spectra for different proton energies were estimated using the track parameters obtained from the program autoTRAK_n. Initially, a background track depth distribution was carried out with pristine detector which was then subtracted from the depth distributions obtained for all the irradiated detectors. A. Study of track shape and track density at different proton energies
From the track density measurements it was observed that a total of about 1010 –1011 neutrons cm−2 can be easily counted using the program autoTRAK_n, because of its inside out approach of detecting a track. This is about two orders higher than the conventional track detection software such as ImageJ,37 AxioVision from Carl Zeiss, etc. A study has also been carried out to correlate the track sizes with pro-
FIG. 3. Major to minor axes distribution of the tracks for 24 MeV proton energy.
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FIG. 4. Neutron spectra generated from protons bombarded on 6 mm thick 9 Be target (a) for 4, 8, and 12 MeV protons and (b) for 16, 20, and 24 MeV protons at 0◦ with respect to beam direction.
B. Neutron spectrum generation for different projectile energies
Figures 4(a) and 4(b) show the neutron spectra at 0◦ produced by the interaction of protons of different energies with the thick 9 Be target. The spectra were generated from the analysis of the 3D track parameters, i.e., track depth and angle. The depth of recoil tracks was correlated with the particle range and by using the SRIM code, the energy of the recoil particle was determined from the range values. From the energy of recoil particles, the neutron energy could be derived by using Eq. (1). The details of the algorithm and validation of the program are reported earlier.24, 33 Figure 4(a) contains the neutron emission spectra from proton energies of 4, 8, 12 MeV and Fig. 4(b) shows the spectra obtained for proton energies of 16, 20, and 24 MeV. The data points in these figures represent the cumulative neutron fluence within the energy bin along with the associated statistical errors. The bin-wise error bars were obtained by regenerating the spectra for about 30 times, every time with a different set of statistically significant number of tracks taken from different set of
Rev. Sci. Instrum. 85, 063501 (2014)
track images. The bin widths in all the spectra were chosen as 1 MeV and the neutron yields were obtained in units of cm−2 μC−1 MeV−1 . A brief outline of the reactions involved in the 9 Be (p,n) process are discussed below to have an understanding on the shape of the neutron spectra obtained in the study. This system preferentially undergoes the reaction via a compound nucleus formation 10 B, but a charge exchange or three body break up reaction may also take place in the range of projectile energies under study (as listed in Table I). Considering 9 Be having the smallest binding energy (1.67 MeV) among all the stable nuclei, the three body break up reactions (p, p n) are possible at a threshold of as low as 1.85 MeV, the typical (p, n) reactions start at a threshold of 2.05 MeV, whereas the neutron production via 9 Be excited state (p, p ) (n) starts at slightly higher energies of 3.61 MeV.31 As the first excited state of 9 B is 4.64 MeV, so for 4 MeV proton energy case the ground state transitions (p, n0 ) only will contribute to the reaction channel under study. Moreover, the Q-value of the reaction is −1.85 MeV. So, the neutron spectrum, due to 4 MeV proton was found to have a large peak at 1 MeV followed by a small contribution at 2 MeV bin and almost nil contribution at higher energies. But at progressively higher proton energies, the excited states started contributing and the neutron spectra were found to be populated progressively at relatively higher energies. At 24 MeV proton energy, neutron production reaction channel opens up with production of 6 Li and 7 Li (threshold energies ∼20 MeV) and can also contribute to the neutron spectrum. These neutrons while traversing the thick 9 Be target might result in generation of low energy neutrons. Consequently, as can be seen in Figs. 4(a) and 4(b) at all proton energies the neutron spectra showed a peak at about 1 MeV followed by a relatively long tail. The effective increase in the total number of neutrons emitted can also be understood by looking at the relative peak heights for different proton energies. The peak heights were found to be increasing progressively with increasing proton energy (Figs. 4(a) and 4(b)). The end point energy in all these neutron spectra was found to be about 2 MeV less than the proton energies due to the Q-value of the neutron emission reactions as mentioned earlier. Figure 5 shows the total neutron yield (μC−1 ) at different projectile energies indicating the increase in the yield with
FIG. 5. Total neutron yield per μC at different proton energies.
