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Measurement of fast-neutron dose using a proton-recoil telescope
NUCLEAR
INSTRUMENTS
AND
METHODS
126
0975)
595;
©
NORTH-HOLLAND
PUBLISHING
CO.
M E A S U R E M E N T OF FAST-NEUTRON DOSE USING A P R O T O N - R E C O I L T E L E S C O P E A. WILLIAMS
Division of Radiation Science, National Physical Laboratory, Teddington, Middlesex, U.K. Received 9 May 1975 A method of measuring fast-neutron dose in hydrogen is proposed which relies on measuring the energy of the protons emitted from an hydrogenous radiator in a proton-recoil telescope.
When a hydrogenous material is irradiated with fast neutrons, the energy deposited in a thin slab within the material, is equal to the energy produced within the slab providing the slab is sufficiently far into the material for charged particle equilibrium to be established, and providing also that the attenuation of the irradiating beam and bremsstrahlung losses can be neglected. Thus, if the energy produced within the slab can be measured, the dose to the slab can be readily obtained. By using a slab of the hydrogenous material in a proton-recoil telescope in the conventional way1'2), the energy and number of protons within the solid angle of the telescope can be measured. From the known angular distribution of the (n, p) reaction, and the mass of the slab, the proton kerma produced in the slab can be obtained, and this is equal to the hydrogen contribution to the kerma the slab would receive were it within an homogeneous medium where chargedparticle equilibrium existed. Note that only the angular distribution of the (n, p) cross section is required and not the absolute value of the cross section. From the observed proton spectrum and the shape of the (n, p) cross section the spectrum of the incident neutrons can be deduced. The dose in tissue can then be calculated from the proton kerma, the composition of
595
the slab and the kerma ratios published by Bach and Caswell3). The energy calibration of the telescope can be carried out using mono-energetic neutrons of known energy from charged-particle reactions, and therefore the dose measurement is not dependent on the W value of the material used to detect the protons. In addition the telescope has negligible gamma sensitivity and measures just the neutron dose. The telescope has a highly directional response and there may be difficulty in using it to determine the total neutron dose where there is a large component due to scattered neutrons. However, it could be used to calibrate ionisation chambers under conditions of low scatter, in which case the telscope is effectively being used to measure a combined value for W (for the gas of the ionisation chamber) and the mass-stoppingpower ratio of chamber wall to gas. References 1) j . B . Marion and J . L . Fowler, Fast neutron physics (Interscience Publishers, New York, 1960) part I, p. 247. 2) H. Liskien and A. Paulsen, Nucl. Instr. and Meth. 69 (1969) 70. a) R. L. Bach and R. S. Caswell, Radiation Research 35 (1968) 1.
INSTRUMENTS
AND
METHODS
126
0975)
595;
©
NORTH-HOLLAND
PUBLISHING
CO.
M E A S U R E M E N T OF FAST-NEUTRON DOSE USING A P R O T O N - R E C O I L T E L E S C O P E A. WILLIAMS
Division of Radiation Science, National Physical Laboratory, Teddington, Middlesex, U.K. Received 9 May 1975 A method of measuring fast-neutron dose in hydrogen is proposed which relies on measuring the energy of the protons emitted from an hydrogenous radiator in a proton-recoil telescope.
When a hydrogenous material is irradiated with fast neutrons, the energy deposited in a thin slab within the material, is equal to the energy produced within the slab providing the slab is sufficiently far into the material for charged particle equilibrium to be established, and providing also that the attenuation of the irradiating beam and bremsstrahlung losses can be neglected. Thus, if the energy produced within the slab can be measured, the dose to the slab can be readily obtained. By using a slab of the hydrogenous material in a proton-recoil telescope in the conventional way1'2), the energy and number of protons within the solid angle of the telescope can be measured. From the known angular distribution of the (n, p) reaction, and the mass of the slab, the proton kerma produced in the slab can be obtained, and this is equal to the hydrogen contribution to the kerma the slab would receive were it within an homogeneous medium where chargedparticle equilibrium existed. Note that only the angular distribution of the (n, p) cross section is required and not the absolute value of the cross section. From the observed proton spectrum and the shape of the (n, p) cross section the spectrum of the incident neutrons can be deduced. The dose in tissue can then be calculated from the proton kerma, the composition of
595
the slab and the kerma ratios published by Bach and Caswell3). The energy calibration of the telescope can be carried out using mono-energetic neutrons of known energy from charged-particle reactions, and therefore the dose measurement is not dependent on the W value of the material used to detect the protons. In addition the telescope has negligible gamma sensitivity and measures just the neutron dose. The telescope has a highly directional response and there may be difficulty in using it to determine the total neutron dose where there is a large component due to scattered neutrons. However, it could be used to calibrate ionisation chambers under conditions of low scatter, in which case the telscope is effectively being used to measure a combined value for W (for the gas of the ionisation chamber) and the mass-stoppingpower ratio of chamber wall to gas. References 1) j . B . Marion and J . L . Fowler, Fast neutron physics (Interscience Publishers, New York, 1960) part I, p. 247. 2) H. Liskien and A. Paulsen, Nucl. Instr. and Meth. 69 (1969) 70. a) R. L. Bach and R. S. Caswell, Radiation Research 35 (1968) 1.