Neutron dosimetry and microdosimetry with track etch based LET spectrometer

Transcription

1 Neutron dosimetry and microdosimetry with track etch based LET spectrometer František Spurný*, Kateřina Brabcová and Iva Jadrníčková Nuclear Physics Institute, Czech Academy of Sciences, Na Truhlářce, 39/64, CZ-886, Prague, Czech Republic. Abstract. There is still need to develop upgrade, and test further methods able to characterise the external exposure to neutrons. This contribution presents further results obtained with the goal to enlarge and upgrade the possibility of neutron dosimetry and microdosimetry with a LET spectrometer based on the chemically etched track detectors (TED). As TED we have used several types of polyallyldiglycolcarbonates (PADC). All were basically etched in 5 N NaOH. To determine the LET value of a particle, the etching rate ratio V (V=v T /v B ; where v B is bulk etching rate and v T is track etching rate) is established through the determination of track parameters. They are measured by means of an automatic optical image analyzer LUCIA NIS. The PADC detectors have been exposed in high-energy neutron beams at ithemba facility, Cape Town, South Africa, and in monoenergetic neutron beams at JRC Geel, Belgium, in neutron beams with energies from.2 to 9.5 MeV. LET spectra obtained are presented, the influence of PADC type s choice are analyzed and discussed. LET spectra obtained permit also to calculate the total dose and dose equivalent in a neutron beam. These data are also presented and compared with the references data submitted by facilities staff. KEYWORDS: Neutron dosimetry, LET spectrometry, Track etch detectors, Polyallyldiglycolcarbonate, chemical etching, automatic track evaluation. Introduction Neutron dosimetry represents a complicated task because of large energy region to be covered (from thermal neutrons up to neutrons with energies of several hundreds MeV). There is therefore still need to develop, upgrade, and test the methods able to enlarge and upgrade the possibility of neutron dosimetry and microdosimetry for characterising the external exposure to neutrons. This contribution presents the results obtained with the LET spectrometer based on the chemically etched track detectors (TED). To determine the LET value of a particle, the etching rate ratio V (V=v T /v B ; where v B is bulk etching rate and v T is track etching rate) is established through the determination of track parameters. LET spectra obtained permit also to calculate the total dose and dose equivalent in a neutron beam. These data are also presented and compared with the references data submitted by facilities staff. 2. Materials and methods 2. Track etch detectors (TED) used for LET spectrometry The spectrometer of linear energy transfer is based on chemically etched polyallyldiglycolcarbonate (PADC) track-etch detector [-3]. In these studies, two types of PADC (Page, Tastrak, both.5mm thick), manufactured in UK have been used. Both were etched in 5 N NaOH, at 7 o C; usual etching time was 8 hours, At these conditions, the thickness of layer removed from each surface of the detectors (bulk etch) is about 7 µm. Before etching, one corner of each sample used is irradiated with 252 Cf fission fragments and another one with alpha particles from 24 Am to check the exact etching conditions. To determine the LET value of a particle, the etching rate ratio V (V=v T /v B ; where v B is bulk etching rate and v T is track etching rate) is established through the determination of track parameters. They are measured by means of an automatic optical image analyzer LUCIA NIS. The obtained V-spectra are then corrected for the critical angle of detection and transformed into the LET spectra using the calibration curves. * Presenting author, spurny@ujf.cas.cz

2 This LET spectrometer enables determining LET of particles approximately from to 7 kev/µm [2-5]. It is not able in a simple way to separate different particle species with the same LET. From the LET spectra, the absorbed dose and dose equivalent can be calculated according to: D LET = (dn / dl). L. dl, () H LET = (dn / dl). L. Q(L). dl, (2) where dn/dl is the number of tracks in the interval dl, L is the LET of a particle, and Q(L) is the ICRP 6 [6] quality factor. 2.2 Irradiation conditions RC Geel irradiations Neutron fluences was characterised with the IRMM recoil proton telescopes (RPT) for the energies, 3.5, 7, 6 and 9.5 MeV at a distance of 25 cm from the neutron producing target, and with the Bonner sphere 3" for the energies.2 and.5 MeV at a distance of 5 cm from the target. Neutron spectra for nominal energies 7 and 9.5 MeV are presented in Figures (courtesy by IRMM staff). Figure. Neutron spectra at energies and 9.5 MeV.... E-3 E-3 E Neutron energy (MeV) E Neutron energy (MeV) Our detectors have been exposed in free-in-air geometry, actual conditions of exposure are specified in Table. Table. Actual reference data for the irradiation of PADC LET spectrometers En [MeV] neutron fluence [n/cm²] H*(), [msv] free in air 9.5.E E E E E+8.5.4E E ithemba exposures 2

