Fast neutron response of alanine probes

0883-2889~89$3.00+ 0.00 Copyright ~ 1989PergamonPress plc

Appl. Radial. Isot. Vol. 40, No. 10-12, pp. 941-944, 1989 Int. J. Radial. Appl. lnstrum. Part A Printed in Great Britain. All rights reserved

Fast Neutron Response of Alanine Probes H. S C H R A U B E , E. W E I T Z E N E G G E R ,

A. W I E S E R a n d D. F. R E G U L L A

Gesellschaft ffir Strahlen- und Umweltforschung (GSF) Mtinchen, D-8042 Neuherberg, F.R.G. The energy response of alanine dosimeter probes with 20% paraffin admixtures for neutron energies between 0.6 and 15 MeV was determined experimentally using accelerator-produced monoenergetic neutrons. Theoretical considerations are made to estimate the energy dependence of the response relative to the response to photons. An optimum mixture of alanine and paraffin is derived which would give an energy-independent neutron tissue dose-response, if the kerma ratio were the only factor influencing the energy response.

Introduction

Alanine is an amino-acid of solid crystalline state. Radiation produces stable free radicals which are measurable quantitatively by electron spin resonance techniques (Bradshaw et al., 1962; Regulla and Definer, 1982). The atomic composition of alanine is similar to that of tissue. This fact and the possibility of fabricating geometrically small probes makes the material attractive for the measurement of tissue dose inside phantoms for both photons and neutrons. A series of neutron data is available from irradiations with polyenergetic high-intensity beams (see e.g. Katsumura et al., 1986), but only few measurements are found in the literature for monoenergetic sources, because of the low sensitivity of the alanine system in relation to the yield of commonly used monoenergetic neutron sources. The aim of the paper is to supplement energy response data from the literature for a type of dosimeter probes which is in use for high-dose 7 dosimetry, to determine the influence of paraffin admixtures, and to make estimations for use in accident monitoring and neutron-therapy dosimetry.

Materials

and Methods

For the experimental study, a batch o f G S F alanine probes was used. A homogeneous mixture of paraffin (20%) and alanine powder (80%) was pressed into rods of 10mm length and 4.9mm diameter with a weight of 250 -2_ 1.5 mg (2a) each. The evaluation was done using a computer-aided ESR spectrometer (Bruker ESP 300). The ESR evaluations were made at room temperature. Calibration of the alanine/ESR dosimetry system was carried out with 6°Co ~ radiation. Traceability to the standards of NPL, Teddington, U.K. involved the use of a NE secondary-standard class ionization chamber dosimeter. The 6°Co irradiations A R ~ 40 .~iz-J

941

of the alanine probes were performed at about I m source distance, also at room temperature. The photon reference irradiations were done with a nominal tissue dose of 10Gy, the neutron irradiations with doses between 1 and 5Gy, depending on the available yield of the respective neutron-producing reaction. The background signal, determined from an unirradiated batch, was equivalent to (0.3 + 0.04) Gy, The standard deviation (68% confidence level) of the reading of the neutron-irradiated samples was between 20 and 100 mGy, and of the photon-irradiated control batch approximately 60 mGy, i,e. 0.5% for the applied radiation dose. Thus, for a 95% confidence level, the lower detection limit for the photon dose determination is approx. 200 mGy. Neutron irradiation

The alanine probes were placed into a PMMA (polymethyl methacrylate = "Perspex") phantom of 10mm thickness with a build-up cover of 1.5 mm (Fig. 1). Six probes were arranged for one irradiation in an annular arrangement around the beam axis of an accelerator-produced neutron beam. The description of the irradiation facility is published elsewhere (Schraube et al., 1978). Because of the low neutron source strength a small distance between the accelerator target and the irradiation phantom had to be chosen. Dosimetry was done with a well-calibrated tissue-equivalent ionization chamber (0.05 cm 3 volume) flushed with tissue-equivalent gas. The outer dimensions of the sensitive part of the chamber were only slightly larger than those of the alanine probes. In the first instance, the total (7 ray plus neutron) dose was determined on the beam axis inside a second phantom, which was constructed identically to the alanine irradiation phantom, with the exception of a cavity for the ion chamber. Then the lateral dose profile was measured free-in-air on the surface of the irradiation phantom in order to prove the calculated

H. SCHRAUBEet

942 Irradiation phantom

/\

Torget

al.

u n d e r secondary charged-particle conditions, then the indication M of the ESR readout system is given by

L

M = ~,, • D~, + %- D~, Beam axis

~"-"~ t

.... "

Akonine probes Side

Fig.

