Baarli, J

Sullivan, A H

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EUROPEAN ORGANIZA'rION FOR NUCLEAR HESEARCH DI/HP/154 Health Physics Group 2 May, 197 2 DOSIMETRY FOR RADIOBIOLOGICAL EXPERIMENTS WITH A 400 MeV NEUTRON BEAM J. Baarli and A.H. Sullivan CERN, Geneva Switzerland ABSTRACT A neutron beam, of 400 MeV maxim1un energy and of about 3 x 105 n/cm2/sec produced in the CERN Synchro-cyclotron, has been used for radiobiological experiments. The dose in the beam has been estimated using tissue-equivalent ionization chambers, and the flux estimated from the activation of carbon in a plastic scintillator. The dose in an absorber placed in the beam builds up very rapidly and reaches a maximum at about 18 cm depth in water. This rapid rate of increase presents problems for the dose estimation to specific organs of the irradiated biological specimens. The contribution to the absorbed dose from the different interaction processes possible with high energy neutrons changes with depth and this is reflected by a marked decrease in the ratio of high energy particle flux to absorbed dose with depth. The implications of the composi-tion both for dosimetry and for the radiobiology are discussed. Presented at International Symposium on Neutron Dosimetry in Biology and Medicine Neuherberg (Munich), 15-18 May 1972 DOSIMETRY FOR RADIOBIOLOGICAL EXPERIMENTS WITH A 400 MeV NEUTRON BEAM J. Baarli and A.H. Sullivan CERN, Geneva Switzerland A B S T R A C T A neutron beam, of 400 MeV maximum energy and of about 3 x 10 5 n/cm2/sec produced in the CERN Synchro-cyclotron, has been used for radiobiological experiments. The dose in the beam has been estimated using tissue-equivalent ionization chambers, and the flux estimated from the activation of carbon in a plastic scintillator. The doBe in an absorber placed in the beam builds up very rapidly and reaches a maximum at about 18 cm depth in water. This rapid rate of increase presents problems for the dose estimation to specific organs of the irradiated biological specimens. The contribution to the absorbed dose from the different interaction processes possible with high energy neutrons changes with depth and this is reflected by a marked decrease in the ratio of high energy particle flux to absorbed dose with depth. The implications of the composi-tion both for dosimetry and for th1~ radiobiology are discussed. 1. INTRGDUCTION DOSIME'I'RY li'Of{ R.ADIOBIOLOG ICAL }:XPEIUMEJ\ TS WITH A 40C MeV NE1TROK BEJ.M The permissible level of high en8rgy particles outside an accelerator shield is essentially determined by Monte-Carlo tyF~ dose equivalent cal cul at ior.s. These calculations imply that QI' decreases as neutron energy increases above 1 MeV and that with nuclear interactions the products can be cocsidered separately. There is little experimental eviderce to support this approach. For this reason it is of interest to carry out biological experi-ments in high energy neutron beams to determine RBE values as an indication of the dependence of QF on neutron energy. This paper dea.ls with the dosimetry applied at CERN for radiobiological experiments in a 40C MeV neutron beam. 2. THE NEUTRON BEAM The 400 MeV neutron beam is produced by bombarding a beryllium target placed in the CERN 600 MeV Synchro-cyclotron such ihat the axis of the beam pipe used is tangential to the circulating protons at the position of the target. The target was a 5 x 5 x 1 cm Be piece placed vertically with the long side in the beam direction. The target position, for the beam pipe used, was 175 cm from the centre of the machine; at this radius circulating protons have a kinetic energy of 397 MeV. The proton beam intensity is of the order of 1 MA. The beam pipe used to collimate the radiation emitted in the forward direction from the target was a 21.6 cm diameter channel through the 4 m thick shield of the machine and this was evacuated. The exit of the beam pipe was approximately 10 m from the target. The layout of the beam area is shown in Fig. 1. The beam contains neutrons emitted in the forward direction from the target which therefore have as a maximum energy of that of the protons. Charged particles originating from the target are deflected by the field of the cyclotron magnet and the amount of stray charged particles passing through the collimator is small. Tests with additional sweeping magnets at the exit of the collimator indicated that the contribution of charged particles to the absorbed dose in the beam was less than 1%. The collimator will al so accept r -rays and evaporation neutrons. An upper 1 imi t of the r-ray component was determined exposing personnel monitoring films in the beam, from which it was estimated that less than 8% of the absorbed des€ could be attributed to }~rays. Measurements of neutrons using moderated indium foils indi-cated that the flux of neutrons of energy below 10 MeV was less than 10% of that of the high energy neutrons. The contribution to the absorbed dose by fast neutrons contaminating the beam would thus be less