Radiation Protection Dosimetry (2014), Vol. 161, No. 1–4, pp. 398 –402 Advance Access publication 2 March 2014
doi:10.1093/rpd/ncu033
RADICAL DISTRIBUTIONS IN AMMONIUM TARTRATE SINGLE CRYSTALS EXPOSED TO PHOTON AND NEUTRON BEAMS M. Marrale1,2, *, A. Longo1,2, A. Barbon3,4, M. Brustolon3,4 and M. Brai1,2 1 Dipartimento di Fisica e Chimica, Universita` di Palermo, Viale delle Scienze, Ed.18, I-90128 Palermo, Italy 2 Gruppo V, INFN, Sezione di Catania, Catania, Italy 3 Dipartimento di Scienze Chimiche, Universita` di Padova, Via F. Marzolo 1, 35131 Padova, Italy 4 Sezione di Padova Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, 35131 Padova, Italy *Corresponding author: [email protected]
INTRODUCTION In the last years an ever-increasing interest on the use of high linear energy transfer (LET) particles for cancer therapy is aroused worldwide. In particular, the hadron therapy with protons or other ions (such as carbon ions) is very effective because of their inverse depth–dose profile and their large energy release inside tumour cells. These features bring about a larger dose release in depth than in entrance allowing to accurately hit the cancer cells and to spare healthy tissues ((1) and references therein). Another radiation therapy that arouses great interest is the neutron capture therapy. This method is based on the introduction of nuclei with high cross section for neutron capture (such as 10B and 157Gd) inside the tumour cells; these nuclei have to be exposed to neutron beams and release particles (alpha particles and 7Li ions in the case of 10B nuclei and Auger electrons in the case of 157 Gd), which are able to lose energy inside the cancer cells themselves(2). The good results of these novel radiation therapies have boosted the analysis of spatial distribution of radiation-induced defects on a micrometric and nanoscopic scale. Therefore, the microdosimetric and nanodosimetric techniques are found to be powerful for the optimisation of these therapies since they can provide information also on the LET of the radiation and therefore on the radiation quality. Various microdosimetric techniques are used such as the tissue-equivalent proportional counters, the chemically etched track detectors,
the gel dosemeters, the thermoluminescent dosemeters through the high-temperature ratio method and solidstate microdosemeters. Recently, also the electron spin resonance (EPR), which is extensively adopted for medical and industrial dosimetry, for retrospective dosimetry and for food control(3 – 12), has been used to assess radiation quality(13 – 15). In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate (AT) single crystals to various radiation beams (60Co gamma-ray photons and thermal neutrons) were reported. The choice of AT is due to its suitable properties, such as high efficiency of radiation-matter energy transfer, tissue equivalence and its rather sharp EPR spectrum ((14) and references therein). Two echo EPR methods can be used to assess the spatial distribution of radicals, i.e. the two pulse echo decays, allowing the measurement of instantaneous diffusion (ID) and the double electron –electron resonance (DEER(16)) spectra. By this method one can measure the distance between paramagnetic centres in solids through the analysis of the electron spin echo modulation induced by the electron– electron dipolar interaction. This technique is today very popular in particular as applied to biological systems as it allows measuring radical –radical distances in the range of 1.5 –8 nm. In a previous study(14) both these techniques were applied on powder-irradiated AT samples.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
The radiation therapy carried out by means of heavy charged particles (such as carbon ions) and neutrons is rapidly becoming widespread worldwide. The success of these radiation therapies relies on the high density of energy released by these particles or by secondary particles produced after primary interaction with matter. The biological damages produced by ionising radiations in tissues and cells depend more properly on the energy released per unit pathlength, which is the linear energy transfer and which determines the radiation quality. To improve the therapy effectiveness, it is necessary to grasp the mechanisms of free radical production and distribution after irradiation with these particles when compared with the photon beams. In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate crystals to various radiation beams (60Co gamma photons and thermal neutrons) were reported. Electron spin resonance analyses were performed by the electron spin echo technique, which allows the determination of local spin concentrations and by double electron–electron resonance technique, which is able to measure the spatial distance distribution (range 1.5– 8 nm) among pairs of radicals in solids. The results of these analyses are discussed on the basis of the different distributions of free radicals produced by the two different radiation beams used.