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TABLE II. Neutron yield and dose values estimated with autoTRAK_n for different proton energies at 0◦ with respect to beam direction. Total Dose Energy current equivalent (MeV) (μC) (mSv μC−1 ) 4 8 12 16 20 24
44.5 42.8 43.1 44.3 43.3 42.0
3.54 ± 1.14 7.85 ± 0.68 20.57 ± 5.80 30.40 ± 4.34 30.47 ± 6.10 35.89 ± 6.72
Neutron yield (μC−1 )
Neutron yield (neutron/proton)
8.48 × 107 ± 2.73 × 107 1.89 × 108 ± 1.63 × 107 4.97 × 108 ± 1.40 × 108 7.08 × 108 ± 1.01 × 108 6.74 × 108 ± 1.35 × 108 7.88 × 108 ± 1.47 × 108
1.36 × 10−5 3.04 × 10−5 7.99 × 10−5 1.13 × 10−4 1.08 × 10−4 1.26 × 10−4
increasing projectile energy. The yield values are listed in Table II, which shows nearly one order enhancement in neutron yield at 24 MeV than with 4 MeV protons. A simple linear fitting (Fig. 5) to the yield values at different projectile energies was made and the equation can be used to predict approximate neutron yield at other proton energies (within the energy regime used in the study). Neutron yield per incident protons of different energies was also calculated and corresponding values were given in Table II. After obtaining the neutron energy distributions, the total dose values were estimated using the energy dependent ICRP-74 DCC values, as discussed in Sec. II C. The neutron dose equivalent values were found to have a linear variation with projectile energy. As expected, an enhancement of about 10 times, i.e., from 3.5 mSv μC−1 at 4 MeV to 35.9 mSv μC−1 at 24 MeV protons was observed (Fig. 6). The linear fit given in Fig. 6 can be used to predict the neutron dose at any other in-between projectile energies. C. Neutron yield at different projectile fluence
The second set of experiments was carried out to investigate the detector response and to test the linearity of neutron yield with projectile beam current. Two different projectile energies, i.e., 12 and 22 MeV, and different beam currents in the range of 25 to 250 nA (as listed in Table III) were chosen. The irradiation time was adjusted to have nearly constant proton fluence on the 9 Be target. Then the irradiated samples were processed and analyzed to obtain the neutron yield
FIG. 6. Variation of neutron dose equivalent (H∗ (10) per μC) with proton energies from 4 to 24 MeV.
(n s−1 ) and to generate the corresponding neutron spectra. The irradiation details like yield and dose values are given in Table III. As can be seen from the table, even for a short time irradiation, the neutron fluence was high enough (∼1011 ) to produce densely overlapping tracks in CR-39 detector, making it difficult for counting. However, it was possible to count these tracks by setting a higher threshold value in the program autoTRAK_n. Such increase in the threshold value, somehow, leads to a slight reduction in the estimated particle fluence and the reduction was found to be about 5–15% depending on energy and beam currents. Finally, the neutron yield and corresponding dose values for both proton energies were estimated and are shown in Figs. 7(a) and 7(b). As shown in Figs. 7(a) and 7(b), a linear trend was found between neutron yield and beam currents for both 12 MeV and 22 MeV energies. The yield at 22 MeV was found to be about 2.5 times higher than that at 12 MeV for all the currents. The yield values are given in Table III. The uncertainty of the measurements was also calculated and the estimated neutron yield per proton was found to be (1.23 ± 0.24) × 10−4 neutron/proton for protons of 22 MeV whereas (1.05 ± 0.18) × 10−4 neutron/proton for 12 MeV protons. Figure 8 shows the plot of neutron dose per μC (mSv μC−1 ) versus beam
TABLE III. Neutron yield and dose values estimated with autoTRAK_n for different proton currents at 12 and 22 MeV proton energies. Energy (MeV)
Current (nA)
Irradiation time (s)
Total charge (μC)
Yield (s−1 )
Dose (mSv μC−1 )
12
25 50 100 150 200 250
600 300 150 60 60 60
140.2 153.4 225.6 94.5 122.1 145.8
1.93 × 108 ± 1.79 × 107 3.74 × 108 ± 4.29 × 107 7.73 × 108 ± 7.89 × 107 8.98 × 108 ± 1.03 × 108 1.27 × 109 ± 1.40 × 108 1.62 × 109 ± 1.75 × 108
8.89 ± 0.83 7.91 ± 0.90 5.06 ± 0.51 8.32 ± 0.95 6.74 ± 0.75 7.24 ± 0.78
22
25 50 100 200
600 300 150 60
131.6 151.0 151.8 137.7
3.54 × 108 ± 5.39 × 107 7.80 × 108 ± 1.62 × 108 1.52 × 109 ± 2.42 × 108 3.69 × 109 ± 6.34 × 108
18.32 ± 2.79 17.41 ± 3.61 17.10 ± 2.73 18.08 ± 3.10
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IV. SUMMARY AND CONCLUSION
Thick target neutron yields generated from the interaction of protons of various energies with a thick 9 Be target have been measured at 0◦ with respect to beam direction using CR39 detectors. A total of 6 different energies, viz., 4, 8, 12, 16, 20, and 24 MeV protons were used to investigate the changes in the neutron spectra due to different reaction channels. The neutron spectra were obtained using an in-house image analyzing program autoTRAK_n which is based on the principle of greylevel variation in and around the recoil tracks developed after chemical etching. The following conclusions were obtained from the present study:
r The total number of tracks counted using auto-
r
r r
r FIG. 7. Variation of neutron yield with beam current at two different proton energies (a) 12 MeV and (b) 22 MeV.
