Radiation Protection Dosimetry Advance Access published November 26, 2013 Radiation Protection Dosimetry (2013), pp. 1–6
doi:10.1093/rpd/nct293
AUTOMATIC NEUTRON DOSIMETRY SYSTEM BASED ON FLUORESCENT NUCLEAR TRACK DETECTOR TECHNOLOGY M. S. Akselrod1,*, V. V. Fomenko1, J. A. Bartz1,2 and T. L. Haslett3 1 Landauer, Inc., 723 1/2 Eastgate St., Stillwater, OK 74074, USA 2 Oklahoma State University, Stillwater, OK 74078, USA 3 Avo Photonics, Inc., 700 Business Center Dr., Horsham, PA 19044, USA
For the first time, the authors are describing an automatic fluorescent nuclear track detector (FNTD) reader for neutron dosimetry. FNTD is a luminescent integrating type of detector made of aluminium oxide crystals that does not require electronics or batteries during irradiation. Non-destructive optical readout of the detector is performed using a confocal laser scanning fluorescence imaging with near-diffraction limited resolution. The fully automatic table-top reader allows one to load up to 216 detectors on a tray, read their engraved IDs using a CCD camera and optical character recognition, scan and process simultaneously two types of images in fluorescent and reflected laser light contrast to eliminate false-positive tracks related to surface and volume crystal imperfections. The FNTD dosimetry system allows one to measure neutron doses from 0.1 mSv to 20 Sv and covers neutron energies from thermal to 20 MeV. The reader is characterised by a robust, compact optical design, fast data processing electronics and user-friendly software.
FNTD TECHNOLOGY AND DETECTORS For the first time, the authors are describing an automatic reader for neutron dosimetry that is based on fluorescent nuclear track detector (FNTD) technology reviewed in detail in Akselrod and Sykora(1). FNTD is a passive integrating type of detector that does not require wires, electronics or batteries during irradiation. The detectors are made of aluminium oxide single crystals doped with carbon and magnesium (Al2O3:C,Mg) made in the form of 8` 4` 0.5 mm3 plates polished on one side. The crystals contain F2þ 2 (2Mg) aggregate oxygen vacancy defects that undergo radiochromic transformation under irradiation(2), provide high quantum yield of luminescence(3), show temperature stability of radiation-induced luminescence up to 6008C(4) and no sensitivity to ambient room light during handling and readout. The FNTD reader technology can operate in two modes of image processing—track counting mode at low doses (0.1–200 mSv) and ‘analogue’ processing mode—which calculates power spectrum integral of fluorescence images as a dosimetric parameter, and is applied in dose range from 10 mSv up to 20 Sv(5, 6). The detectors can be read non-destructively multiple times(5) and nevertheless are reusable after thermal annealing or optical bleaching(2, 5). The energy and angular response of the detectors were investigated for monoenergetic and broad spectrum neutrons in the range from 40 keV to 19.5 MeV(7). The angular response of the dosemeters was studied in bare AmBe neutron fields(8). Major advantages of FNTDs over conventional CR-39 plastic nuclear track detectors(9) include the following: no need for post-irradiation chemical etching,
a significantly wider range of measured doses and LETs (0.5–1800 keV mm21)(6, 10) and higher spatial imaging resolution and larger measured track density. FNTDs are capable of processing charged particle fluence in excess of 106 cm22 without saturation (track overlap)(10, 11). Most of OSL and TL neutron detectors are not sensitive to fast neutrons and can operate only in albedo configuration(12) whereas FNTDs are sensitive to low-LET recoil protons and are able to detect fast protons and neutrons with energies up to 200 MeV(11). In comparison with superheated emulsions (bubble detectors), which have a very high sensitivity to neutrons but are bulky(13), FNTDs are much more compact and are not sensitive to temperature fluctuations.
