Dosimetric Properties of Neutron Beams Produced by 16-60 MeV Deuterons on Beryllium

Beams of fast neutrons were produced by a variable-energy cyclotron, using the _{4}{}^{9}{\rm Be}$ (d, n) _{5}{}^{10}{\rm B}$ reaction. The Be target thickness was 12.1 mm; Benelex material was used for collimating the neutron beams. A field of 5 × 5 cm at 125-cm target-surface distance was used for most measurements; the phantom employed was a Lucite-walled tank filled with tissue-equivalent (T-E) liquid. The absorbed dose rate in terms of rad/min/μA was measured at deuteron (d) energies between 16-60 MeV and found to follow an ${\rm E}^{2.67}$ relationship; the results are in agreement with previous data at the lower energies. Central axis depth-dose data were measured using d energies of 16, 30 and 50 MeV; the depth of the 1/2 maximum dose was found to increase with energy from 8.5 cm at 16 MeV to 12.0 cm at 50 MeV. Positions of maximum dose buildup were measured with an extrapolation chamber; at the higher energies these maxima were found to occur at depths greater than from 60 Co γ-rays. The effectiveness of the collimator was measured for 16, 30 and 50 MeV d. Data obtained in the phantom indicated that outside the defined beam the dose was 5% of the maximum, independent of energy. Measurements outside the phantom indicated that 40% of that was attributable to transmission through the collimator. An estimate of the n/γ ratio was made using a paired ion chamber system (T-E and carbon); while calculations were done for 16 MeV, more data on the response of the carbon chamber to the higher energy neutrons are needed to obtain the ratio at 50 MeV. At 16 MeV the entrance gamma dose is 5% of the neutron dose and at 10 cm it is 14%. Preliminary indications are that these numbers do not increase much at higher energies.