RADICAL DISTRIBUTIONS IN ORGANIC SINGLE CRYSTALS
In the present paper a new approach for the direct determination of the ID contribution will be described, allowing an easier treatment of the data. Moreover, preliminary results will be shown, indicating that the DEER method, promising for the assessment of the radiation quality, can be applied conveniently to single crystal samples. In fact, these latter give EPR spectra with narrower and more resolved bands, depending on the crystal orientation. This issue is particularly important for DEERwhich requires a separate microwave excitation of different spin packets.
Hahn decays For Hahn decays a classical two-pulse sequence tp t 2tp t echo was used; the pulse tp was 20 ns long and the delay t was varied from 200 ns to 8 ms. ID was obtained by measuring the decay rates for different tilting angles of the excitation pulse. The tilting angle of the pulses was controlled by varying the pulse power(14). Data were analysed as explained in the next section.
DEER experiments DEER signal(17) was obtained from a probing refocusing echo sequence:
Preparation of samples AT crystals have been prepared by crystalisation from water: a hot saturated water solution of AT has been left to slowly cool down in a quite environment in a crystaliser open to air. After 3–5 d, 3–5-mm-long single crystals appeared and were removed from the mother solution. After washing, the crystals were cut into pieces of 4 mm size. The orientation of the crystal pieces was preserved. Samples were glued at the end of a perspex rod with the long crystal axis parallel to the rod axis. The rod was attached to a goniometer, allowing the rotation about a direction perpendicular to the magnetic field and permitting to measure the angle between the crystal axis and static magnetic field. Irradiation Irradiations of crystal samples with 60Co gamma-ray photons, 1.25-MeV mean energy, were carried out with the IGS-3 irradiator in the Dipartimento Energia, Ingegneria dell’Informazione e Modelli Matematici (DEIM), Universita` di Palermo. The total dose was equal to 1.0 kGy. Thermal neutron irradiations were performed at the thermal column of the LENA reactor, University of Pavia (Pavia, Italy) at fluence of 2` 1014 cm22. The corresponding kerma in AT is 0.24 kGy. Instrumentation and pulse experiments Continuous-wave (cw) and pulse EPR spectra were obtained by a Bruker ELEXSYS spectrometer equipped with a 5-mm dielectric cavity inside a CF935 Oxford cryostat cooled by liquid nitrogen vapours. Quantitative determination of the macroscopic concentration was obtained by double integration of the whole spectrum recorded at very high attenuation (45 dB equivalent to 6 mW) to avoid saturation. Reproducibility of the spectra was within 10 % error. Hahn decays were acquired to obtain the ID, and thus the local spin concentration, whereas the DEER spectra were acquired to obtain the spatial spin distribution.
tp=2 t tp t echo t1 tp echo By using a pumping pulse at a different frequency, swept in time between the first echo and the second tp pulse, a modulation of the echo intensity appeared, proportional to spin–spin interaction strength(14). A delay t1 ¼1000 ns was used, which limits the detectable DEER frequencies, and thus the distances to 5 nm. Probing pulses tp=2 and tp were both 54-ns long, whereas the pumping pulse was of 40 ns. The crystal was oriented at 308 from the c-axis in the a–c plain in order to have two intense bands distant 7 G in the cw-EPR spectrum. For DEER measurements the pump and the probe resonance frequencies were centred around the maximum of the two bands. The offset frequency was 28 MHz to minimise overlapping in the pumping/probing spin packets. The data were analysed following the indications given in ref. (16) and by using DEER Analysis 2006 software(18).
Analysis of Hahn decays The local concentration of radicals CA was accessed by measuring the ID contribution to the echo decay(14). The classical analysis involves the fitting of the Hahn decay profile, composed of a dumped modulation function: I ðtÞ ¼ Im ðtÞ Id ðtÞ
ð1Þ
where Im ðtÞ describes the modulation of the echo intensity (ESEEM effect) and Id ðtÞ the decay function of the signal due to any relaxation process(14). ID provides an additional relaxation channel that reduces the measured phase memory time (TM). The two contributions are normally written as X ck t cosð2pnk tÞ 1þ 1 exp Im ðtÞ ¼ 2 dk k¼1;3