current (nA). It can be seen from Fig. 8 that the dose per μC (or projectile) remains almost flat at all currents and for both energies. The dose values were found to be 18 mSv μC−1 and 7.35 mSv μC−1 for 22 MeV and 12 MeV projectile energies, respectively.
r
TRAK_n was found to be of the order of 1010–11 neutrons cm−2 , which are nearly 2 orders higher than the counting limit of the conventional image analyzing softwares. The track size distribution was not found to be linearly dependent on the projectile energies and, hence, should not be considered as a suitable parameter for neutron spectrum generation. Nearly, 92% of the tracks were found to be circular in nature. The linear fitting equations obtained in this study can be used to predict the neutron yield and dose values at any point within the energy regime studied. All the neutron spectra obtained from this reaction due to different proton energies ranging from 4 to 24 MeV, in general, showed a characteristic peak at about 1 MeV followed by a relatively long tail. Higher the proton energy, the neutron spectra were found to be harder. Neutron yield per unit proton charge was found to increase linearly with increasing proton energy and the variation in the yield is nearly ten times for 24 MeV compared to that at 4 MeV protons. A linear increase in neutron yield with beam current was also observed for 2 different proton energies studied in this work, i.e., 12 and 22 MeV. The dose values (mSv μC−1 ) followed a nearly linear relationship with proton energy.
This work demonstrates successful use of CR-39 detector for the measurement of fast neutron spectrum. The method is found to be simple and effective in terms of easy irradiation, processing, and analysis. Moreover, it does not involve the complex unfolding procedures in generating the neutron spectra. Further work is in progress to study the effect of improved track profile obtained with recently developed rapid etching technique (Microwave-induced chemical etching)39, 40 on the energy resolution of the neutron spectra. ACKNOWLEDGMENTS
FIG. 8. Variation of dose equivalents (H∗ (10) per μC) with beam current at two different proton energies.
The authors thank the staffs of Pelletron Linac Accelerator Facility for their constant support during the experiment and are thankful to Shri A. Mahadakar, Target Lab/TIFR for providing the targets during the experiments. Authors are
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grateful to Dr. R.M. Tripathi, Head, Health Physics Division and Dr. D.N. Sharma, Director, Health, Safety & Environment Group, BARC for the inspiration in carrying out these studies. 1 K.
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T. Routti, Comput. Phys. Commum. 4, 33 (1972). Luszik-Bhadra, E. Dietz, F. D’Errico, S. Guldbakke, and M. Matzke, Radiat. Meas. 28, 473 (1997). 22 F. D’Errico, M. Weiss, M. Luszik-Bhadra, M. Matzke, L. Bernardi, and A. Cecchi, Radiat. Meas. 28, 823 (1997). 23 K. Oda, H. Encho, H. Yoneda, H. Miyake, I. Urabe, K. Kobayashi, and I. Kimura, Nucl. Sci. Technol. 28(7), 608 (1991). 24 S. Paul, S. P. Tripathy, G. S. Sahoo, T. Bandyopadhyay, and P. K. Sarkar, Nucl. Instrum. Methods, Phys. Res. A 729, 444 (2013). 25 M. Caresana, M. Ferrarinia, A. Pola, S. Agosteo, F. Campi, and A. Porta, Nucl. Instrum. Methods, Phys. Res. A 620, 368 (2010). 26 A. R. El-Sersy and S. A. Eman, Eur. Phys. J. A 44, 397 (2010). 27 A. Abdel-Naby, F. A. El-Akkad, and A. M. Abdel-Moneim, Int. J. Radiat. Appl. Radiat. Isot. 43, 1291 (1992). 28 F. Spurny, J. Bednar, L. Johansson, and A. Sätherberg, Radiat. Meas. 26, 645 (1996). 29 G. S. Sahoo, S. P. Tripathy, S. Jena, D. S. Joshi, and T. Bandyopadhyay, Eur. Phys. J.: Appl. Phys. 65, 10701 (2014). 30 V. R. Johnson, F. Ajzenberg, and M. J. Wilsonlaubenstein, Phys. Rev. 79, 187 (1950). 31 W. B. Howard, S. M. Grimes, T. N. Massey, S. I. Al-Quraishi, D. K. Jacobs, C. E. Brient, and J. C. Yanch, Nucl. Sci. Eng. 138(2), 145 (2001). 32 M. A. Lone and A. J. Ferguson, Nucl. Instrum. Methods 189, 515 (1981). 33 S. Paul, S. P. Tripathy, and P. K. Sarkar, Nucl. Instrum. Methods, Phys. Res. A 690, 58 (2012). 34 S. P. Tripathy, S. Paul, G. S. Sahoo, V. Suman, C. Sunil, D. S. Joshi, and T. Bandyopadhyay, Nucl. Instrum. Methods, Phys. Res. B 318, 237 (2014). 35 S. Mayer, M. Boschung, H. Hoedlmoser, Th. Buchillier, C. Bailat, and B. Bitterli, Radiat. Meas. 47(8), 634 (2012). 36 ICRP Publication 74, Ann. ICRP 26(3–4) (1996). 37 See http://rsb.info.nih.gov/ij/ for freely available image analysing software ImageJ. 38 G. W. Phillips, J. E. Spann, J. S. Bogard, T. VoDinh, D. Emfietzoglou, R. T. Devine, and M. Moscovitch, Radiat. Prot. Dosim. 120, 457 (2006). 39 S. P. Tripathy, R. V. Kolekar, C. Sunil, P. K. Sarkar, K. K. Dwivedi, and D. N. Sharma, Nucl. Instrum. Methods, Phys. Res. A 612, 421 (2010). 40 G. S. Sahoo, S. P. Tripathy, and T. Bandyopadhyay, Nucl. Instrum. Methods, Phys. Res. A 739, 83 (2014). 21 M.
