Experimental Microdosimetry in High Energy Radiation Fields

Transcription

1 Experimental Microdosimetry in High Energy Radiation Fields F. Spurny 1, J. Bednar 1, J.-F. Bottollier-Depois 2, A.G. Molokanov 3, B. Vlcek 1 1 Department of Radiation Dosimetry, Nuclear Physics Insitute, Czech Academy of Sciences, 1886 Prague, 2 Dosimetry Department, Institute for Nuclear Safety and Protection, Fontenay-aux-Roses 3 Joint Institute for Nuclear Research, Dubna, Moscow region, Russia INTRODUCTION High energy radiation fields are usual rather complex, both as far as the types of particles are concerned and theirs energies. It is usually not quite evident to establish their characteristics through the methods available for low energy radiation, i. e. by determining dosimetry quantities for each particle present in a field separately. An alternative approach is to establish directly the distributions of microdosimetric quantities and deduce the dosimetry characteristics from them. Such approach permits also to establish simultaneously not only quantitative but also qualitative characteristics of a high energy radiation beam and/or field. Two methods of experimental microdosimetry have been used for the measurement in high energy radiation beams and fields: The classical method of experimental microdosimetry, i.e. a tissue equivalent low pressure proportional counter (TEPC), and The linear energy transfer (LET) spectrometer based on a chemically etched polyallyldiglycolcarbonate (PADC) as a track etched detector (TED). Among other purposes, these two methods were compared also with the goal to get an idea, how the TED LET spectrometer should be able to determine microdosimetry distributions in the conditions, where the use of TEPC is not possible (beams at high instantaneous dose rates and complex time structures). Experimental microdosimetry methods were used in on-board aircraft radiation fields, in high energy radiation reference fields, and in the beams of protons with energies up to 2 MeV (Dubna, Moscow). EXPERIMENTAL Microdosimetry methods of measurements The both methods mentioned above and discussed below measure primarily the numbers of events with defined value of or the lineal energy y, f(y), (TEPC) or the numbers of events with defined value of the linear energy transfer, F(L) (PADC LET spectrometer). However, in the microdosimetry it is common to present these spectra as the products of event distribution and corresponding values of y, or L, i.e. y*f(y), y*d(y), and y*h(y). In such presentation the area under a curve is directly proportional to the contribution of events with different values of y, or L to chosen dosimetric quantity. As far as the dose equivalent is concerned, the quality factors from the recent ICRP Recommendations 6 (3,4) are mostly used. As far as two methods of experimental microdosimetry used, they can be briefly characterised as follows: Tissue equivalent proportional counter (TEPC) The use of TEPC for experimental microdosimetry is based on the simulation of small tissue element by the low pressure gas filling (1). Several TEPC were constructed, some of them are commercially available. We used NAUSICAA equipment developed in France (2). Its sensor is a cylindrical TEPC. Its sensitive volume is filled with a propane based tissue equivalent gas mixture at the pressure corresponding to a tissue target with a diameter of 3 µm. The counter wall thickness is equivalent to 1 mm of tissue. The NAUSICAA equipment measures directly the lineal energy between.15 and 15 kev/µm. The instrument is calibrated with a 6 Co photon source for low linear energy transfer (LET) region (below ~1 kev/µm), and in the field of AmBe neutron source for high LET region (above ~1 kev/µm). The actual performance is regularly checked by means of an internal 244 Cm alpha particle source. TEPC NAUSSICA measures primarily the spectra of lineal energy y behind the 1 mm of tissue equivalent material. It can be used in the fields of radiation with the instantaneous dose equivalent rates between ~ 1µSv/h and several msv/h. Microdosimetric spectra obtained with this equipment at some standard radiation sources are shown in the Figures 1 and 2. While in a photon beam, only events up to about 1 kev/µm are seen, the range of the energy deposition events is much more larger in the case of AmBe neutron source. There, both components of the field are well presented: the component with low lineal energy, up to about 1 kev/µm, corresponding mostly to photons also present in the field of this source, and the component with high lineal energy, above 1 kev/µm, corresponding to neutrons. 1