399
ð2Þ
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
MATERIALS AND METHODS
M. MARRALE ET AL.
Id ðtÞ ¼ A0 sinu1 sin2 ðu2 =2Þexpð2t=TM Þ ¼ I0 ðuÞexpð2t=TM Þ
ð3Þ
with 1=TM ¼ WSD þ WID , sin2 u1 . ¼ WSD þ WID ðdBÞ
ð4Þ
2p WID ¼ pffiffiffi g2 hCA ¼ kCA 9 3
ð5Þ
where h is the Planck constant and g the gyromagnetic ratio. Because of the signal-to-noise ratio, and because of the large number of free parameters, automatic optimisation lead to an unrealistic determination of the ID contribution, in particular for the sample irradiated with neutrons. The authors have tried to carry out a new analysis based on the fact that the echo intensity at a determined power attenuation (expressed in dB) can be written in the following way: I ðt; dBÞ ¼ Im I0 ðuÞe½WSD þWID ðdBÞt
ð6Þ
where WSD is the rate related to the spectral diffusion contribution and WID to the ID contribution. For very low power (and high attenuation values HdB such as 23 dB which correspond to 5 mW) this approximation holds: I ðt; HdBÞ ¼ Im I0 ðuÞeWSD t
ð7Þ
and, therefore, the following ratio depends only on the ID: I ðt; dBÞ eWID ðdBÞt I ðt, HdBÞ
Figure 1. ID as determined by the ratio with the decay obtained by HdB values. Linear best fits are also reported. For the gamma-irradiated sample, the contribution to instantaneous diffusion at the maximum of the echo is in perfect line with that found in the authors’ previous work(14) by considering samples irradiated at the same dose.
ð8Þ
The function is a simple exponential decay.
RESULTS AND DISCUSSION The authors have carried out the analysis of the Hahn echo decay using the ratio of the echo decay trends acquired with different attenuation as described in the ‘Materials and Methods’ section. The use of this method was successful with respect to the standard method (reproduction of the whole echo decay) as a reduced number of free parameters was required. Moreover, by the standard method a partial mixing of
the nuclear modulation decay with the total echo decay was probably present. Figure 1 reports the results of this study of the ID based on the ratio method for AT crystals exposed to 60 Co gamma-ray photons and thermal neutrons. Fitting of the slopes in Figure 1 allowed to obtain a much steeper trend for the sample irradiated by neutrons, associated with a higher local concentration of radicals in the case of irradiation with neutrons with respect to gamma irradiation. This contrasts with the macroscopic determination of the spin concentration, in which the gamma-irradiated sample is 7+1 times higher than that irradiated by neutrons. These preliminary results show that thermal neutrons produce a local concentration of free radicals much larger than that induced by gamma photons, i.e. the radicals are not uniformly distributed inside the crystal. Further investigations are in progress to determine quantitatively more precisely the value of the microscopic concentration of free radicals.
400
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
where WSD takes into account the spectral diffusion contribution and WID takes into account the ID contribution. The local concentration of radicals CA was determined using WID(14)
RADICAL DISTRIBUTIONS IN ORGANIC SINGLE CRYSTALS
CONCLUSIONS
The different distribution of radicals produced by the two types of beams is shown even more clearly by DEER results. Figure 2 shows the distance distribution for single crystal samples exposed to 60Co gamma-ray photons and thermal neutrons obtained by analysing the DEER traces. The radical –radical distances Rrr are expected to correspond to pairs of radicals (created by the same incident particle) separated by an integer or a combination of lattice constants. The peak intensity in Figure 2 corresponds to the distribution of Rrr values. The peaks are connected to the most probable distances that are discrete as identifiable only at discrete crystal positions. As one can see, in samples exposed to thermal neutrons the largest peak corresponding to the prevailing Rrr is at a smaller distance than for gamma photons. This is due to the fact that the high LET particles produced after interaction with neutrons (mainly protons from the reaction 14N(n,p)14 C) release a larger amount of their energy in the proximity of the primary reactions with respect to the gamma-irradiation case. This technique therefore can be used to discriminate between the two typologies of radiation field. To model the Rrr distributions inside the crystals one should take into account the crystal parameters and the energy distribution of the various ionising particles. The positions of the peaks found in this work are different from those observed in the previous work of the group(14) because in the case of a single crystal the orientations are not fully randomised, whereas in the case of powders the orientations are completely random and this difference can affect the analysis of the distance distribution of free radicals. Furthermore, experiments are in progress to evaluate the effects on distribution with increasing absorbed dose. Work is in progress to this aim.
