Ion losses in a plane-parallel ionisation chamber irradiated with a pulsed electron beam

General collection efficiency for liquid isooctane and tetramethylsilane in pulsed radiation

The general collection efficiency in pulsed radiation was studied for isooctane (C8H18) and tetramethylsilane (Si(CH3)4). These two liquids were used as sensitive media in a parallel-plate liquid ionization chamber with a 1 mm sensitive layer. Measurements were carried out using 20 MV photon radiation from a linear accelerator with a pulse repetition frequency of 30 pulses/second and a pulse length of 3.5 microseconds. The general collection efficiency was determined for polarizing voltages in the interval 1000-2000 V for isooctane and 500-2000 V for tetramethylsilane and for pulse doses in the interval 0.06-1.9 mGy/pulse. An air ionization chamber was used as a pulse dose reference monitor. The experimental results were compared with those predicted by the equation for the general collection efficiency for gases in pulsed radiation, using the permittivity of each of the liquids. It was found that for general collection efficiencies down to 80% the differences between the predicted and experimental general collection efficiencies in the two liquids were within +/- 1% at electric field strengths exceeding 10(6) V m-1.

Design studies related to an in vivo neutron activation analysis facility for measuring total body nitrogen

Design studies relating to an in vivo prompt capture neutron activation analysis facility measuring total body nitrogen are presented. The basis of the design is a beryllium-graphite neutron collimator and reflector configuration for (alpha, n) type radionuclide neutron sources (238PuBe or 241AmBe), so as to reflect leaking, or out-scattered, neutrons towards the subject. This improves the ratio of thermal neutron flux to dose and the spatial distribution of thermal flux achieved with these sources, whilst retaining their advantage of long half-lives as compared to 252Cf based systems. The common problem of high count-rate at the detector, and therefore high nitrogen region of interest background due to pile-up, is decreased by using a set of smaller (5.1 cm diameter x 10.2 cm long) NaI(Tl) detectors instead of large ones. The facility described presents a relative error of nitrogen measurement of 3.6% and a nitrogen to background ratio of 2.3 for 0.45 mSv skin dose (assuming ten 5.1 cm x 10.2 cm NaI(Tl) detectors).