2 .8 6 Co.8 AmBe FAR y*f(y) y*f(y) y*f(y); y*d(y); y*h6(y) y*d(y) y*h6(y) y*f(y); y*d(y); y*h6(y) y*d(y) y*h6(y) Y, kev/µm Figure 1: Microdosimetric spectra measured with TEPC NAUSSICA in a 6 Co photon beam Y, kev/µm Figure 2: Microdosimetric spectra measured with TEPC NAUSSICA at an AmBe neutron source LET spectrometer based on a solid state nuclear track detector (SSNTD) SSNTD s are also devices whose response, i.e. the revealing of a particle track through chemical etching, is related to the energy deposition density, the dimensions of critical volume are estimated to be several nm (5). The dependence of track parameters on the type and the linear energy transfer (LET) of a particle permits to determine the LET of particle creating a track, in some cases also both the particle identification and its spectrometry. We have developed a LET spectrometer based on the chemically etched SSNTD, a polyallyldiglycolcarbonate (PADC), including the automatic evaluation of particle track parameters (5-7). PADC available from Pershore Moulding, England (curing time 32 hours, thickness.5 mm) was used. The detector samples have been etched in a 5N NaOH at 7 C. Each sample was before etching irradiated in a corner with 252 Cf fission fragments, in another one with 241 Am alpha particles to check etching conditions. The optimal conditions corresponded to 18 hours of etching (one-side removed layer about 17µm). To determine LET-value of a particle, the etch rate ratio V (=V T /V B ; where V B is bulk etching rate and V T is track etching rate) was primarily established through the determination of track parameters. They were measured by means of an automatic optical image analyzer LUCIA II based on a Leitz microscope. The value of V was calculated by at least two ways of track parameters combination, the final optimisation is performed through the comparison of the removed layer thickness recalculated from V-value and that directly measured through fission fragment tracks diameter. V-spectra obtained are corrected for the critical angle of the registration and transformed to LET spectra on the basis of the heavy charged particles calibration checked through high dose electron irradiation. The spectrometer permits to establish LET of a track between 1 and 7 kev/µm in tissue. There are practically no limitations on the use of this spectrometer from the point of view of the dose equivalent rate. The background and the track s superposition limit its use for the dose equivalent determination on the region between ~1 msv and about 1 msv. Microdosimetric spectra obtained with this PADC LET spectrometer in the cases of some high energy and other radiation are shown in Figures 3 and 4..8 Protons 1 GeV.16 AmBe FAR L*d(L) L*d(L) L*h6(L).6 L*h6(L) L*h6(L); L*d(L).4.8 L*h6(L); L*d(L) Figure 3: Microdosimetric spectra measured with SSNTD LET spectrometer in the beam of 1 GeV protons Figure 4: Microdosimetric spectra measured with SSNTD LET spectrometer in the beam of AmBe neutrons When comparing the microdosimetric spectra obtained with two methods, one can see the difference of y (or LET) range covered. While TEPC spectra start in the region corresponding to the radiation with low 2