ACKNOWLEDGMENTS The research described in this paper was funded by the Italian Ministry of Research and University and by the ‘Neutron dOsimetry and Radiation quality Measurements by ESR and TL’ (NORMET) Project (Gruppo V-INFN Project. Project Leader: M. Marrale) funded by the Istituto Nazionale di Fisica Nucleare (INFN) and by the Universita` di Palermo. The research here described is related to the Project Biologically weighted quantities in radiotherapy (BIOQUART) EMRP European Metrology Research Programme (Project Leader: Hans Rabus). REFERENCES
401
1. Schardt, D., Elsa¨sser, T. and Schulz-Ertner, D. Heavyion tumor therapy: physical and radiobiological benefits. Rev. Mod. Phys. 82, 383 (2010). 2. Barth, R. F. A critical assessment of boron neutron capture therapy: an overview. J. Neuro-Oncol. 62, 1 (2003).
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
Figure 2. Distance distribution for crystal samples exposed to 60Co gamma photons and thermal neutrons.
This paper extends to the single crystal case, a similar work done on AT powder irradiated with different beams, with assessment of microscopic radical concentration by determining the amount of ID contribution and obtaining the inter-radical distance distributions by double microwave irradiation. In this paper single crystals of AT have been exposed to 60Co photons and neutrons. The results confirm that advanced pulse EPR techniques allow the direct measurement of the local free radical concentration and provide information about the distribution of free radicals in the material. A more convenient approach is proposed than in ref. (14) for a direct determination of the ID contribution, allowing an easier treatment of the data and which shows larger concentration of free radical in the case of neutron irradiation with respect to photon irradiation. As far as the DEER technique is concerned, it is noted that the study of irradiated single crystals has advantages and disadvantages with respect to powders. An advantage is that for particular orientations of the crystal in the magnetic field the EPR spectrum is given by intense and well-separated lines, allowing the necessary separate microwave double excitation. A further advantage could derive from the dependence of DEER results on the crystal orientation in the magnetic field, as this could in principle provide much more information on the effects of different radiation beams. In particular, it was observed that the spatial spin distribution has largest peak centred at lower distances for neutron irradiation than for gamma irradiation. However, this would require a demanding thorough analysis of the DEER response exploring a number of different orientations of the crystal.
M. MARRALE ET AL.
11.
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and 10 MeV electrons analysed by means of ESR technique: a preliminary study. Radiat. Meas. 46, 822– 826 (2011). Trompier, F., Bassinet, C., Wieser, A., De Angelis, C., Viscomi, D. and Fattibene, P. Radiation-induced signals analysed by EPR spectrometry applied to fortuitous dosimetry. Ann. Ist. Super. Sanita` 45, 287– 296 (2009). Wieser, A. Review of reconstruction of radiation incident air kerma by measurement of absorbed dose in tooth enamel with EPR. Radiat. Prot. Dosim. 149, 71– 78 (2012). Marrale, M., Brai, M., Triolo, A., Bartolotta, A. and D’Oca, M. C. Power saturation of ESR signal in ammonium tartrate exposed to 60Cog-ray photons, electrons and protons. Radiat. Res. 166, 802– 809 (2006). Marrale, M., Brai, M., Barbon, A. and Brustolon, M. Analysis of the spatial distribution of free radicals in ammonium tartrate by pulse EPR techniques. Radiat. Res. 171, 349–359 (2009). Marrale, M., Longo, A., Brai, M., Barbon, A., Brustolon, M. and Fattibene, P. Pulsed EPR analysis of tooth enamel samples exposed to UV and g-radiations. Radiat. Meas. 46, 789–792 (2011). Jeschke, G. DEER distance measurements on proteins. Annu. Rev. Phys. Chem. 63, 419–446 (2012). Brustolon, M. and Barbon, A. Chapter: Pulse EPR of paramagnetic centers in solid phases. EPR of Free Radicals in Solids. Kluwer Academic Publishers (2003). Jeschke, G. DEERanalysis (2006). Available on www.epr. ethz.ch/software (accessed on 13 June 2012).