Front view

view

1. Experimental arrangement of the irradiation phantom at the accelerator target.

where D N is the n e u t r o n dose in the reference material with the c o m p o s i t i o n of muscle tissue ( I C R U , 1977), D~ is the p h o t o n dose, a n d ~, and % the overall response for n e u t r o n s and p h o t o n s , respectivel). The reading M can be normalized to a reading M c from an i r r a d i a t i o n in a p h o t o n calibration field (e.g. "'Co) with a reference dose D c. Thus, the response R of the alanine detector in the mixed n e u t r o n ~ p h o t o n field relative to its response in the calibration field is M

b e a m profile, and to introduce corrections where necessary. Figure 2 shows the fluence (or dose) distribution across the p h a n t o m . The 5.3-MeV irradiation condition exhibits a relatively large i n h o m o g e n e i t y of the field across the probes because of the n o n - i s o t r o p y of the n e u t r o n - p r o d u c i n g reaction 2D(:d, ~n)3He. F o r each condition the unweighted average over the irradiated area was used to derive the dose in the p r o b e relative to the central-axis dose. The relative p h o t o n c o n t r i b u t i o n was determined by the twinc h a m b e r m e t h o d ( C o p p o l a a n d Schraube, 1978: M i j n h e e r et al., 1984). The n e u t r o n b e a m was m o n i tored by using a tissue-equivalent transmission chamber. Reference p h o t o n irradiations were performed with the same p h a n t o m s at a 6°Co medical irradiation facility and the dose m e a s u r e m e n t done in the same way as for the n e u t r o n irradiations.

Derivation of the Neutron Dose-Response Characteristic In the context of this p a p e r the interesting characteristic of the alanine detector is its dose response, i.e. its reading per unit dose in tissue. If the detector is positioned into a n e u t r o n r a d i a t i o n field with a (small) p h o t o n c o n t r i b u t i o n 1.0

8 o~ 0.8 0.7 D

i ~

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-5.3MeV I

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i

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Lateral

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d (mm)

Fig. 2. Relative neutron dose distribution across the surface of the irradition phantom for the 4 neutron energies in use, vs the lateral distance from the beam axis. The dashed area indicates the position of the alanine probes.

tl)

R=--=k.Dy+h.Dc; Dc

(2)

where k is now the n e u t r o n a n d h the p h o t o n response relative to its response to the ;'-ray reference radiation field. If it is assumed that the response to the p h o t o n s in the n e u t r o n field is the same as to the p h o t o n s in the 6°Co reference field, then h becomes unit)', and the required relative n e u t r o n response k -

R

Do

D,,

D~'

(3)

In some literature the G-value is given, or the q u o t i e n t GN/GE. Here, G is the n u m b e r of stable radicals produced by an energy of 100eV in the dosimeter material, where subscripts N and E stands for n e u t r o n s and tsecondary) electrons, respectively. k m a y be derived from the G-values by the e q u a t i o n k

G~ Kr,~

G~ K. . . .

(4)

where kmauKt,,sis the quotient of n e u t r o n kerma in the dosimeter material to that in I C R U tissue.

Experimental Results Figure 3 shows the n e u t r o n d o s e - r e s p o n s e k (reference material: I C R U - m u s c l e tissue) of alanine with 2 0 % paraffin admixture, relative to its response to 6°Co p h o t o n s , for monoenergetic neutrons. The error bars indicate estimated c o m b i n e d r a n d o m (for the detector readout) and n o n - r a n d o m (for absolute dose d e t e r m i n a t i o n ) uncertainties at a 63% confidence level. The dashed line is an eye-guided curve t h r o u g h the experimental data. The experimental data from the literature are restricted to those which h a d been determined for detectors with 20% paraffin admixture, u n d e r monoenergetic irradiation conditions. Additionally, however, the data gained at the H a m m e r s m i t h Hospital Cyclotron ( S i m m o n s and Bewley, 1976: K a t z and Berman, 1976) with a b r o a d spectral distribution between approx. 1 a n d 16 MeV are added, The spectral distribution for this condition is available from the literature (Parnell et al., 1971)

Fast neutron response of alanine probes and was used here to calculate the mean energy