READOUT INSTRUMENT Non-destructive readout of FNTDs is performed using a fluorescence laser scanning confocal imaging with nearly diffraction-limited resolution(1). The image is formed from 750-nm fluorescence intensity excited with a 638-nm diode laser. The automatic, table-top reader (Figures 1 and 2) allows one to load 216 detectors on a tray after removing them from a dosimeter slide (Figure 3), read their engraved IDs using a CCD-camera and optical character recognition (Figure 4), sequentially scan all detectors and process obtained fluorescence images (Figure 5) with user-configurable parameters. One of the current OSL-FNTD dosimeter slide configurations, as depicted in Figure 3, consists of three Al2O3:C OSL sensors and FNTD crystal installed with polished
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
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*Corresponding author: [email protected]
M. S. AKSELROD ET AL.
Figure 4. ID engraved on a back, unpolished side of FNTD crystal. Three features are identified for further imaging: detector edges, bevel position and engraved ID. The detected OCR ID is also shown.
Figure 2. Schematic diagram of 3D opto-mechanical scanning of FNTDs.
Figure 5. Fluorescence image (100` 100 mm) with neutroninduced recoil proton tracks recognised and counted by the FNTD reader image processing software.
Figure 3. Dosimeter slide with three OSL sensors and FNTD crystal on top of three converters (Li-glass, PE and PTFE).
side down on top of three radiation converters: Li-based glass for thermal neutron detection, polyethylene (PE) for fast neutrons and Teflonw (PTFE) for gamma signal subtraction at high photon doses(5, 6).
The simplified overall optical diagram of the reader was presented elsewhere(1, 6), and its latest variant of 3D scanning arrangement is depicted in Figure 2. To increase the reader productivity, imaging is performed semi-continuously, in strips, consisting of a configurable number of images (12 is the default). The fast axis of scanning (x) is performed by a galvanometer with the slow axis of image scanning ( y) been done by the high precision translation stage. One important aspect of the reader operation is the need to image detectors at a precise and consistent depth of 2.0+0.2 mm below the polished surface. To achieve such precision, the reader automatically
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Figure 1. Automatic FNTD dosimetry system.
FNTD NEUTRON DOSIMETRY SYSTEM
NEUTRON DOSIMETRY PERFORMANCE In this paper, the authors are focusing on the FNTD instrument’s ability to measure very low doses (,1 mSv), linearity of dose dependence at low doses and neutron measurements in mixed neutron –photon fields. Different ratios of 241AmBe or 252Cf neutrons (bare and moderated) and photons (137Cs, H150 and M150 X rays) were used to assess the reader performance against the US ANSI N13.11-2009 standard(14). The linearity of dose dependence was tested according to the draft of ISO-21909-1 standard(15). Because of relatively low doses, all measurements were performed in track counting mode with sensitivities for each radiation source determined by preliminary calibration. FNTD crystals for these tests were randomly selected from a large production lot, and no individual calibration of each detector was performed. Linearity of dose dependence and uncertainties The linearity performance characteristic was determined after irradiation of dosemeters with bare AmBe neutrons on 40` 40` 15 cm PMMA phantom with doses of 0.2, 0.5, 1, 2, 5, 10, and 30 mSv at Landauer calibration laboratory. Although the
number of detectors (i) irradiated at each dose ( j ) is not strictly defined in the ISO standard, it is an important consideration for satisfaction of the standard: j + lj =Ht; j 1:2: 0:8 L ¼ H
ð1Þ
j is the average measured dose In this equation, H from lot j. Ht,j is the delivered dose for lot j. lj is calculated in units of dose as follows:
sj t n lj ¼ pffiffiffi ; n
ð2Þ
where sj is the standard deviation of measurements for n detectors, used in lot j, tn is the two-sided Student’s t value for the 0.95 confidence value for a sample of size n. To investigate the role of lj, the authors assumed j ¼ Ht; j and set the delivered dose to 0.5 mSv. In H this case, the measurements uncertainty l,j/Hj from equation (1) becomes 0.2. The si term in lj can be estimated based on Poisson statistics of total number of counted tracks as: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCA ¼ Hj ðD C AÞ1=2 ; si ¼ Hj ð3Þ DCA where D represents the delivered dose in mSv, C represents the sensitivity in tracks mm22 mSv21 and A represents the scan area in mm2. Solving equation (2) for si, inserting it into equation (3) and solving for area A result in a useful relationship allowing one to optimise the scan area and the scan time (Figure 6). To increase the instrument productivity and decrease the scanning time per detector, the adaptive readout mode was developed and tested, in which the scanning regime was automatically adjusted for each detector based on photon dose obtained from OSL sensors of the badge. By analysing experimental data and theoretical estimation based on equation (3), it was determined that the linearity performance characteristics can be satisfied at readout rate of 4 min per detector with 2 mm2 of scan area for batches of 10 detectors. The results of the linearity tests presented in Figure 7 and Table 1 are compliant with the draft of the ISO standard.