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3. Regulla, D. F. ESR spectrometry: a future-oriented tool for dosimetry and dating. Appl. Radiat. Isot. 62, 117– 127 (2005). 4. Marrale, M., Longo, A., Spano`, M., Bartolotta, A., D’Oca, M. C. and Brai, M., Sensitivity of alanine dosimeters with gadolinium exposed to 6 MV photons at clinical doses. Radiat. Res. 175, 821–826 (2011). 5. Marrale, M., Gennaro, G., Brai, M., Basile, S., Bartolotta, A. and D’Oca, M. C. Exposure of Gd2O3alanine and Gd2O3-ammonium tartrate ESR dosimeters to thermal neutrons: experiments and Monte Carlo simulations. Radiat. Meas. 43, 471–475 (2008). 6. Marrale, M., Brai, M., Gennaro, G., Bartolotta, A. and D’Oca, M. C. The effect of gadolinium on the ESR response of alanine and ammonium tartrate exposed to thermal neutrons. Radiat. Res. 169, 232–239 (2008). 7. Brai, M., Marrale, M., Gennaro, G., Bartolotta, A., D’Oca, M. C. and Rosi, G. Improvement of ESR dosimetry for thermal neutron beams through the addition of gadolinium. Phys. Med. Biol. 52, 5219– 5230 (2007). 8. Marrale, M. ESR spectroscopy for analyzing the spatial distribution of free radicals in ammonium tartrate. Nuovo Cimento C 36, 161–170 (2013). 9. Parlato, A., Calderaro, E., Bartolotta, A., D’Oca, M. C., Brai, M., Marrale, M. and Tranchina, L. Application of the ESR spectroscopy to estimate the original dose in irradiated chicken bone. Radiat. Phys. Chem. 76, 1466– 1469 (2007). 10. Marrale, M., Longo, A., D’Oca, M. C., Bartolotta, A. and Brai, M. Watch glasses exposed to 6 MV photons
doi:10.1093/rpd/ncu033
RADICAL DISTRIBUTIONS IN AMMONIUM TARTRATE SINGLE CRYSTALS EXPOSED TO PHOTON AND NEUTRON BEAMS M. Marrale1,2, *, A. Longo1,2, A. Barbon3,4, M. Brustolon3,4 and M. Brai1,2 1 Dipartimento di Fisica e Chimica, Universita` di Palermo, Viale delle Scienze, Ed.18, I-90128 Palermo, Italy 2 Gruppo V, INFN, Sezione di Catania, Catania, Italy 3 Dipartimento di Scienze Chimiche, Universita` di Padova, Via F. Marzolo 1, 35131 Padova, Italy 4 Sezione di Padova Istituto Nazionale di Fisica Nucleare, Via Marzolo 8, 35131 Padova, Italy *Corresponding author: [email protected]
INTRODUCTION In the last years an ever-increasing interest on the use of high linear energy transfer (LET) particles for cancer therapy is aroused worldwide. In particular, the hadron therapy with protons or other ions (such as carbon ions) is very effective because of their inverse depth–dose profile and their large energy release inside tumour cells. These features bring about a larger dose release in depth than in entrance allowing to accurately hit the cancer cells and to spare healthy tissues ((1) and references therein). Another radiation therapy that arouses great interest is the neutron capture therapy. This method is based on the introduction of nuclei with high cross section for neutron capture (such as 10B and 157Gd) inside the tumour cells; these nuclei have to be exposed to neutron beams and release particles (alpha particles and 7Li ions in the case of 10B nuclei and Auger electrons in the case of 157 Gd), which are able to lose energy inside the cancer cells themselves(2). The good results of these novel radiation therapies have boosted the analysis of spatial distribution of radiation-induced defects on a micrometric and nanoscopic scale. Therefore, the microdosimetric and nanodosimetric techniques are found to be powerful for the optimisation of these therapies since they can provide information also on the LET of the radiation and therefore on the radiation quality. Various microdosimetric techniques are used such as the tissue-equivalent proportional counters, the chemically etched track detectors,
the gel dosemeters, the thermoluminescent dosemeters through the high-temperature ratio method and solidstate microdosemeters. Recently, also the electron spin resonance (EPR), which is extensively adopted for medical and industrial dosimetry, for retrospective dosimetry and for food control(3 – 12), has been used to assess radiation quality(13 – 15). In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate (AT) single crystals to various radiation beams (60Co gamma-ray photons and thermal neutrons) were reported. The choice of AT is due to its suitable properties, such as high efficiency of radiation-matter energy transfer, tissue equivalence and its rather sharp EPR spectrum ((14) and references therein). Two echo EPR methods can be used to assess the spatial distribution of radicals, i.e. the two pulse echo decays, allowing the measurement of instantaneous diffusion (ID) and the double electron –electron resonance (DEER(16)) spectra. By this method one can measure the distance between paramagnetic centres in solids through the analysis of the electron spin echo modulation induced by the electron– electron dipolar interaction. This technique is today very popular in particular as applied to biological systems as it allows measuring radical –radical distances in the range of 1.5 –8 nm. In a previous study(14) both these techniques were applied on powder-irradiated AT samples.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