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 063501 (2014)
Measurement of neutron spectra generated from bombardment of 4 to 24 MeV protons on a thick 9 Be target and estimation of neutron yields Sabyasachi Paul,1 G. S. Sahoo,1 S. P. Tripathy,1,a) S. C. Sharma,2 Ramjilal,2 N. G. Ninawe,2 C. Sunil,1 A. K. Gupta,2 and T. Bandyopadhyay1 1 2
Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India
(Received 22 April 2014; accepted 15 May 2014; published online 4 June 2014) A systematic study on the measurement of neutron spectra emitted from the interaction of protons of various energies with a thick beryllium target has been carried out. The measurements were carried out in the forward direction (at 0◦ with respect to the direction of protons) using CR-39 detectors. The doses were estimated using the in-house image analyzing program autoTRAK_n, which works on the principle of luminosity variation in and around the track boundaries. A total of six different proton energies starting from 4 MeV to 24 MeV with an energy gap of 4 MeV were chosen for the study of the neutron yields and the estimation of doses. Nearly, 92% of the recoil tracks developed after chemical etching were circular in nature, but the size distributions of the recoil tracks were not found to be linearly dependent on the projectile energy. The neutron yield and dose values were found to be increasing linearly with increasing projectile energies. The response of CR-39 detector was also investigated at different beam currents at two different proton energies. A linear increase of neutron yield with beam current was observed. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4880202] I. INTRODUCTION
Intense fast neutron sources are produced primarily by charged particle induced nuclear reactions using protons as projectile with low Z thick target elements.1–3 The emitted fast neutrons from these reactions are important for different medical applications like accelerator based boron neutron capture therapy (BNCT) for cancer treatment3–5 and for studies on shielding6 and dosimetric7, 8 applications. For this purpose, a thick 9 Be target has been bombarded with proton energies ranging from a few MeV to few tens of MeV to produce neutrons.9–13 In this energy range of protons, a set of different reaction channels open up and produce neutrons having broad spectrum of energy with a peak at lower energy followed by a tail with decreasing neutron yield at higher energies. Among these, the low energy neutrons play an important role for the cancer treatment, especially for BNCT, as the relative biological effectiveness (RBE) is significantly high due to a better interaction probability and preferential absorption of low energy neutrons in the organs and tissues.14 So, to study the neutron spectral yield generated from the reaction of protons of various energies with a thick 9 Be target, a systematic investigation has been carried out at the BARCTIFR Pelletron Accelerator facility, Mumbai, India. The set of discrete reactions for the system under study, i.e., 9 Be + 1 H reaction, is well known and is listed in Table I. The neutron yield and dose values produced by these reactions depend primarily on the projectile energy. For the measurement of the neutrons generated from this reaction, several methods using active and passive techniques such as time of flight, Bonner Sphere Spectrometer, activation analysis, etc., have a) Author to whom correspondence should be addressed. Electronic
addresses: [email protected] and [email protected]
0034-6748/2014/85(6)/063501/7/$30.00
been established which often involve bulky and expensive instrumentation. In addition, these measurements (except time of flight) involve complex spectrum unfolding procedures using codes like MAXED,15 ANDI-03,16 FRUIT,17 BUNKI,18 GAMCD,19 LOUHI,20 etc., to generate the neutron spectrum which further require a well-defined response matrix and initial guess spectrum.21–23 Considering this difficult instrumentation, time consuming analysis, and complex unfolding procedures, the passive technique using CR-39 detectors and image analysis programs as an alternative for neutron spectrometry has gained popularity in the recent past.24 Researchers have developed response matrices for Bonner Sphere Spectrometry using boron coated CR-39 detectors25 with experimental validation. A few works have also been reported regarding the generation of neutron spectrum using CR-39 detectors.21, 22, 24, 26, 27 Studies have also been carried out for estimation of dose responses of CR-39 detector in neutron fields by measuring the linear energy transfer (LET) spectra28 and also by correlating with the optical properties.29 A comprehensive literature survey on the 9 Be (p,n) reaction reveals that the first quantitative estimation of neutron yields from this projectile target combination has been reported in the early 1950s30 using a thin beryllium target with projectile energies from threshold to 4 MeV. Since then several experiments were carried out with protons to study the neutron yield and angular distributions with different target thickness at various projectile energies using mainly time of flight measurements.10, 31, 32 But reports on spectrum generation using passive detectors like CR-39 for various proton energies from 4 MeV to 24 MeV are rare and hence an effort has been made in this work to systematically study the emitted particle spectra using CR-39 detectors. For the present study, the CR-39 detectors were chosen mainly due to the following reasons: (i) passive technique and no
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TABLE I. Neutron producing reactions for 24 MeV projectile energy.