Low limit of detection To determine the low limit of detection (LLD) parameter, 15 freshly bleached FNTDs were scanned and processed with the optimal set of parameters determined in the section Linearity of dose dependence and uncertainties. Another 15 detectors were irradiated and scanned after 10 mSv of AmBe neutrons. An LLD value for pure neutron fields was calculated
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performs surface mapping of each detector and calculates the vertical (z) position of the objective along each strip of images. Reflected excitation laser light is used to locate the surface and appropriately sets the vertical (z) position of the objective along each scan line of the image. In addition, the image captured from reflected excitation light, collected simultaneously with the fluorescence image, allows surface and a crystal defects to be identified and masked in order to remove false-positive track. The latest advances in FNTD instrumentation and image processing include the following: faster rate of imaging and data acquisition—down to 1.0 s (from previous 12 s) per 100` 100 mm image; improvements in image processing to reliably identify recoil protons and alpha particle tracks; better discrimination of non-radiation-induced defects and surface contamination; and adaptive scanning depending on measured (OSL) photon dose. The reader is characterised by a compact optical design, fast data processing electronics and userfriendly software designed for three levels of access: technician, senior dosimetrist and technical support engineer. The reader is flexible in the selection of scan settings. Users with adequate permissions can specify the converter areas to be scanned such as the amount of area under each converter, the scanning speed and the scan depth, thus allowing the system to be customised for a variety of dosimetric applications. For each detector, a series of diagnostic routines is performed prior, during and after the scan.
M. S. AKSELROD ET AL.
based on recommendations of ANSI N13.11(14) as follows: LLD ¼
2½tp; n1 s0 þ ðtp; n1 s1 =H1 Þ2 H00 2
1 ðtp; n1 s1 =H1 Þ
;
ð4Þ
where tp,n21 is Student’s t value for (n 2 1) degrees of freedom at the confidence level value p ¼ 0.95, s0 is standard deviation of the unirradiated dosimeter readings, s1 is standard deviation of the irradiated dosimeter readings, H1 is average of the irradiated dosimeter readings and H00 is average of the unirradiated dosimeter readings without background subtracted. As shown in Table 2, an LLD of better than 0.05 mSv has been demonstrated. Performance in mixed neutron–photon fields Performance of the FNTD dosimetry system was tested according to the category V-A of US ANSI standard(14). Dosemeters for this test were irradiated with varying mixtures of neutrons and photons. The total dose delivered during testing was in the range from 1.5 to 50 mSv with neutron-to-photon ratios ranging from 1:3 to 3:1. The neutrons were delivered at Pacific Northwest National Laboratory (PNNL) and were chosen randomly between D2O-moderated 252 Cf and bare 252Cf sources. The photon portions of the total dose were delivered by Landauer calibration facility and were chosen from a selection of standard X-ray and g-ray sources. At least 3 of the 15
Figure 7. Measured neutron doses as a function of delivered doses (top). 1:1 line represented as a dashed line. Error bars represent 1 standard deviation. Detectors were irradiated with 241 AmBe neutrons. In the bottom chart, measured dose and standard deviations were normalised to the delivered dose.
Table 1. Tabulated results of linearity processed according to the draft of ISO-21909-1. Delivered dose, mSv 0.20 0.50 1.00 2.00 5.00 10.00 30.00
Measured dose, mSv
Standard deviation, mSv
Lþ
L2
0.19 0.47 1.01 2.07 5.28 9.71 33.77
0.10 0.19 0.12 0.17 0.60 1.31 1.09
1.21 1.16 1.08 1.09 1.13 1.06 1.15
0.89 0.92 1.00 1.03 1.05 0.88 1.10
dosemeters were irradiated using a high-energy photon source (average energy 500 keV). Compliance with the standard for category V is based on the average bias (B) and standard deviation of the biases (S) of the 15 irradiated detectors such that: B2 þ S 2 L2 ;
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ð5Þ
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Figure 6. Estimated dependence of scan area necessary to satisfy the linearity performance characteristic as a function of number of detectors irradiated. The 0.5 mSv dose is used in the calculation of these trends. Each trend is associated with a measure of relative uncertainty lj/Hj (see legend). Satisfactory performance of the system for 10 detectors, and 4 min of scan time is estimated with uncertainty to be near 0.15.