The radiation therapy carried out by means of heavy charged particles (such as carbon ions) and neutrons is rapidly becoming widespread worldwide. The success of these radiation therapies relies on the high density of energy released by these particles or by secondary particles produced after primary interaction with matter. The biological damages produced by ionising radiations in tissues and cells depend more properly on the energy released per unit pathlength, which is the linear energy transfer and which determines the radiation quality. To improve the therapy effectiveness, it is necessary to grasp the mechanisms of free radical production and distribution after irradiation with these particles when compared with the photon beams. In this work some preliminary results on the analysis of the spatial distributions of the free radicals produced after exposure of ammonium tartrate crystals to various radiation beams (60Co gamma photons and thermal neutrons) were reported. Electron spin resonance analyses were performed by the electron spin echo technique, which allows the determination of local spin concentrations and by double electron–electron resonance technique, which is able to measure the spatial distance distribution (range 1.5– 8 nm) among pairs of radicals in solids. The results of these analyses are discussed on the basis of the different distributions of free radicals produced by the two different radiation beams used.
RADICAL DISTRIBUTIONS IN ORGANIC SINGLE CRYSTALS
In the present paper a new approach for the direct determination of the ID contribution will be described, allowing an easier treatment of the data. Moreover, preliminary results will be shown, indicating that the DEER method, promising for the assessment of the radiation quality, can be applied conveniently to single crystal samples. In fact, these latter give EPR spectra with narrower and more resolved bands, depending on the crystal orientation. This issue is particularly important for DEERwhich requires a separate microwave excitation of different spin packets.
Hahn decays For Hahn decays a classical two-pulse sequence tp t 2tp t echo was used; the pulse tp was 20 ns long and the delay t was varied from 200 ns to 8 ms. ID was obtained by measuring the decay rates for different tilting angles of the excitation pulse. The tilting angle of the pulses was controlled by varying the pulse power(14). Data were analysed as explained in the next section.
DEER experiments DEER signal(17) was obtained from a probing refocusing echo sequence:
Preparation of samples AT crystals have been prepared by crystalisation from water: a hot saturated water solution of AT has been left to slowly cool down in a quite environment in a crystaliser open to air. After 3–5 d, 3–5-mm-long single crystals appeared and were removed from the mother solution. After washing, the crystals were cut into pieces of 4 mm size. The orientation of the crystal pieces was preserved. Samples were glued at the end of a perspex rod with the long crystal axis parallel to the rod axis. The rod was attached to a goniometer, allowing the rotation about a direction perpendicular to the magnetic field and permitting to measure the angle between the crystal axis and static magnetic field. Irradiation Irradiations of crystal samples with 60Co gamma-ray photons, 1.25-MeV mean energy, were carried out with the IGS-3 irradiator in the Dipartimento Energia, Ingegneria dell’Informazione e Modelli Matematici (DEIM), Universita` di Palermo. The total dose was equal to 1.0 kGy. Thermal neutron irradiations were performed at the thermal column of the LENA reactor, University of Pavia (Pavia, Italy) at fluence of 2` 1014 cm22. The corresponding kerma in AT is 0.24 kGy. Instrumentation and pulse experiments Continuous-wave (cw) and pulse EPR spectra were obtained by a Bruker ELEXSYS spectrometer equipped with a 5-mm dielectric cavity inside a CF935 Oxford cryostat cooled by liquid nitrogen vapours. Quantitative determination of the macroscopic concentration was obtained by double integration of the whole spectrum recorded at very high attenuation (45 dB equivalent to 6 mW) to avoid saturation. Reproducibility of the spectra was within 10 % error. Hahn decays were acquired to obtain the ID, and thus the local spin concentration, whereas the DEER spectra were acquired to obtain the spatial spin distribution.