Reaction 9 Be + p
→ n + p + 2α → n + p + 8B 9 Be + p → n + 9 B 9 Be + p → α + p + 6 Li∗ → α + n+ p 9 Be + p → p + 9 Be∗ → α + 5 He → n + α 9 Be + p → α + 6 Li∗ → p + 5 He → n + α 9 Be + p → α + 6 Li∗ → n + 5 Li 9 Be + p → α + 6 Li∗ → n + 5 Li 9 Be + p → n + d + 7 Be 9 Be + p → n + 3 He + 6 Li 9 Be + p → n + 2p + 7 Li 9 Be + p → 2n + 8 B 9 Be + p → 2n + p + 7 Be 9 Be + p
Rev. Sci. Instrum. 85, 063501 (2014) 9 Be
(p,n) reactions up to
Q-value (MeV)
Threshold (MeV)
− 1.573 − 1.665 − 1.850 − 2.185 − 2.46 − 3.245 − 3.525 − 3.54 − 18.339 − 18.452 − 18.918 − 20.427 − 20.563
1.749 1.851 2.057 2.312 2.74 3.608 3.920 3.93 20.389 20.516 21.034 22.711 22.863
requirement for associated instrumentation, (ii) customizable size, easy and convenient irradiation procedure, (iii) insensitivity to low LET radiations, (iv) permanent mode of track registration, (v) easy track development procedure and imaging. The difficulty in measuring neutron spectrum by CR-39 detectors was due to the lack of information on track length and angle of neutron induced recoil tracks from the twodimensional (2D) track images. This is now made possible by the recently developed image analysing program autoTRAK_n.24, 33, 34 Therefore, CR-39 detectors were used along with this image analysing program for obtaining the neutron spectra in this study. This code estimates the neutron spectrum from the track parameters in an easy and effective way using two simple energy dependent folding parameters, viz., track to neutron response function35 and ICRP-74 dose conversion coefficients (DCC).36 The detailed algorithm of spectrum generation from the track parameters was explained earlier.24, 33 In the present study, neutron spectral yield for 9 Be (p, n) reaction at 0◦ with respect to the beam direction with different proton energies was determined. The dose related parameters and detector responses at different beam currents were also investigated. II. MATERIALS AND METHODS A. Irradiations
The CR-39 detectors (12 × 12 × 1.5 mm3 , Intercast, Parma, Italy) were irradiated at the 6 m irradiation port above the analyzing magnet of BARC-TIFR Pelletron Accelerator facility where maximum proton current can be extracted. A new setup was designed for this irradiation experiment and the schematic of the setting is given in Fig. 1. As described in the figure, the proton beam of fixed energy passes through a 4 mm Ta collimator and then interacts with the 6 mm 9 Be target. The neutrons emitted from this reaction irradiated the CR-39 detector kept in a sample holder of dimensions 2.0 cm (diameter) × 2.2 cm (depth). The irradiations were performed at a distance of 1.75 cm from the 9 Be target along the beam direction. This distance factor has been taken into
FIG. 1. Schematic illustration of the irradiation setup.
account for all the further calculations of the neutron yield and the dose values at the source (9 Be target). This whole assembly is a detachable unit enabling easy and quick loading and removal of detectors without disturbing the vacuum in the beamline. This feature helps in performing several experiments during the same run (beam time) by varying the type and energy of ions and to study their reactions with different types of targets. This was not so far achievable without this new assembly. This system has been used in this experiment where the samples were exposed at different projectile (proton) energies ranging from 4 to 24 MeV with an energy interval of 4 MeV. The detectors were exposed at 0◦ with respect to beam direction for 1 min duration at 70 nA beam current which was found to be sufficient to produce statistically significant number of tracks. In this condition, a total of (2.70 ± 0.05) × 1014 protons interacted with the 9 Be target as measured by a current integrator attached to the Faraday cup. Another experiment was performed to study the linearity of detector response and neutron yield with beam current where two different energies, viz., 12 and 22 MeV were chosen for irradiation. Different proton beam currents ranging from 25 to 250 nA were applied, but the total proton fluence in each irradiation was kept constant (∼1015 protons) by adjusting the irradiation time accordingly.