FNTD NEUTRON DOSIMETRY SYSTEM Table 2. Data in mSv used for the calculation of LLD obtained for one set of 15 unirradiated detectors and 4 sets of 15 detectors irradiated with AmBe neutrons at different level of dose H1. Delivered dose, mSv Average dose read from detectors (H1) Standard deviation of irradiated detectors (s1) Average dose of unirradiated detectors ðH00 Þ Standard deviation for unirradiated detectors (s0) LLD
0.5
1.0
2.0
5.0
0.566 0.127 0.0076 0.0096 0.0453
1.032 0.106 0.0076 0.0096 0.0370
2.070 0.189 0.0076 0.0096 0.0366
5.060 0.483 0.0076 0.0096 0.0367
ACKNOWLEDGEMENTS
Figure 8. Results for relative bias and standard deviation as a fraction of total delivered neutron and photon dose for 1000 randomly selected sets of 15 dosemeters out of 300 irradiated badges with mixed fields according to the US ANSI N13.11-2009. The semicircle with L ¼ 0.3 represents the compliance boundary.
The authors thank their colleagues at Landauer— Stillwater—T. U. Underwood and J. Allen for their help with the preparation of dosemeters and T. McNamee of Landauer-Glenwood and PNNL staff for performing high volume of neutron and photon irradiations for the tests. FUNDING
where L ¼ 0.3. Here the calculation of L is based on the total dose each badge measures with respect to the total delivered dose (neutrons plus photons). The first test for compliance with category V-A was performed on 15 badges as a blind test, and the authors reported the results without knowing the delivered dose. The FNTD system successfully passed the test. For the second test, 300 badges were irradiated with the total dose ranging from 1 to 50 mSv of AmBe neutrons and 137Cs or X-ray (M150 and H150) photons in the ratios of 1:5, 1:3, 1:1 and 3:1. An automatic software program was developed to randomly select 1000 sets of readings from 15 dosemeters out of the 300 irradiated badges, and results were processed for compliance with the condition of equation (5). The performance of the FNTD reader, presented in Figure 8 as a scatter plot, was in all cases compliant with requirements for the general category of mixed neutron –photon field measurements. CONCLUSIONS The first table-top automatic FNTD neutron dosimetry system was successfully tested for LLD, linearity and ability to measure neutrons in mixed neutron–photon
Only internal Landauer funding was used in this work. No government funding was provided. REFERENCES 1. Akselrod, M. S. and Sykora, G. J. Fluorescent nuclear track detector technology—a new way to do passive solid state dosimetry. Radiat. Meas. 46, 1671– 1679 (2011). 2. Sykora, G. J. and Akselrod, M. S. Photoluminescence study of photochromically and radiochromically transformed Al2O3:C,Mg crystals used for fluorescent nuclear track detectors. Radiat. Meas. 45, 631–634 (2010). 3. Sanyal, S. and Akselrod, M. S. Anisotropy of optical absorption and fluorescence in Al2O3:C,Mg crystals. J. Appl. Phys. 98, 033518 (2005). 4. Akselrod, M. S. and Akselrod, A. E. New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging of tracks produced by heavy charge particles. Radiat Prot. Dosim. 119, 218–221 (2006). 5. Sykora, G. J., Salasky, M. and Akselrod, M. S. Properties of novel fluorescent nuclear track detectors for use in passive neutron dosimetry. Radiat. Meas. 43, 1017–1023 (2008). 6. Sykora, G. J. and Akselrod, M. S. Spatial frequency analysis of fluorescent nuclear track detectors irradiated in mixed neutron– photon fields. Radiat. Meas. 45(10), 1197–1200 (2010).
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fields satisfying US and ISO standards. This new neutron dosimetry system provides advantages over other technologies including environmental stability of the detector material, wide range of detectable neutron energies and doses, detector re-readability and re-usability and all-optical readout. A new adaptive image processing algorithm reliably removes false-positive tracks associated with surface and bulk crystal imperfections.
M. S. AKSELROD ET AL.
12.
13. 14. 15.