10 ~

f O~E(E).E.dE E. = " /,

J

(5)

o

~,~ =

Caiculational Results The experimental studies were restricted to fast neutrons with energies higher than approx. 0.5 MeV. In practice, however, a considerable low-energy spectral fluence component may be present, either inside a phantom, or in an accidental radiation field. Although speaking in terms of dose, this component is reduced because of the decreasing fluence-to-dose conversion with decreasing neutron energy, the response of the detectors to lower energetic neutrons is also important. In the first instance the ratio of kerma in the dosimeter material to that in ICRU-tissue was calculated for pure alanine vs the neutron energy (Fig. 4). It is observed that the kerma ratio increases with decreasing energy below ! keV. This is due to the higher nitrogen component in alanine compared with tissue, and to the fact that the energy released to the material is caused predominantly by the exothermal )"N(n, p) reaction. F r o m Fig. 4 it can also be seen that, above 10 keV, P M M A acts as a nearly perfect phantom material for pure alanine, as the kerma in

~" .~ ca ¢r

10-- 1

101

Itltlt!d

102

I[iilllli

103

J Llifl[~

Lilt!Ill

104 105

Ii/lllll~ Illllll~

106 107 108

Energy (eVI Fig. 4. Kerma value in some alanine-paraffin mixtures and in polymethyl methacrylate (PMMA) relative to kerma in tissue, as a function of neutron energy.

both materials is the same. Therefore, the dose on both sides of the material boundaries is the same, as well. Below 1 keV, however, the kerma in P M M A drops dramatically because of the lack of nitrogen. The kerma ratios were also calculated for various paraffin admixtures to the alanine. In Fig. 5 the kerma ratio is plotted vs the relative alanine content (by weight) at 1.1. eV, 11 keV and 1.05 MeV. The lines for energies above 10keV and below 2 e V intersect at approx. 27% alataine and 73% paraffin mixture. Returning to Fig. 4 one can see that for this mixture the kerma ratio is nearly constant from thermal energy up to 20 MeV. This means that an alanine probe of this mixture would read the dose (in I C R U tissue) independently of the neutron energy, in case the LET of the deliberated particles is constant. Figure 6 gives more details of the kerma ratio for fast neutrons in linear representation, showing that, due to resonances in the neutron cross sections, this ratio can vary within + 15% above 500 keV neutron energy. In order to estimate the L E T effect the following simple consideration is made: protons from the ~4N(n, p) reaction possess an energy close to 0.6 MeV. This is nearly the same as the mean energy imparted to recoil protons by 1.2 MeV neutrons. So, in a first

.~

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resulting values are En = 7.2 MeV (compared with 7.5 MeV taken from Simmons and Bewley, 1976) and £', = 7.7 MeV. At this energy the experimental response for the spectral distribution is drawn. Also data gained in reactor beams are included into Fig. 3; the responses are plotted at the mean energies of the neutron field, as stated by the authors.

0.6

o~o.12%~ -

100/0 o t a n l n e + :' 9 0 % paraffin ."

o

k(E) was taken from the dashed line in Fig. 3. The

=

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._.

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and the alanine response weighted mean energy

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943

I z

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~ ~ - - - - - -

immons ond Bewley (1976) Katz ond Bermonn (1876) Ikezoe et a(. (1984) Kotsumura et al. (1985) This work I I I I I I 6 8 10 12 14 16

N e u t r o n energy En (MeV)

Fig. 3. Neutron dose response, k, of the alanine probes (with 20% paraffin admixture) relative to the response to 6°Co photons.

k.

3.5 3.0

~-

2.5

o

2.0 1.5

o

E

0.5 0

J

1.05 MeV

I 0.2 ReLative

I 0.4 atonine

-- ' ~'~ "="'~'~ ~

I 0.6

I 0.8

I 1.0

comDonent

Fig. 5. Neutron kerma ratio vs the relative alanine camponent for 1.1 eV, 11 keV and 1.05 MeV neutron energy.

944

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8 0 % aLanine + 20%

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2Q

Energy (MeV)

Fig.

6.

Kerma

ratio vs neutron energy representation of Fig. 4.

in

linear

estimate, the protons, generated at low neutron energies, should have a GN value close to that at 1.2 MeV neutron energy. On the other hand, the m a x i m u m o f LET for p r o t o n s appears at roughly 100-keV p r o t o n energy. This c o m p a r e s with 200 keV neutron energy, at which a m i n i m u m o f the G,. value might be expected. This is, to some extent, in agreement with observations o f Lawson and Porter (1975), who found a m i n i m u m G-value for p r o t o n s at a b o u t 500 keV for the FeSO4 standard Fricke solution. If this i n f o r m a t i o n is applied to the neutron response in Fig. 3. the following speculative prediction may be made: using the mixture o f 27% instead o f 80% alanine, the relative neutron dose response k would increase by about 25% due to the higher hydrogen content, but the sensitivity to radiation (both for p h o t o n s and neutrons) decreases by a factor o f 3 using the same mass of the detector probe, due to the reduced alanine content. The m i n i m u m o f the neutron response appears somewhere between 0.1 and 1 MeV. At lower n e u t r o n energies the response increases slightly because o f the sliding transition from low energetic recoil p r o t o n s to 600keV o f the J~N(n, p) reaction. Below 10eV neutron energy the response should remain constant. It should be stressed again that this is speculative and would require experimental verification.