3 density of deposited energy, PADC LET spectrometer measures the spectra only in the region corresponding to the energy deposition by protons and heavier charged particles. To compare both types of microdosimetric distributions we were obliged to treat from the spectra obtained with TEPC separately the part above 1 kev/µm. RADIATION FIELDS STUDIED Two methods of experimental microdosimetry described were tested in following radiation fields: The reference fields behind the shielding of high energy particle accelerators. Such fields have been recently formed behind the shielding of the phasotron of the Joint Institute for Nuclear Research at Dubna (8) and at the SPS facility at CERN (9). The most of our studies have been performed at CERN. There, the high energy stray radiation reference fields have been realised at the beam H6 of the SPS facility. A secondary beam of positively charged particles (protons: pions; 2:1; 12 GeV/c) is allowed to enter into collision with a thick copper target ( 7 mm x 5 mm). The target is located in one of two shielding configuration, built for the purpose of the experiment, with a concrete (8 mm) or an iron (4 mm) top desk as already described (9). Different measurement positions are available on the top and on the side of both shielding configurations. The time structure of the beam is that within a pulse cycle lasting 14.4 s the particles are impinging on the target 2.58 s. High energy stray radiation reference fields are monitored with a beam monitor-precise ionisation chamber (PIC). Radiation fields behind shielding are composed of low LET component (mostly muons) and high LET component in which the contribution of neutrons to the dose equivalent is dominant. The information on particle spectra is available from the calculation by means of FLUKA Monte Carlo code and/or from the measurements with different types of spectrometers. The neutron component is composed of two parts, one which peaked at the energy of about 1 MeV, another one with the maximum a liitle below 1 MeV. The second part is relatively much more important in the field behind concrete shielding. The mean equivalent dose rates reach up to 1 msv/h, instantaneous values are up to 25 times higher (see the time structure of the beam). The radiation fields on board of subsonic and/or supersonic aircraft. Many studies of the exposure on board aircraft have been realised last years after the publication of ICRP 6 recommendations (4) which recommended to estimate and controlled the exposure of the air crew members. Their goal was to collect more information on this exposure to the radiation on board. The results obtained have been compared, analysed and discussed in many publications and some review articles (1,11). We have performed such measurements since early 9`s (12). Many different dosimetry systems were exploited, among them also methods discussed in this contribution. The radiation field on aircraft board is also composed of two components: the low LET component, to which contribute mostly electrons, high energy protons and photons, and the high LET component in which the contribution of neutrons is dominant. It was found out (1-12), that the neutron spectrum on board is similar to that behind the concrete shielding in the CERN high energy reference field facility. It should be also mentioned that the values of the total (both components included) equivalent dose rate on the aircraft board are generally below 1 µsv/h for subsonic transport, approach to 15 µsv/h at the highest flight levels of supersonic transport. The contributions of both components are roughly the same. High energy proton beams. The irradiation was realised in the beams of protons of the phasotron of the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research (LNP JINR), Dubna, Russia.; and these of the synchrotron at the Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia. The monitoring of irradiation conditions at Dubna was ensured through the ionisation chamber measurements. The detectors sets were irradiated perpendicularly to the beam, the proton tissue kerma was about 15 mgy at the beam entrance. At 155 MeV primary energy, two samples were exposed: one with the minimum thickness of the degrader, i.e. with the mean energy of about 134 MeV; and the second one with the mean residual proton energy about 7 MeV. At 2 MeV primary energy, three samples were exposed: the first with the minimum thickness of the degrader, with the mean residual proton energy of about 18 MeV; the second one with the mean residual proton energy of about 13 MeV; and the third with the mean residual proton energy about 7 MeV. The monitoring of irradiation conditions at Moscow is assured by means of a transmission current transformer calibrated against activation detectors ( 12 C(p,pn) 11 C). Two samples were exposed in the ITEP synchrotron beam, one at the average residual proton energy of about 2 MeV, the second at the average residual proton energy of about 7 MeV. The typical dose rates in high energy proton beams are of the order of.1 mgy.s -1, i.e. several mgy per hour. The use of an usual TEPC in such conditions is impossible, only the samples of PADC LET spectrometer were exposed in high energy proton beams. 3

4 RESULTS; DISCUSSION Comparison of two methods studied in high energy radiation fields. It should be emphasised, that in all high energy radiation fields studied, we concentrated our analysis to the region, in which both methods overlap, i.e. between 1 and 1 kev/µm in tissue. It should be also stressed, that the events observed in this region correspond to the interactions of secondary particles created by primary radiation, mostly neutrons close or into the sensitive volume of both equipment s. First, we have compared the microdosimetric distributions determined by means of both methods. The results obtained are presented in Figures 5-8. CERN top concrete CERN Top concrete y*f(y) L*h6(L); L*d(L).6.3 L*d(L) L*h6(L) L*F(L).12.6 y*h6(y); y*d(y); y*f(y) y*d(y) y*h6(y) Figure 5: Microdosimetric spectra measured with PADC LET spectrometer in CERN high energy reference field, top concrete shield Figure 6: : Microdosimetric spectra measured with TEPC NAUSSICA in CERN high energy reference field, top concrete shield Concorde, 6 flight hours Concorde Paris - New York AR.15.6 L*d(L) y*f(y) L*h6(L) y*d(y) L*h6(L); L*d(L);.1.5 y*h6(y); y*d(y); y*f(y).4.2 y*h6(y) Figure 7: Microdosimetric spectra measured with PADC LET spectrometer on board of Concorde Figure 8: Microdosimetric spectra measured with TEPC NAUSSICA on board of Concorde First of all, it should be mentioned, that the numbers of intervals of y (TEPC) and/or L (PADC-TED) are different. The TEPC performance permits to divide the region above 1 kev/µm to 15 intervals. The accuracy of track parameters determination and resulting resolution power of the PADC TED LET spectrometer permit the division to only 6 intervals. One can see in the figures, that, in spite of this difference, the microdosimetric spectra obtained by both methods are similar for both high energy radiation fields studied, i.e. on aircraft board and/or behind the high energy accelerator shielding. More quantitative analysis of both types of microdosimetric spectra are in progress. The microdosimetric event spectra enables to calculate the dose D, and the dose equivalent, H, corresponding to the particles, the tracks of which are revealed. The integral values of the dose, D, resp. the dose equivalent, H, are, in the case of LET spectra, obtained as: D = f(l)*l*dl; resp. (1) H = f(l)*l*qf(l)*dl; (2) where f(l) is the number of tracks in a LET interval; L is the value of LET; and QF(L) is the quality factor corresponding to the value of L. 4