Evaluation of aluminum oxide fluorescent and OSL detectors in proton radiotherapy beams. Radiat. Meas. 46, 1974–1978 (2011). Yukihara, E. G., Mittani, J. C., Vanhavere, F. and Akselrod, M. S. Development of new optically stimulated neutron dosimeters. Radiat. Meas. 43, 309–314 (2008). d’Ericco, F. Radiation dosimetry and spectroscopy with superheated emulsions. Nucl. Instrum. Meth. B 184, 229– 254 (2001). ANSI N13.11– 2009 American National Standard for Dosimetry—Personnel Dosimetry Performance— Criteria for Testing. (2009). ISO-21909-1. Passive neutron dosimetry systems—Part 1: performance and test requirements for personal dosimetry. April 2013 (draft).
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7. Sykora, G. J., Akselrod, M. S. and Vanhavere, F. Performance of fluorescence nuclear track detectors in monoenergetic and broad spectrum neutron fields. Radiat. Meas. 44, 988– 991 (2009). 8. Sykora, G. J. and Akselrod, M. S. Novel fluorescent nuclear track detector technology for mixed neutrongamma fields. Radiat. Meas. 45(3–6), 594 –598 (2010). 9. Benton, E. V., Oswald, R. A., Frank, A. L. and Wheeler, R. V. Proton-recoil neutron dosimeter for personnel monitoring. Health Phys. 40, 801 –809 (1981). 10. Sykora, J., Akselrod, M. S., Benton, E. R. and Yasuda, N. Spectroscopic properties of novel fluorescent nuclear track detectors for high and low LET charged particles. Radiat. Meas. 43, 422–426 (2008). 11. Bartz, J. A., Sykora, G. J., Underwood, T. H., Nichiporov, D. N., Sawakuchi, G. O. and Akselrod, M. S.
doi:10.1093/rpd/nct293
AUTOMATIC NEUTRON DOSIMETRY SYSTEM BASED ON FLUORESCENT NUCLEAR TRACK DETECTOR TECHNOLOGY M. S. Akselrod1,*, V. V. Fomenko1, J. A. Bartz1,2 and T. L. Haslett3 1 Landauer, Inc., 723 1/2 Eastgate St., Stillwater, OK 74074, USA 2 Oklahoma State University, Stillwater, OK 74078, USA 3 Avo Photonics, Inc., 700 Business Center Dr., Horsham, PA 19044, USA
For the first time, the authors are describing an automatic fluorescent nuclear track detector (FNTD) reader for neutron dosimetry. FNTD is a luminescent integrating type of detector made of aluminium oxide crystals that does not require electronics or batteries during irradiation. Non-destructive optical readout of the detector is performed using a confocal laser scanning fluorescence imaging with near-diffraction limited resolution. The fully automatic table-top reader allows one to load up to 216 detectors on a tray, read their engraved IDs using a CCD camera and optical character recognition, scan and process simultaneously two types of images in fluorescent and reflected laser light contrast to eliminate false-positive tracks related to surface and volume crystal imperfections. The FNTD dosimetry system allows one to measure neutron doses from 0.1 mSv to 20 Sv and covers neutron energies from thermal to 20 MeV. The reader is characterised by a robust, compact optical design, fast data processing electronics and user-friendly software.
