Collimation of 14 MeV neutron beams

The feasibility of measuring total body carbon by counting neutron inelastic scatter gamma rays

A technique is examined for measuring the mass of carbon of both ambulatory and non-ambulatory subjects. The method is based upon the detection of the 4.43 MeV gamma rays emitted from carbon nuclei when the body is irradiated with fast neutrons. The supine subject is irradiated laterally by a horizontal collimated beam of 14 MeV neutrons and the emitted gamma rays are counted by a shielded NaI(Tl) detector placed underneath the subject. The method has been calibrated for all sizes of subjects from 30 to 90 kg and there appears to be no significant interference from the other bulk elements of the body. Body carbon has been measured in six normal volunteers; body nitrogen was also determined by measuring the 13N induced in each subject in a second irradiation. Body fat was estimated from the measured carbon and nitrogen, and the values compared with those derived from skinfold thicknesses. The technique is proposed as a method of estimating long-term energy expenditure.

Tolerance of rodent tails to necrosis after "daily" fractionated X rays or D-T neutrons

The rat tail has been used as a model system to study the necrosis of an organized tissue following fractionated doses of collimated D-T neutrons or 290 kV X rays. RBE values for tail tolerance - 10 per cent of tails necrosing after the early skin reactions - rise from about 1-7 (single doses) to about 3-1 (16 fractions in 22 days). Neutron tolerance doses are almost independent of fractionation from 2 to 16 fractions. The tissues at risk are shown to be rather hypoxic. Early skin reaction levels can be used to predict the fraction of tails that will necrose. Early peak reactions for a given fraction of necrotic tails were slightly higher for neutrons than for X rays, and this difference was consistent for all the dose fractionation schedules employed.

Evaluation of a continuously variable collimator for 14 MeV neutrons

This paper presents an evaluation of a continuously variable collimator for 14 MeV neutrons. Beam profiles measured in air and a phantom are used to demonstrate the relative importance of patient scatter and collimator throat scatter as they limit the performance characteristics of the collimator. The collimator consists basically of two sets of jaws each composed of 30 cm of steel followed by 20 cm of polyethylene. A solution is presented for the asymmetry in vertical and horizontal collimating characteristics resulting from the use of thick collimators. Beam profiles obtained with the collimator are comparable to published values for a fixed field 14 MeV neutron collimator.