B. Track development and imaging
After the irradiation, the neutron-induced recoil tracks in CR-39 detectors were developed using the conventional chemical etching (CE) technique, i.e., 6.25 N NaOH at 70 ◦ C for 6 h. The developed tracks were then captured using a 5 MP camera, attached to an optical microscope (Carl-Zeiss) at a magnification of 200×. The light intensity was set in a
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way that the centre part of each track receives a higher luminescence compared to the boundary part. For the background correction, a pristine detector was also etched and imaged using the same procedure.
C. Image analysis
The in-house image analysis program autoTRAK_n has been used for the analysis of the 2D track parameters like major and minor axes, eccentricity, track density and also the 3D parameters like track depth, and the particle incidence angle. The program works on the principle of variation in the grey values in and around each track.33 Once the recoil track depth and incidence angles were obtained by the program, the recoil particle energy and hence the incident neutron energy was calculated using Eq. (1): ER = En
4A cos2 θ, (A + 1)2
(1)
where ER and En are the recoil particle and incident neutron energies, respectively, A is the recoil particle mass, and θ is the recoil angle. Finally, this neutron energy distribution was folded with the energy dependent neutron to track response function35 of CR-39 detectors to estimate the total number of neutrons, as discussed in our earlier work.24 Furthermore, the neutron ambient dose equivalent, H∗ (10) was estimated for the neutron spectra at different projectile energies by multiplying the spectrum with the ICRP-74 energy dependent DCC using Eq. (2): Dose =
n
(φi × DCCi × 10−9 ) mSv,
(2)
i=1
where i is the energy bin index, φ i and DCCi are the corresponding neutron fluence (cm−2 ), and ICRP-74 dose conversion coefficients (pSv cm2 ) at ith energy bin. After the complete analysis of neutron spectra and total neutron dose, the quantities such as neutron yield and neutron dose per projectile (proton) were determined for different proton energies.
FIG. 2. Size distribution of the tracks at different proton energies ranging from 4 to 24 MeV.
jectile incident energy. The distribution of major axes for all projectile energies is shown in Fig. 2. As can be seen from these distributions, the median track diameter is about (3.5 ± 0.8) μm. The inset of Fig. 2 shows the variation in the mode value of track diameters with various proton energies. This corroborates the fact that these track parameters are not suitable to correlate with the incident projectile or neutron energy. Similar observations were reported by Phillips et al.38 Therefore, the track depth and angle are determined to generate the neutron spectra, as discussed in Sec. III B. However, most of the tracks, as expected, were found to be circular in nature. A typical distribution pattern of major to minor axes for the neutron-induced recoil tracks due to 24 MeV protons is presented in Fig. 3. The median value for the track size is found to be about (3.46 ± 0.81) μm for major axis and (3.10 ± 0.71) μm for the minor axis. Considering a Gaussian distribution of the track sizes at 95% confidence interval, it has been found that about 92% of the tracks were circular in nature, which are close to the line at 45◦ (Fig. 3).
III. RESULTS AND DISCUSSION
The neutron spectra for different proton energies were estimated using the track parameters obtained from the program autoTRAK_n. Initially, a background track depth distribution was carried out with pristine detector which was then subtracted from the depth distributions obtained for all the irradiated detectors. A. Study of track shape and track density at different proton energies
From the track density measurements it was observed that a total of about 1010 –1011 neutrons cm−2 can be easily counted using the program autoTRAK_n, because of its inside out approach of detecting a track. This is about two orders higher than the conventional track detection software such as ImageJ,37 AxioVision from Carl Zeiss, etc. A study has also been carried out to correlate the track sizes with pro-
FIG. 3. Major to minor axes distribution of the tracks for 24 MeV proton energy.
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FIG. 4. Neutron spectra generated from protons bombarded on 6 mm thick 9 Be target (a) for 4, 8, and 12 MeV protons and (b) for 16, 20, and 24 MeV protons at 0◦ with respect to beam direction.