tp=2 t tp t echo t1 tp echo By using a pumping pulse at a different frequency, swept in time between the first echo and the second tp pulse, a modulation of the echo intensity appeared, proportional to spin–spin interaction strength(14). A delay t1 ¼1000 ns was used, which limits the detectable DEER frequencies, and thus the distances to 5 nm. Probing pulses tp=2 and tp were both 54-ns long, whereas the pumping pulse was of 40 ns. The crystal was oriented at 308 from the c-axis in the a–c plain in order to have two intense bands distant 7 G in the cw-EPR spectrum. For DEER measurements the pump and the probe resonance frequencies were centred around the maximum of the two bands. The offset frequency was 28 MHz to minimise overlapping in the pumping/probing spin packets. The data were analysed following the indications given in ref. (16) and by using DEER Analysis 2006 software(18).
Analysis of Hahn decays The local concentration of radicals CA was accessed by measuring the ID contribution to the echo decay(14). The classical analysis involves the fitting of the Hahn decay profile, composed of a dumped modulation function: I ðtÞ ¼ Im ðtÞ Id ðtÞ
ð1Þ
where Im ðtÞ describes the modulation of the echo intensity (ESEEM effect) and Id ðtÞ the decay function of the signal due to any relaxation process(14). ID provides an additional relaxation channel that reduces the measured phase memory time (TM). The two contributions are normally written as X ck t cosð2pnk tÞ 1þ 1 exp Im ðtÞ ¼ 2 dk k¼1;3
399
ð2Þ
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
MATERIALS AND METHODS
M. MARRALE ET AL.
Id ðtÞ ¼ A0 sinu1 sin2 ðu2 =2Þexpð2t=TM Þ ¼ I0 ðuÞexpð2t=TM Þ
ð3Þ
with 1=TM ¼ WSD þ WID , sin2 u1 . ¼ WSD þ WID ðdBÞ
ð4Þ
2p WID ¼ pffiffiffi g2 hCA ¼ kCA 9 3
ð5Þ
where h is the Planck constant and g the gyromagnetic ratio. Because of the signal-to-noise ratio, and because of the large number of free parameters, automatic optimisation lead to an unrealistic determination of the ID contribution, in particular for the sample irradiated with neutrons. The authors have tried to carry out a new analysis based on the fact that the echo intensity at a determined power attenuation (expressed in dB) can be written in the following way: I ðt; dBÞ ¼ Im I0 ðuÞe½WSD þWID ðdBÞt
ð6Þ
where WSD is the rate related to the spectral diffusion contribution and WID to the ID contribution. For very low power (and high attenuation values HdB such as 23 dB which correspond to 5 mW) this approximation holds: I ðt; HdBÞ ¼ Im I0 ðuÞeWSD t
ð7Þ
and, therefore, the following ratio depends only on the ID: I ðt; dBÞ eWID ðdBÞt I ðt, HdBÞ
Figure 1. ID as determined by the ratio with the decay obtained by HdB values. Linear best fits are also reported. For the gamma-irradiated sample, the contribution to instantaneous diffusion at the maximum of the echo is in perfect line with that found in the authors’ previous work(14) by considering samples irradiated at the same dose.
ð8Þ
The function is a simple exponential decay.
RESULTS AND DISCUSSION The authors have carried out the analysis of the Hahn echo decay using the ratio of the echo decay trends acquired with different attenuation as described in the ‘Materials and Methods’ section. The use of this method was successful with respect to the standard method (reproduction of the whole echo decay) as a reduced number of free parameters was required. Moreover, by the standard method a partial mixing of
the nuclear modulation decay with the total echo decay was probably present. Figure 1 reports the results of this study of the ID based on the ratio method for AT crystals exposed to 60 Co gamma-ray photons and thermal neutrons. Fitting of the slopes in Figure 1 allowed to obtain a much steeper trend for the sample irradiated by neutrons, associated with a higher local concentration of radicals in the case of irradiation with neutrons with respect to gamma irradiation. This contrasts with the macroscopic determination of the spin concentration, in which the gamma-irradiated sample is 7+1 times higher than that irradiated by neutrons. These preliminary results show that thermal neutrons produce a local concentration of free radicals much larger than that induced by gamma photons, i.e. the radicals are not uniformly distributed inside the crystal. Further investigations are in progress to determine quantitatively more precisely the value of the microscopic concentration of free radicals.