3 Two exposures have been realised in ithemba radiation beams, one in the neutron beam with the characteristic energy MeV, another one with characteristic energy of 2 MeV. Two positions were chosen in both cases, one in the direction of beam, the second deviated by 6 o from that. The neutron spectra for all these four cases are presented in Figure 2 (courtesy of ithemba staff). Our detectors packages have been in both cases irradiated to the level corresponding to H*() value equal to msv. Figure 2. MeV and 2 MeV neutron spectra 8 mm Li MeV p 6 mm Li + MeV p (ΦE/Φ) / MeV (ΦE/Φ) / MeV E / MeV 5 5 E / MeV 3. Results 3.. Results of RC Geel irradiations The exposed detectors have been treated as described above. As an example, the distributions in LET of dose D and dose equivalent H, respectively, are for Page and Tastrak detectors, shown in Figure 3 and Figure 4, respectively. The distributions are presented in the way usual in microdosimetry, i.e. as L*D(L) for the dose and L*(H(L) for the dose equivalent. In such case the area under the curve is proportional to the contribution of particles with a LET to the total value of the quantity. One can see in these figures that recoil protons with LET below MeV.cm 2.g - largely predominate in the contribution for neutron energies up to 3.5 MeV; at higher energies, the contribution of higher LET particles (helium nuclei, heavier recoils) starts to become more and more important. Figure 3. Distributions of the dose for different neutron energies, detectors Page and Tastrak Page Tastrak L*D 6 MeV MeV,5 MeV L*D 6 MeV MeV,5 MeV LET [MeV.cm 2.g - ] LET [MeV.cm 2.g - ] 3

4 Figure 4. Distributions of the dose equivalent for different neutron energies, detectors Page and Tastrak Page Tastrak.5 6 MeV MeV,5 mev.5 6 MeV MeV,5 MeV.. LET [MeV.cm 2.g - ] LET [MeV.cm 2.g - ] Integral values of dose equivalent H LET measured with different PADC LET spectrometers and obtained from LET spectra through the equations () and (2) are presented in Table 2. One can see there that the actually measured values of dose equivalent differ from the reference values of ambient dose equivalent H*n(), they are there expressed also relatively to the reference value as η. The differences observed depend both on the neutron energy as on the type of PADC LET spectrometer considered. Table 2. Total values of dose equivalent measured with different PADC LET spectrometers, their comparison with reference values. En [MeV] PAGE.5 mm T H LET [msv] η H LET [msv] η ± 6.3 *).5 ±.9 **) 29.9 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 3..9 ± ± ± ±.3.24 ± ±.9.36 ±.2.35 ±.2.2 ± ±.87.5 ±.8 *) σ, **) only uncertainties of H LET considered not those of reference values Some general conclusions can be drawn from the values of H LET presented in the Table 2:. A general decrease of H LET in comparison to H*n() with decreasing neutron energy can be stated; with the some exceptions (neutron energy of and ). It could be attributed to the registration efficiency of different secondary particles. As already mentioned, for the energies up to 3.5 MeV, recoiled protons dominate in the contribution to registered tracks (see Figure 3 and Figure 4). With increasing neutron energy, their registration efficiency increase, however also their energy. Starting from heavier recoils start to dominate in the particles registered, protons start to have to high energy to be registered. 2. The thresholds of particle s registration in PADC LET spectrometers used is ~ kev/µm for Page, ~5 kev/µm Tastrak [3-5]. The influence of this difference can be also seen in the Table 2, general less pronounced that could be expected. 4

5 3.2. Results of ithemba exposures Relative microdosimetry distributions of L*(H(L) are for both PADC materials used and both energies for perpendicularly incident neutrons presented in Figure 5. Figure 5. Distributions of the dose equivalent for perpendicularly incident MeV and 2 MeV neutrons ithemba distributions - MeV ithemba distributions - 2 MeV.2 Page -H T.5 - H.2 Page - H T5 - H LET [MeV.cm 2.g - ] LET, MeV.cm -2.g - One can see there that:. The maximum in the distributions is for 2 MeV neutrons slightly shifted to lower values of LET. 2. The relative importance of particles with LET values lower than ~ 4 MeV.cm -2.g - is clearly higher for 2 MeV neutrons. Both these tendencies demonstrate that the energies of secondary particles registered by LET spectrometers with neutron energy increases, and consequently theirs LET diminishes. Table 3. Results of evaluation of PADC LET spectrometers exposed in ithemba neutron beams Neutron energy Angle Quantity, unit Page Tastrak.5 mm D LET, mgy.8 ± ±. MeV 2 MeV 6 6 H LET, msv 7.82 ± ±.33 QF 9.8 ± ± 2.2 D LET, mgy.44 ±.2.4 ±.9 H LET, msv 4.4 ± ±.33 QF 9.4 ± ± 2.6 D LET, mgy.84 ±.49 ±.6 H LET, msv 4.55 ± ±.26 QF 5.4 ±.3 7. ± 2.5 D LET, mgy.95 ±.34 ±.3 H LET, msv 3.56 ± ±.2 QF 3.7 ± ± 2.9 5