FNTD TECHNOLOGY AND DETECTORS For the first time, the authors are describing an automatic reader for neutron dosimetry that is based on fluorescent nuclear track detector (FNTD) technology reviewed in detail in Akselrod and Sykora(1). FNTD is a passive integrating type of detector that does not require wires, electronics or batteries during irradiation. The detectors are made of aluminium oxide single crystals doped with carbon and magnesium (Al2O3:C,Mg) made in the form of 8` 4` 0.5 mm3 plates polished on one side. The crystals contain F2þ 2 (2Mg) aggregate oxygen vacancy defects that undergo radiochromic transformation under irradiation(2), provide high quantum yield of luminescence(3), show temperature stability of radiation-induced luminescence up to 6008C(4) and no sensitivity to ambient room light during handling and readout. The FNTD reader technology can operate in two modes of image processing—track counting mode at low doses (0.1–200 mSv) and ‘analogue’ processing mode—which calculates power spectrum integral of fluorescence images as a dosimetric parameter, and is applied in dose range from 10 mSv up to 20 Sv(5, 6). The detectors can be read non-destructively multiple times(5) and nevertheless are reusable after thermal annealing or optical bleaching(2, 5). The energy and angular response of the detectors were investigated for monoenergetic and broad spectrum neutrons in the range from 40 keV to 19.5 MeV(7). The angular response of the dosemeters was studied in bare AmBe neutron fields(8). Major advantages of FNTDs over conventional CR-39 plastic nuclear track detectors(9) include the following: no need for post-irradiation chemical etching,
a significantly wider range of measured doses and LETs (0.5–1800 keV mm21)(6, 10) and higher spatial imaging resolution and larger measured track density. FNTDs are capable of processing charged particle fluence in excess of 106 cm22 without saturation (track overlap)(10, 11). Most of OSL and TL neutron detectors are not sensitive to fast neutrons and can operate only in albedo configuration(12) whereas FNTDs are sensitive to low-LET recoil protons and are able to detect fast protons and neutrons with energies up to 200 MeV(11). In comparison with superheated emulsions (bubble detectors), which have a very high sensitivity to neutrons but are bulky(13), FNTDs are much more compact and are not sensitive to temperature fluctuations.
READOUT INSTRUMENT Non-destructive readout of FNTDs is performed using a fluorescence laser scanning confocal imaging with nearly diffraction-limited resolution(1). The image is formed from 750-nm fluorescence intensity excited with a 638-nm diode laser. The automatic, table-top reader (Figures 1 and 2) allows one to load 216 detectors on a tray after removing them from a dosimeter slide (Figure 3), read their engraved IDs using a CCD-camera and optical character recognition (Figure 4), sequentially scan all detectors and process obtained fluorescence images (Figure 5) with user-configurable parameters. One of the current OSL-FNTD dosimeter slide configurations, as depicted in Figure 3, consists of three Al2O3:C OSL sensors and FNTD crystal installed with polished
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Alabama at Birmingham on May 31, 2014
*Corresponding author: [email protected]
M. S. AKSELROD ET AL.
Figure 4. ID engraved on a back, unpolished side of FNTD crystal. Three features are identified for further imaging: detector edges, bevel position and engraved ID. The detected OCR ID is also shown.
Figure 2. Schematic diagram of 3D opto-mechanical scanning of FNTDs.
Figure 5. Fluorescence image (100` 100 mm) with neutroninduced recoil proton tracks recognised and counted by the FNTD reader image processing software.
Figure 3. Dosimeter slide with three OSL sensors and FNTD crystal on top of three converters (Li-glass, PE and PTFE).
side down on top of three radiation converters: Li-based glass for thermal neutron detection, polyethylene (PE) for fast neutrons and Teflonw (PTFE) for gamma signal subtraction at high photon doses(5, 6).
The simplified overall optical diagram of the reader was presented elsewhere(1, 6), and its latest variant of 3D scanning arrangement is depicted in Figure 2. To increase the reader productivity, imaging is performed semi-continuously, in strips, consisting of a configurable number of images (12 is the default). The fast axis of scanning (x) is performed by a galvanometer with the slow axis of image scanning ( y) been done by the high precision translation stage. One important aspect of the reader operation is the need to image detectors at a precise and consistent depth of 2.0+0.2 mm below the polished surface. To achieve such precision, the reader automatically
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Figure 1. Automatic FNTD dosimetry system.