Summary and Conclusions The response relative to tissue dose in alanine with 20% paraffin admixture to monoenergetic fast neutrons ranges from 0.36 to 0.7 c o m p a r e d with the response to ('°Co p h o t o n s , in the energy range from 0.5 to 15 MeV. The lower detection limit for 6°Co 7 radiation is at about 0 . 2 0 G y and consequently between 300 and 500 m G y for neutrons for the energy range under study. The lower detection limit thus does not yet

meet the requirements for accidental dosimetr,,, i.e. to measure neutron doses down to approx, lU mGy. The kerma calculations lead to the assumption that the use o f alanine in p h a n t o m s for measurement of dose distributions in neutron radiation therapy ma,, suffer from the overresponse to neutrons below 100 eV, due to the higher nitrogen content. In the case o f the presence of p h o t o n s , the p h o t o n dose has to be determined separatel_~ b,, a neutron-insensitive detector, as the p h o t o n dose is overweighted by roughl', a factor o f 1.5 3. As the energy distribution changes with the position in the p h a n t o m , this change has to be taken into account by calculating the average detector response, if accurate measurements are required. The effect of overresponse below I00 eV can be avoided by using a mixture o f 27% alanine with 73°.0 paraffin. In this case the dose in alanine is the same as the dose in tissue. The sensitivity of the probes is reduced, however, approximately by a factor o f 3.

References Bradshaw W. W., Cedena D. G.. Crawford (5. W. and Spetzler H. A. (1962) The use of alanine as a solid dosimeter. Radial. Res. 17, 11-21. Coppola M. and Schraube H. l1978) In A European Neutron Dosimetrv Intercomparison Proiect (Eds Broerse J. J.. Burger G. and Coppola M.). EUR 6004, pp. 62 94. lCRU-Report 26 (I977) Neutron Dosimetry /or Biolm,,3 and Medicine. International Commission on Radiation Units and Measurements. Washington, D.C. Ikezoe Y., Sara S., Onuki K , Morishita N., Nakamura T., Katsumura Y. and Tabata Y. (19841 Alanine dosimetry of 14 MeV neutrons in FNS target room. J. Sci Technol. 21, 722 724. Katsumara Y., Tabata Y., Seguchi T., Hayakawa N., Yoshida K. and Tamura N. (1985) Fast neutron irradiation effects I. Dosimetrv. Radial. Phvs Chem. 26, 211 220. Katsumura Y., Tabata Y., Seguchi T., Morishita N. and Kojima T. 11986) Fast neutron irradiation effects--Ill. Sensitivity of alanine systems for fast neutron having an energy of ca 1 MeV. Radial. Phys. Chem. 28, 337 341. Katz R. and Berman F {1976'~Un dosimetre a large gamme, peu sensible a la qualite du rayonnement: I'alanine. l'II1 Congres Intern. Sac. Franc. Radioprotection. Lawson R. C. and Porter D. (1975) The response of the ferrous sulphate dosemeter to neutrons. Phys. Med. Biol. 20, 420--430. Mijnheer B. J.. Williams J. R. and Broersc J. J. (1984) Standardization of dosimetry for fast neutron therapy. J. Eur. Radiother. 5, 195 205. Parnell C. J., Page B. C and Chaudrhi M. A. (1971) A comparison of hear 5 water and her}Ilium as cyclotron targets for fast neutron production for radiotherapeutic applications. Br J. Radml. 44, 63 66. Regulla D. and Definer U. (I982) Dosimetry by ESR spectroscopy of alanine, bzt. J Appl. Radiat. lsot. 33, 1101 1114. Schraube H , Morhart A.. Schraube G. and Burger G. (1986) In ,4 European Neutron Dosirnetrv hltercomparison ProNct. (Eds Broerse J. J.. Burger G. and Coppola M.), EUR 6004, p p 9 31. Simmons J. A. and Bewle,, D K. (1976) The relative effectiveness of fast neutrons in creating stable free radicals. Radiat. Re.s. 65, 197--201.