B. Neutron spectrum generation for different projectile energies
Figures 4(a) and 4(b) show the neutron spectra at 0◦ produced by the interaction of protons of different energies with the thick 9 Be target. The spectra were generated from the analysis of the 3D track parameters, i.e., track depth and angle. The depth of recoil tracks was correlated with the particle range and by using the SRIM code, the energy of the recoil particle was determined from the range values. From the energy of recoil particles, the neutron energy could be derived by using Eq. (1). The details of the algorithm and validation of the program are reported earlier.24, 33 Figure 4(a) contains the neutron emission spectra from proton energies of 4, 8, 12 MeV and Fig. 4(b) shows the spectra obtained for proton energies of 16, 20, and 24 MeV. The data points in these figures represent the cumulative neutron fluence within the energy bin along with the associated statistical errors. The bin-wise error bars were obtained by regenerating the spectra for about 30 times, every time with a different set of statistically significant number of tracks taken from different set of
Rev. Sci. Instrum. 85, 063501 (2014)
track images. The bin widths in all the spectra were chosen as 1 MeV and the neutron yields were obtained in units of cm−2 μC−1 MeV−1 . A brief outline of the reactions involved in the 9 Be (p,n) process are discussed below to have an understanding on the shape of the neutron spectra obtained in the study. This system preferentially undergoes the reaction via a compound nucleus formation 10 B, but a charge exchange or three body break up reaction may also take place in the range of projectile energies under study (as listed in Table I). Considering 9 Be having the smallest binding energy (1.67 MeV) among all the stable nuclei, the three body break up reactions (p, p n) are possible at a threshold of as low as 1.85 MeV, the typical (p, n) reactions start at a threshold of 2.05 MeV, whereas the neutron production via 9 Be excited state (p, p ) (n) starts at slightly higher energies of 3.61 MeV.31 As the first excited state of 9 B is 4.64 MeV, so for 4 MeV proton energy case the ground state transitions (p, n0 ) only will contribute to the reaction channel under study. Moreover, the Q-value of the reaction is −1.85 MeV. So, the neutron spectrum, due to 4 MeV proton was found to have a large peak at 1 MeV followed by a small contribution at 2 MeV bin and almost nil contribution at higher energies. But at progressively higher proton energies, the excited states started contributing and the neutron spectra were found to be populated progressively at relatively higher energies. At 24 MeV proton energy, neutron production reaction channel opens up with production of 6 Li and 7 Li (threshold energies ∼20 MeV) and can also contribute to the neutron spectrum. These neutrons while traversing the thick 9 Be target might result in generation of low energy neutrons. Consequently, as can be seen in Figs. 4(a) and 4(b) at all proton energies the neutron spectra showed a peak at about 1 MeV followed by a relatively long tail. The effective increase in the total number of neutrons emitted can also be understood by looking at the relative peak heights for different proton energies. The peak heights were found to be increasing progressively with increasing proton energy (Figs. 4(a) and 4(b)). The end point energy in all these neutron spectra was found to be about 2 MeV less than the proton energies due to the Q-value of the neutron emission reactions as mentioned earlier. Figure 5 shows the total neutron yield (μC−1 ) at different projectile energies indicating the increase in the yield with
FIG. 5. Total neutron yield per μC at different proton energies.
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TABLE II. Neutron yield and dose values estimated with autoTRAK_n for different proton energies at 0◦ with respect to beam direction. Total Dose Energy current equivalent (MeV) (μC) (mSv μC−1 ) 4 8 12 16 20 24
44.5 42.8 43.1 44.3 43.3 42.0
3.54 ± 1.14 7.85 ± 0.68 20.57 ± 5.80 30.40 ± 4.34 30.47 ± 6.10 35.89 ± 6.72
Neutron yield (μC−1 )
Neutron yield (neutron/proton)
8.48 × 107 ± 2.73 × 107 1.89 × 108 ± 1.63 × 107 4.97 × 108 ± 1.40 × 108 7.08 × 108 ± 1.01 × 108 6.74 × 108 ± 1.35 × 108 7.88 × 108 ± 1.47 × 108
1.36 × 10−5 3.04 × 10−5 7.99 × 10−5 1.13 × 10−4 1.08 × 10−4 1.26 × 10−4
increasing projectile energy. The yield values are listed in Table II, which shows nearly one order enhancement in neutron yield at 24 MeV than with 4 MeV protons. A simple linear fitting (Fig. 5) to the yield values at different projectile energies was made and the equation can be used to predict approximate neutron yield at other proton energies (within the energy regime used in the study). Neutron yield per incident protons of different energies was also calculated and corresponding values were given in Table II. After obtaining the neutron energy distributions, the total dose values were estimated using the energy dependent ICRP-74 DCC values, as discussed in Sec. II C. The neutron dose equivalent values were found to have a linear variation with projectile energy. As expected, an enhancement of about 10 times, i.e., from 3.5 mSv μC−1 at 4 MeV to 35.9 mSv μC−1 at 24 MeV protons was observed (Fig. 6). The linear fit given in Fig. 6 can be used to predict the neutron dose at any other in-between projectile energies. C. Neutron yield at different projectile fluence
The second set of experiments was carried out to investigate the detector response and to test the linearity of neutron yield with projectile beam current. Two different projectile energies, i.e., 12 and 22 MeV, and different beam currents in the range of 25 to 250 nA (as listed in Table III) were chosen. The irradiation time was adjusted to have nearly constant proton fluence on the 9 Be target. Then the irradiated samples were processed and analyzed to obtain the neutron yield
FIG. 6. Variation of neutron dose equivalent (H∗ (10) per μC) with proton energies from 4 to 24 MeV.