FNTD NEUTRON DOSIMETRY SYSTEM
NEUTRON DOSIMETRY PERFORMANCE In this paper, the authors are focusing on the FNTD instrument’s ability to measure very low doses (,1 mSv), linearity of dose dependence at low doses and neutron measurements in mixed neutron –photon fields. Different ratios of 241AmBe or 252Cf neutrons (bare and moderated) and photons (137Cs, H150 and M150 X rays) were used to assess the reader performance against the US ANSI N13.11-2009 standard(14). The linearity of dose dependence was tested according to the draft of ISO-21909-1 standard(15). Because of relatively low doses, all measurements were performed in track counting mode with sensitivities for each radiation source determined by preliminary calibration. FNTD crystals for these tests were randomly selected from a large production lot, and no individual calibration of each detector was performed. Linearity of dose dependence and uncertainties The linearity performance characteristic was determined after irradiation of dosemeters with bare AmBe neutrons on 40` 40` 15 cm PMMA phantom with doses of 0.2, 0.5, 1, 2, 5, 10, and 30 mSv at Landauer calibration laboratory. Although the
number of detectors (i) irradiated at each dose ( j ) is not strictly defined in the ISO standard, it is an important consideration for satisfaction of the standard: j + lj =Ht; j 1:2: 0:8 L ¼ H
ð1Þ
j is the average measured dose In this equation, H from lot j. Ht,j is the delivered dose for lot j. lj is calculated in units of dose as follows:
sj t n lj ¼ pffiffiffi ; n
ð2Þ
where sj is the standard deviation of measurements for n detectors, used in lot j, tn is the two-sided Student’s t value for the 0.95 confidence value for a sample of size n. To investigate the role of lj, the authors assumed j ¼ Ht; j and set the delivered dose to 0.5 mSv. In H this case, the measurements uncertainty l,j/Hj from equation (1) becomes 0.2. The si term in lj can be estimated based on Poisson statistics of total number of counted tracks as: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCA ¼ Hj ðD C AÞ1=2 ; si ¼ Hj ð3Þ DCA where D represents the delivered dose in mSv, C represents the sensitivity in tracks mm22 mSv21 and A represents the scan area in mm2. Solving equation (2) for si, inserting it into equation (3) and solving for area A result in a useful relationship allowing one to optimise the scan area and the scan time (Figure 6). To increase the instrument productivity and decrease the scanning time per detector, the adaptive readout mode was developed and tested, in which the scanning regime was automatically adjusted for each detector based on photon dose obtained from OSL sensors of the badge. By analysing experimental data and theoretical estimation based on equation (3), it was determined that the linearity performance characteristics can be satisfied at readout rate of 4 min per detector with 2 mm2 of scan area for batches of 10 detectors. The results of the linearity tests presented in Figure 7 and Table 1 are compliant with the draft of the ISO standard.
Low limit of detection To determine the low limit of detection (LLD) parameter, 15 freshly bleached FNTDs were scanned and processed with the optimal set of parameters determined in the section Linearity of dose dependence and uncertainties. Another 15 detectors were irradiated and scanned after 10 mSv of AmBe neutrons. An LLD value for pure neutron fields was calculated
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performs surface mapping of each detector and calculates the vertical (z) position of the objective along each strip of images. Reflected excitation laser light is used to locate the surface and appropriately sets the vertical (z) position of the objective along each scan line of the image. In addition, the image captured from reflected excitation light, collected simultaneously with the fluorescence image, allows surface and a crystal defects to be identified and masked in order to remove false-positive track. The latest advances in FNTD instrumentation and image processing include the following: faster rate of imaging and data acquisition—down to 1.0 s (from previous 12 s) per 100` 100 mm image; improvements in image processing to reliably identify recoil protons and alpha particle tracks; better discrimination of non-radiation-induced defects and surface contamination; and adaptive scanning depending on measured (OSL) photon dose. The reader is characterised by a compact optical design, fast data processing electronics and userfriendly software designed for three levels of access: technician, senior dosimetrist and technical support engineer. The reader is flexible in the selection of scan settings. Users with adequate permissions can specify the converter areas to be scanned such as the amount of area under each converter, the scanning speed and the scan depth, thus allowing the system to be customised for a variety of dosimetric applications. For each detector, a series of diagnostic routines is performed prior, during and after the scan.
M. S. AKSELROD ET AL.
based on recommendations of ANSI N13.11(14) as follows: LLD ¼
2½tp; n1 s0 þ ðtp; n1 s1 =H1 Þ2 H00 2
1 ðtp; n1 s1 =H1 Þ
;
ð4Þ
where tp,n21 is Student’s t value for (n 2 1) degrees of freedom at the confidence level value p ¼ 0.95, s0 is standard deviation of the unirradiated dosimeter readings, s1 is standard deviation of the irradiated dosimeter readings, H1 is average of the irradiated dosimeter readings and H00 is average of the unirradiated dosimeter readings without background subtracted. As shown in Table 2, an LLD of better than 0.05 mSv has been demonstrated. Performance in mixed neutron–photon fields Performance of the FNTD dosimetry system was tested according to the category V-A of US ANSI standard(14). Dosemeters for this test were irradiated with varying mixtures of neutrons and photons. The total dose delivered during testing was in the range from 1.5 to 50 mSv with neutron-to-photon ratios ranging from 1:3 to 3:1. The neutrons were delivered at Pacific Northwest National Laboratory (PNNL) and were chosen randomly between D2O-moderated 252 Cf and bare 252Cf sources. The photon portions of the total dose were delivered by Landauer calibration facility and were chosen from a selection of standard X-ray and g-ray sources. At least 3 of the 15
Figure 7. Measured neutron doses as a function of delivered doses (top). 1:1 line represented as a dashed line. Error bars represent 1 standard deviation. Detectors were irradiated with 241 AmBe neutrons. In the bottom chart, measured dose and standard deviations were normalised to the delivered dose.