6 Total dosimetry and microdosimetry characteristics are presented in Table 3. Several tendencies can be drawn from these values:. The values of H LET for both and 2 MeV are lower than reference value of H*(), the differences are more important for 2 MeV neutrons, and for angle 6 o. 2. As could be expected from the higher LET threshold for Tastrak, the differences between H LET and H*() are for that material more important than for Page. For the same reason, the quality factor is always higher for Tastrak material. 3. In agreement with the tendencies already mentioned in the case of figures, quality factors are, due to more important contribution of low LET particles, clearly lower for 2 than for MeV neutrons. 3. Discussion The results presented in previous chapters can be briefly summarised as follows:. H LET measured with PADC TED LET spectrometers varies with the PADC type, and with the neutron energy. Their values are usually lower than reference values of H*(). 2. For the same neutron energy, the variations of H LET do not always follow expectation based on the LET threshold values [3-5]. 3. The responses relatively to H*() reference values depend on the neutron energy in rather complicated way, see Figure 6. In spite of some punctual excesses, general tendency shows the increase of the ratio mentioned from ~. to about, followed by the constancy up to the decrease starting a little below MeV. The punctual excesses can be attributed to the fact that for monoenergetical neutrons the response can vary very rapidly even for close energies. 4. From the last point of view it could be of interest to compare these data with that we have accumulated in previous years for polyenergetic neutron sources. We have shown that the ratio H LET /H*() varies [3]: For AmF neutrons (averaged E N ~.5 MeV) between.5 and.3 (last figure when covered by polyethylene); For AmBe neutrons (averaged E N ~ 4.2 MeV) between.5 and.9; and For CERF reference field concrete shield (averaged E N ~ 49.5 MeV) between.6 and.9. In the last case, we have performed during years four studies with the same lot of detectors as those used in JRC Geel and ithemba. The ratios of H LET /H*() have been much more consistent, they varied between.68 and.82 for Page, between.88 and. for Tastrak [7]. These values correspond well to tendencies mentioned. In the future we will try to validate our experimental results also through MC calculations. Figure 6. Variations of ratios H LET /H*() with neutron energy for Page and Tastrak PADC TED LET spectrometers neutron response Response relatively to H*()ref. Page T,5.. Neutron energy [MeV] 6

7 4. Conclusions. A new set of data on the response of PADC TED LET spectrometer to neutrons with energies from.2 to 2 MeV has been obtained. 2. The ratio of measured dose equivalent, H LET, and reference, H*(), depends on the neutron energy in quite complex way. The increase of this ratio is observed in few MeV region, followed by the region with more or less constant value of it, the results obtained agree with those obtained previously for polyenergetic neutron sources. 3. Nevertheless, even accelerator-produced neutrons are not fully monoenergetic, particularly at higher energies (see Figure and Figure 2). More detailed analysis of the effects which could be connected to that are in progress, we are also starting to try to validate our experimental results through MC calculations Acknowledgements The studies have been supported through Grant of the GACR No. 22/4/795, NPI research project AVZ4855, Ministry of Education, Youth and Sports CR n. P5OC32, and COST 724 project. We acknowledge the colleagues from DOBIES ESA project, particularly F. Vanhavere for the organisation of JRC Geel exposures, T. Berger for the organisation of that at ithemba. REFERENCES [] SPURNÝ, F., BEDNÁŘ, J., JOHANSSON, L., SÄTHERBERG, A. LET spectra of secondary particles in CR 39 track etch detectors. Radiat. Meas. 26 (996) [2] SPURNÝ, F., JADRNÍČKOVÁ, I., MOLOKANOV, A.G., BAMBLEVSKI, V.P. Upgrading of LET track-etch spectrometer: Calibration and uncertainty analysis. Radiat. Meas. 4 (25) [3] JADRNÍČKOVÁ I. Spectrometry of linear energy transfer and its use in radiotherapy and radiation protection in high-energy particle fields. PhD thesis, Prague, September 26. [4] SPURNÝ F., MOLOKANOV A.G., BAMBLEVSKI V.P. Spectrometry of linear energy transfer, its development and use. Rad. Prot. Dos. (24) [5] JADRNÍČKOVÁ, I., SPURNÝ, F. LET spectrometry with track etch detectors use in highenergy radiation fields. Radiat. Meas. 43 (28), [6] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, 99 Recommendations of the International Commission on Radiological Protection, ICRP Publications 6, Annal of ICRP 2, -3 (99). [7] SPURNÝ, F., BRABCOVÁ, K., and JADRNÍČKOVÁ I. Response of thermoluminescent detectors and LET spectrometer based on track etching at CERF high-energy radiation fields: Results of studies 26/27, Report DRD NPI ASCR 599/28, Prague, April 28. 7