than 2% of the total. The spectrum of the high energy neutrons was not measured; however, measurements under similar conditions using 480 and 576 MeV protons(l, 2 ) to bombard a beryllium target show a broad peak in the neutron spectrum some 100 MeV below the proton energy, and a slowly increasing intensity with decreasing energy to about 100 MeV. The intensity profile across the beam and its divergence were checked using a 30 cm3 ionization chamber mounted on a remote controlled X-Y scanner. At 4 m from the exit of the beam colli-mator the beam was reasonably uniform over a 25 cm diameter, as shown in Fig. 2. The intensity in the beam varied by about 14% per metre a.long the beam path, a figure which coincides with the expected divergence of the beam from geometric considerations. 3. DOSIMETRY The absorbed dose in the beam was measured using a parallel-plate tissue equivalent ionization chamber( 3 ) the construction of which is shown in Figo 3. As the dose is uniform over the beam cross-section, the measured dose will be independent of the dia-meter of the collecting electrode provided it is less than the beam diameter. The dose varies rapidly with depth in an absorber, requiring that the spacing between the plates is small such that the depth at which the sensitive volume is situated can be accurately defined. The dimensions chosen for the chamber were 60 mm diameter with 3 mm spacing. The dose was interpreted from a relation: - 2 -D where S is the sensitivity 1.24 (rad/h), i the mean ionization current measured in picoamps using negative and positive polariza-tion, and p the absolute gas pressure in kg/cm2 at 20°c. f 1 and f 2 are correction factors necessary to allow for the response of the chamber to high energy particle radiations. Tissue equivalent materials have an excess of carbon whereas tissue is abundant in oxygen. The mean atomic number and the hydrogen content of tissue equivalent plastic is adjusted to be the same as soft tissue whereas the macroscopic inelastic nuclear cross-section depends on the inverse third root of atomic weight. Hence more nuclear interactions are to be expected in tissue equiva-lent materials than in the tissue which it is meant to represent. Table 1 Interactions per gram per 100 high energy particles/cm2 I Material I CNO I H Muscle 1.09 0.31 TE plastic 1.15 0. 31 TE gas 1.14 o. 29 Water 1.06 0.33 Polythene 1.12 0.43 -The interaction frequency of high energy neutrons with heavy nuclei and with hydrogen for various common dosimetric materials is shown in the table. This indicates that the dosimeter will overestimate the dose from nuclear interactions to tissue by less than 5% and to water-equivalent biological materials by less than 8%. - 3 -A correction given by f Do) fl == ,1 - 0.05 D is applied where D 0 is the dose at zero absorber depth and D the dose to be corrected. Another correction due to the differences in the energy distribution between nuclear fragments is also required. At the present time a quantitative correction is not possible but it is to be expected that more energy would be deposited locally from a carbon interaction than from oxygen. The other correction factor f 2 takes into account the efficiency of the chamber which includes recombination losses and variation of the energy required to form an ion pair( 3 , 4 )_ The 60 chamber is calibrated with a Co gamma source which has been standardized against a known radium source. The measured and cal-culated sensitivities agree if the energy required to form an ion pair is assumed to be 31.6 eV. This energy is not expected to change by more than 1 or 2% in the high energy neutron field. Although heavy charged fragments arc present which would lose about twice the above energy per ion pair on average, their contribution to the absorbed dose is small, as protons, high energy a-particles and minimum ionizing radiation are expected to make the major contribution( 5 ). The voltage and pressure of the chamber are varied to estimate the corrections for recombination that might be necessary. This correction varies up to 5%. 4. DOSE AS A FUNCTION OF DEPTH The depth-dose distribution in water was measured by moving the tissue equivalent chamber along the axis of the .beam while 2 immersed in a water tank. For depths less than 1 g/cm a chamber similar to the tissue equivalent chamber, but with air filling and 'l a mylar entrance window of 3.4 mg/cm~ was used. The reading of this chamber was normalized to the TE chamber at a depth of ') 570 mg/cm~. The dosimeter readings were corrected for beam divergence and chamber response as described above. - 4 -5. FLUX MESUREMENTS Neutron flux measurements are made by measuring the activity 11 . 