5 In the case of lineal energy spectra the equations are similar, with the substitution of y for L. The integral values obtained in this way are presented in Table 1. Table 1: High LET integral dosimetric quantities obtained by means of two methods of experimental microdosimetry tested *) Quantity Method Concorde CERN-TC Dose, TEPC 58±9 42±5 in µgy PADC-TED 61±1 36±5 Dose equiv., TEPC 63±9 378±5 in µsv PADC-TED 77±12 33±45 Quality TEPC 1.7±.7 9.±.7 Factor PADC-TED 12.7± ±1.2 *) for 1 hours of flight at Concorde board, and 1 6 PIC monitor counts at CERN top concrete reference field One can see in Table 1, that the integral values of dosimetric quantities obtained by both methods agree rather well, at least in the limits of uncertainties. Such agreement has brought to us another support to try to use PADC LET spectrometer in rather intense high energy proton (radiotherapy) beams. High energy proton beams First, it should be reminded that the LET of primary protons is too low, about.7 kev/µm at 1 MeV. Such particles can not be registered in PADC LET spectrometer directly. The spectrometer register only the secondary particles with higher LET created in the detector sheet and/or its surrounding through the interactions of primary protons. The data we are obtaining characterise the contribution of such particles to the total energy transferred. PADC LET spectrometer samples were always irradiated as a set. Each set contained two sheets of track detectors, the upper one was covered by one half with polyethylene (PE). In such situation there are four different «radiators» in the front of a detector sheet: no radiator (bare), 2 mm of polyethylene (PE), upper sheet of detector (bare-cr), and upper sheet of detector behind PE (PE-CR). First we wanted to appreciate whether there is an influence of the immediate cover of a PADC sheet on the dose and/or dose equivalent distributions in LET. We have found out that such influence is minimal. A typical example is shown in the Figure 7 in the case of 18 MeV protons and the dose equivalent., the same behaviour was also observed for other sets at all energies mentioned above. 4. Radiator influence on H6(L) 1 Microdosimetric distributions H6(L) 2. L*H6(L) L*D(L) bare PE bare-cr PE-CR Figure 7: Radiator influence on the dose equivalent distributions in LET (E p 18MeV, ICRP 6 QF) 2 L*H6(L) L*D(L) Figure 8: Microdosimetry distributions, set irradiated with 18 MeV protons 1 5

6 More information on the contribution of particles with the different LET values to the total values can be obtained from microdosimetric distributions L*D(L), resp. L*H(L). These distributions are for the detectors set irradiated by 18 MeV protons presented in the Figure 8, for the dose as well as for the equivalent dosis with ICRP 6 quality factors. One can see there, that both the dose and dose equivalent distributions are a little different as compared to the distributions found out in the samples irradiated in high energy radiation fields. Besides, we have observed also some differences in these distributions for different proton energies. Their extent will be more discussed in the connection with integral values of dosimetric quantities. The integral values of dosimetric characteristics were calculated by means of the equations (1) and (2) for three different LET thresholds, 1, 12, and 14 kev/µm. The general characteristics were not quantitatively changed by this choice, we shall discuss the results obtained for the full LET range covered by our spectrometer, i.e. from 1 to 7 kev/µm. First, it should be mentioned that the results obtained show clearly, that, for the same entrance proton energy, the contribution of secondary high LET particles to the dose quantities increases with the decreasing residual proton energy, i.e. with the depth in a tissue-like material. Table 1 gives an example in the case of the entrance energy of 2 MeV.. Table 1: Integral dosimetric quantities of secondary high LET particles in the beam with primary proton energy 2 MeV (Entrance dose due to ionisation losses of primary protons 15 mgy - JINR phasotron) Dosimetric quantity Unit Residual E p, MeV Dose mgy Dose Equivalent (ICRP26) msv Dose Equivalent (ICRP6) msv Quality factor (ICRP26) Quality factor (ICRP6) This increase is not only absolute but also relative, i.e. when the increase of dose due to ionisation losses with decreasing proton energy is taken into account The quantitative data on the increase mentioned was taken from our own measurements in the beams with a Si-diode, obtained relative contributions of secondary high LET particles to the ionisation loss absorbed doses are given in Table 4. Table 2: Relative contributions of high LET secondary particles to the ionisation loss proton absorbed dosis Primary Beam Residual E p Relative contribution of high LET particles MeV to the dose due to ionisation losses, % MeV - JINR MeV - ITEP Relative standard uncertainty about ± 15 % (2σ) in all cases. One can see there that these contributions are of the order of few percents at the beam entrance, they increase substantially with the depth in a tissue-like material. As far as the values of this contribution at the entrance are concerned, there are a little higher than we observed in some of our previous studies (13,14), mostly due to the improving our method leading to the decrease of the lowest limit of LET taken into account. To give an idea about this effect, when the threshold is shifted to 14 kev/µm, the relative contribution decreases to about 6 % of the value given in Table 2. The increase of the relative contribution with the depth in a tissue-like material would have origin in the formation of still additional secondary particles through the primary proton interactions (lower energy protons, neutrons). Their influence would change not only the relative contribution as a whole, but also its qualitative characteristics, i.e. the LET spectrum of high LET particles seen by means of our spectrometer. We tried to analyse such possibility first through the analysis of the changes in the average values of quality factors corresponding to registered particle tracks. The tendencies observed are presented in Figures 5 and 6. 6