Table 1. Tabulated results of linearity processed according to the draft of ISO-21909-1. Delivered dose, mSv 0.20 0.50 1.00 2.00 5.00 10.00 30.00
Measured dose, mSv
Standard deviation, mSv
Lþ
L2
0.19 0.47 1.01 2.07 5.28 9.71 33.77
0.10 0.19 0.12 0.17 0.60 1.31 1.09
1.21 1.16 1.08 1.09 1.13 1.06 1.15
0.89 0.92 1.00 1.03 1.05 0.88 1.10
dosemeters were irradiated using a high-energy photon source (average energy 500 keV). Compliance with the standard for category V is based on the average bias (B) and standard deviation of the biases (S) of the 15 irradiated detectors such that: B2 þ S 2 L2 ;
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ð5Þ
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Figure 6. Estimated dependence of scan area necessary to satisfy the linearity performance characteristic as a function of number of detectors irradiated. The 0.5 mSv dose is used in the calculation of these trends. Each trend is associated with a measure of relative uncertainty lj/Hj (see legend). Satisfactory performance of the system for 10 detectors, and 4 min of scan time is estimated with uncertainty to be near 0.15.
FNTD NEUTRON DOSIMETRY SYSTEM Table 2. Data in mSv used for the calculation of LLD obtained for one set of 15 unirradiated detectors and 4 sets of 15 detectors irradiated with AmBe neutrons at different level of dose H1. Delivered dose, mSv Average dose read from detectors (H1) Standard deviation of irradiated detectors (s1) Average dose of unirradiated detectors ðH00 Þ Standard deviation for unirradiated detectors (s0) LLD
0.5
1.0
2.0
5.0
0.566 0.127 0.0076 0.0096 0.0453
1.032 0.106 0.0076 0.0096 0.0370
2.070 0.189 0.0076 0.0096 0.0366
5.060 0.483 0.0076 0.0096 0.0367
ACKNOWLEDGEMENTS
Figure 8. Results for relative bias and standard deviation as a fraction of total delivered neutron and photon dose for 1000 randomly selected sets of 15 dosemeters out of 300 irradiated badges with mixed fields according to the US ANSI N13.11-2009. The semicircle with L ¼ 0.3 represents the compliance boundary.
The authors thank their colleagues at Landauer— Stillwater—T. U. Underwood and J. Allen for their help with the preparation of dosemeters and T. McNamee of Landauer-Glenwood and PNNL staff for performing high volume of neutron and photon irradiations for the tests. FUNDING
where L ¼ 0.3. Here the calculation of L is based on the total dose each badge measures with respect to the total delivered dose (neutrons plus photons). The first test for compliance with category V-A was performed on 15 badges as a blind test, and the authors reported the results without knowing the delivered dose. The FNTD system successfully passed the test. For the second test, 300 badges were irradiated with the total dose ranging from 1 to 50 mSv of AmBe neutrons and 137Cs or X-ray (M150 and H150) photons in the ratios of 1:5, 1:3, 1:1 and 3:1. An automatic software program was developed to randomly select 1000 sets of readings from 15 dosemeters out of the 300 irradiated badges, and results were processed for compliance with the condition of equation (5). The performance of the FNTD reader, presented in Figure 8 as a scatter plot, was in all cases compliant with requirements for the general category of mixed neutron –photon field measurements. CONCLUSIONS The first table-top automatic FNTD neutron dosimetry system was successfully tested for LLD, linearity and ability to measure neutrons in mixed neutron–photon
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