400
Downloaded from http://rpd.oxfordjournals.org/ at University of Michigan on October 22, 2014
where WSD takes into account the spectral diffusion contribution and WID takes into account the ID contribution. The local concentration of radicals CA was determined using WID(14)
RADICAL DISTRIBUTIONS IN ORGANIC SINGLE CRYSTALS
CONCLUSIONS
The different distribution of radicals produced by the two types of beams is shown even more clearly by DEER results. Figure 2 shows the distance distribution for single crystal samples exposed to 60Co gamma-ray photons and thermal neutrons obtained by analysing the DEER traces. The radical –radical distances Rrr are expected to correspond to pairs of radicals (created by the same incident particle) separated by an integer or a combination of lattice constants. The peak intensity in Figure 2 corresponds to the distribution of Rrr values. The peaks are connected to the most probable distances that are discrete as identifiable only at discrete crystal positions. As one can see, in samples exposed to thermal neutrons the largest peak corresponding to the prevailing Rrr is at a smaller distance than for gamma photons. This is due to the fact that the high LET particles produced after interaction with neutrons (mainly protons from the reaction 14N(n,p)14 C) release a larger amount of their energy in the proximity of the primary reactions with respect to the gamma-irradiation case. This technique therefore can be used to discriminate between the two typologies of radiation field. To model the Rrr distributions inside the crystals one should take into account the crystal parameters and the energy distribution of the various ionising particles. The positions of the peaks found in this work are different from those observed in the previous work of the group(14) because in the case of a single crystal the orientations are not fully randomised, whereas in the case of powders the orientations are completely random and this difference can affect the analysis of the distance distribution of free radicals. Furthermore, experiments are in progress to evaluate the effects on distribution with increasing absorbed dose. Work is in progress to this aim.
ACKNOWLEDGMENTS The research described in this paper was funded by the Italian Ministry of Research and University and by the ‘Neutron dOsimetry and Radiation quality Measurements by ESR and TL’ (NORMET) Project (Gruppo V-INFN Project. Project Leader: M. Marrale) funded by the Istituto Nazionale di Fisica Nucleare (INFN) and by the Universita` di Palermo. The research here described is related to the Project Biologically weighted quantities in radiotherapy (BIOQUART) EMRP European Metrology Research Programme (Project Leader: Hans Rabus). REFERENCES
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Figure 2. Distance distribution for crystal samples exposed to 60Co gamma photons and thermal neutrons.
This paper extends to the single crystal case, a similar work done on AT powder irradiated with different beams, with assessment of microscopic radical concentration by determining the amount of ID contribution and obtaining the inter-radical distance distributions by double microwave irradiation. In this paper single crystals of AT have been exposed to 60Co photons and neutrons. The results confirm that advanced pulse EPR techniques allow the direct measurement of the local free radical concentration and provide information about the distribution of free radicals in the material. A more convenient approach is proposed than in ref. (14) for a direct determination of the ID contribution, allowing an easier treatment of the data and which shows larger concentration of free radical in the case of neutron irradiation with respect to photon irradiation. As far as the DEER technique is concerned, it is noted that the study of irradiated single crystals has advantages and disadvantages with respect to powders. An advantage is that for particular orientations of the crystal in the magnetic field the EPR spectrum is given by intense and well-separated lines, allowing the necessary separate microwave double excitation. A further advantage could derive from the dependence of DEER results on the crystal orientation in the magnetic field, as this could in principle provide much more information on the effects of different radiation beams. In particular, it was observed that the spatial spin distribution has largest peak centred at lower distances for neutron irradiation than for gamma irradiation. However, this would require a demanding thorough analysis of the DEER response exploring a number of different orientations of the crystal.
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