7 QF (2 6) above 12keV/µm above 14 kev/µm above 16 kev/µm Ep, MeV Figure 9: QF(26) as a function of the residual proton energy QF(6) above 12 kev/µm above 14 kev/µm above 16 kev/µm Ep, MeV Figure 1: QF(6) as a function of the residual proton energy One can see there that the both quality factors decrease with the decreasing residual proton energy, i.e. with the increasing depth in a tissue-like material. The decrease is similar for both recommended sets of QF values, it could signal that the changes is caused mainly by the increase of the contribution of particles with lower LET values. We have tried therefore to analyse the changes in the relative contribution of particles with LET lower than about 9 kev/µm (proton edge) to the total values of dosimetric characteristics. We have observed that, actually, the relative contribution of particles with the LET below proton edge increases with the decreasing residual proton energy, i.e. with the depth in a tissue-like material. CONCLUSIONS First, the contribution presents an attempt to compare two principally different methods of experimental microdosimetry. We believe that this comparison can be considered as encouraging. The qualitative agreement has been observed in the case of microdosimetric distributions, the values of integral dosimetric quantities agree quite well also quantitatively. Further analysis are necessary to reach more deep understanding of both methods. Experimental studies should be also supported by theoretical calculations, which should particularly help to analyse the influence of critical volumes dimensions on the microdosimetric distributions. They are estimated to be of the order of nm for TED, of µm in the case of the TEPC. There are two aspects which have to be discussed in the connection with the results obtained with PADC LET spectrometer in high energy (radiotherapy) proton beams: the reasons for the increase of the contribution of high LET secondary particles to the absorbed dose, and the importance of this effect for some applications. The reasons for the contribution mentioned and its changes with the proton energy and with the depth in the material consist in the formation of secondary and other generations particles through nuclear interactions and their energy transfer to the matter. The confirmation of this influence could be analysed through an appropriate transport calculations, we would like to follow this direction in the next future. As far as the second aspect is concerned, the protons of energies studied are, due to their ionisation losses a radiation with low LET. Our results show, however, that, when all radiation transfer and absorption processes are taken into account, the situation is not so straightforward. As far as the radiation protection is concerned, the contribution of secondary high LET particles change average quality factors. While for only ionisation losses the quality factor is about 1., for the high LET secondaries can approach up to more than 1. (see Table 1). It was calculated that this phenomenon can change the average QF in the human body by a factor more than 2 (16). As far as the proton radiotherapy is concerned, the dose due to the secondary high LET particles change characteristics of the beam. Quantitatively, the effect represents few percent at the beam entrance, it is higher in the depth. Besides, the biological effectiveness increases with LET in the radiotherapy as well. Probably in not such extent as it is supposed in the radiation protection through the values of quality factors, nevertheless, this effect could be responsible for the increased radiotherapy effectiveness of high energy protons recognized (15). For all these reasons the effect described should be studied more completely and from several aspects. 7

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