(n s−1 ) and to generate the corresponding neutron spectra. The irradiation details like yield and dose values are given in Table III. As can be seen from the table, even for a short time irradiation, the neutron fluence was high enough (∼1011 ) to produce densely overlapping tracks in CR-39 detector, making it difficult for counting. However, it was possible to count these tracks by setting a higher threshold value in the program autoTRAK_n. Such increase in the threshold value, somehow, leads to a slight reduction in the estimated particle fluence and the reduction was found to be about 5–15% depending on energy and beam currents. Finally, the neutron yield and corresponding dose values for both proton energies were estimated and are shown in Figs. 7(a) and 7(b). As shown in Figs. 7(a) and 7(b), a linear trend was found between neutron yield and beam currents for both 12 MeV and 22 MeV energies. The yield at 22 MeV was found to be about 2.5 times higher than that at 12 MeV for all the currents. The yield values are given in Table III. The uncertainty of the measurements was also calculated and the estimated neutron yield per proton was found to be (1.23 ± 0.24) × 10−4 neutron/proton for protons of 22 MeV whereas (1.05 ± 0.18) × 10−4 neutron/proton for 12 MeV protons. Figure 8 shows the plot of neutron dose per μC (mSv μC−1 ) versus beam
TABLE III. Neutron yield and dose values estimated with autoTRAK_n for different proton currents at 12 and 22 MeV proton energies. Energy (MeV)
Current (nA)
Irradiation time (s)
Total charge (μC)
Yield (s−1 )
Dose (mSv μC−1 )
12
25 50 100 150 200 250
600 300 150 60 60 60
140.2 153.4 225.6 94.5 122.1 145.8
1.93 × 108 ± 1.79 × 107 3.74 × 108 ± 4.29 × 107 7.73 × 108 ± 7.89 × 107 8.98 × 108 ± 1.03 × 108 1.27 × 109 ± 1.40 × 108 1.62 × 109 ± 1.75 × 108
8.89 ± 0.83 7.91 ± 0.90 5.06 ± 0.51 8.32 ± 0.95 6.74 ± 0.75 7.24 ± 0.78
22
25 50 100 200
600 300 150 60
131.6 151.0 151.8 137.7
3.54 × 108 ± 5.39 × 107 7.80 × 108 ± 1.62 × 108 1.52 × 109 ± 2.42 × 108 3.69 × 109 ± 6.34 × 108
18.32 ± 2.79 17.41 ± 3.61 17.10 ± 2.73 18.08 ± 3.10
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IV. SUMMARY AND CONCLUSION
Thick target neutron yields generated from the interaction of protons of various energies with a thick 9 Be target have been measured at 0◦ with respect to beam direction using CR39 detectors. A total of 6 different energies, viz., 4, 8, 12, 16, 20, and 24 MeV protons were used to investigate the changes in the neutron spectra due to different reaction channels. The neutron spectra were obtained using an in-house image analyzing program autoTRAK_n which is based on the principle of greylevel variation in and around the recoil tracks developed after chemical etching. The following conclusions were obtained from the present study:
r The total number of tracks counted using auto-
r
r r
r FIG. 7. Variation of neutron yield with beam current at two different proton energies (a) 12 MeV and (b) 22 MeV.
current (nA). It can be seen from Fig. 8 that the dose per μC (or projectile) remains almost flat at all currents and for both energies. The dose values were found to be 18 mSv μC−1 and 7.35 mSv μC−1 for 22 MeV and 12 MeV projectile energies, respectively.
r
TRAK_n was found to be of the order of 1010–11 neutrons cm−2 , which are nearly 2 orders higher than the counting limit of the conventional image analyzing softwares. The track size distribution was not found to be linearly dependent on the projectile energies and, hence, should not be considered as a suitable parameter for neutron spectrum generation. Nearly, 92% of the tracks were found to be circular in nature. The linear fitting equations obtained in this study can be used to predict the neutron yield and dose values at any point within the energy regime studied. All the neutron spectra obtained from this reaction due to different proton energies ranging from 4 to 24 MeV, in general, showed a characteristic peak at about 1 MeV followed by a relatively long tail. Higher the proton energy, the neutron spectra were found to be harder. Neutron yield per unit proton charge was found to increase linearly with increasing proton energy and the variation in the yield is nearly ten times for 24 MeV compared to that at 4 MeV protons. A linear increase in neutron yield with beam current was also observed for 2 different proton energies studied in this work, i.e., 12 and 22 MeV. The dose values (mSv μC−1 ) followed a nearly linear relationship with proton energy.
This work demonstrates successful use of CR-39 detector for the measurement of fast neutron spectrum. The method is found to be simple and effective in terms of easy irradiation, processing, and analysis. Moreover, it does not involve the complex unfolding procedures in generating the neutron spectra. Further work is in progress to study the effect of improved track profile obtained with recently developed rapid etching technique (Microwave-induced chemical etching)39, 40 on the energy resolution of the neutron spectra. ACKNOWLEDGMENTS
FIG. 8. Variation of dose equivalents (H∗ (10) per μC) with beam current at two different proton energies.
The authors thank the staffs of Pelletron Linac Accelerator Facility for their constant support during the experiment and are thankful to Shri A. Mahadakar, Target Lab/TIFR for providing the targets during the experiments. Authors are
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