12 11 of C in plastic ecintillators. The reaction C(n,2n) C has a threshold at 20 MeV and the cross-section stays reasonably constant at about 22 mb up to high energies. The scintillators used had a diameter of 6 cm and were 1 cm thick. The induced 11c 2ctivity was counted on a photomultiplier system and the 20.4 min activity decay followed. Carbon-11 is also induced by proton interactions, where the cross-section ranges up to 90 mb at 50 MeV. Hence measure-ments of carbon activation as a function of depth in water will be influenced by secondary proton production. The carbon activation varies by only about 10% over the first 30 g/cm2 in water. For these reasons the depth-dose distribution in Fig. 4 has been normalized to the neutron flux density as measured by carbon activation with an assumed 22 mb cross-section in a plastic scintillator 1 cm thick, which for the beam used gave a flux 5 2 density of 3.2 x 10 n/cm /sec, and the dose has been expressed as rad per incident particle/cm2 • 6. BIOLOGICAL IRRADIATION The main interest of high energy neutron radiobiology is to irradiate the materials in the region where the nuclear interactions are the major contribution to the dose. This occurs when the samples are in the pure beam with a minimum of material in front. Under these conditions there is a maximum gradient of about 10% per mm in the depth-dose variation and the exact mass of material in front of the site of interest has to be determined. For the same reason only small biological objects can be utilized in order to optimize homogeneous irradiation. So far eye lenses and testes of mice and bean root meristems have been irradiated. The accuracy of the dose estimate to these samples is thought to depend more on uncertainties of position in the beam than on the absolute dosimetry, but by careful positioning an overall dose estimate within 10% has been feasible. The same biological systems have also been studied after exposure to the same beam at maximum build-up in tissue-like - 5 -absorbers. At this position the secondary charged particles are in equilibrium with their primaries and the positioning of the biological specimens is less critical. 7. DISCUSSION The rapid variation of dose with depth in an absorber indicates that the secondary radiation composition is also rapidly changing. Initially the dose is from charged short-range second-aries from nuclear interactions which becomes diluted with proton ionization as the depth increases. Some reservation should be placed on the interpretation of high energy particle flux density measured in the beam, but the values of dose per unit incident flux of 0.55 and 1.2 x 10-S rad/n/cm2 at 1 g and 20 g/cm2 depth 8 2 in water compare with the 0.7 and 1.7 x 10- rad/n/cm calculated for 520 MeV neutrons. The dose measured in the depth-dose curve is neither that in tissue nor in water, the short-range secondary contribution will be that from tissue equivalent plastic whereas the proton contribution to the dose will be that largely origin-ating in water. Also the depth-dose curve will be a function of beam and detector diameter as the secondary radiation has an angular distribution. Hence only refined calculations and weasurements can be expected to have any precise agreement. A quantity of interest to the radiobiology is the amount of energy deposited in an interaction with a heavy nucleus. Consider-" ing the flux per unit dose at very small depths (ru 3 mg/cm~) in the absorber and knowing the interaction frequencies with heavy elements and with hydrogen, an estimate of the average energy q deposited locally (within a few mg/cm~) per nuclear interaction can be made and is found to be 17 MeV. Tl1 is f i gur<' cornr ares favourably with the same value estimated for s Lopp<~d neg;ati v«· pions, (\) 20 MeV ( 7 ) - (i -REFERENCES 1. V.P. Dzhelepov, lzv. Akad., Nauk. SSSR, Ser.Fis.~' 573 (1955). 2. SC Users Handbook, Vol. I, CERN, Chapter F. 3. A.H. Sullivan, CERN Report 69-1 (1969). 4. J. Baarli and A.II. Sullivan, Phys. Med. Biol., 14, 269 (1969). 5. R.G. Alsmiller et al., ORNL-2924 (1970). 6. T.W. Armstrong and B.L. Bishop, Rad. Res., 47, 581 (1971). 7. A.II. Sullivan and J. Baarli, Phys. Med. Biol., _!l, 435 (1968). - 7 -FIGURE CAPTIONS Fig. 1. The beam layout for the 400 MeV neutron beam from the CERN 600 MeV Synchro-cyclotron. Fig. 2. Relative intensity across the 400 MeV neutron beam measured at 14 m from target. Fig. 3. The construction of a tissue equivalent ionization · chamber for absorbed dose measurements in the 400 MeV neutron beam. Fig. 4. Depth-dose distribution of the 400 MeV neutron beam expressed as dose per incident high energy particle flux. - 8 -\ ~ ~! ca \~ I \ E m ..a c 0 '-..... :J <I> c 20 5 96 l )-,-, ... -"' z 1111 15 ... z 1111 > ... c( 10 .... 1111 ~ 5 15 10 5 0 5 10 15 cm FIG. 2 '"! I /...,. /; ( ,. r-' f" ( r' { .._,,.. r' .... ,r \ ( ., ( ; ,.", .. •;;.,...J---'-----.'t\'; I . -I l, ... _; \' "' {' I 't ~\ 'f" f f ) .I ) I ( ) f" I '\ \)/J ~_r,~·-..J~·) ,;_,~I ;1',··11"; ( r' >'I)\', '-\ ) ""' -) ' \ I ':i .__ . '\ r i· (" , 0 )~·)"'. v;,,, " \I\ 1 ~ ,.. ~ ) r I ( i.l I ·"' J 0 . (!) .... M E "' ....... "' _, ,u ... Oii: cc a. ... z "' 0 u z Oii: "' a.0 cc Oii: -9 15.10 -9 10.10 5 10 15 20 FIG.4 25 30 9/cm2 

Dosimetry for radiobiological experiments with a 400 MeV neutron beam

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