Spherical Proportional Counter Research Articles

Microdosimetric Measurements of Pretherapeutic Heavy Ion Beams

Microdosimetric quantities for pretherapeutic neon, carbon, and helium ion beams have been measured in the plateau and throughout the spread Bragg-peak region of the depth-dose distribution. The beams studied were: He/sup 2 +/ (230 MeV/amu spread to 5 and 8 cm), C/sup 6 +/ (400 MeV/amu spread to 4 cm), and Ne/sup 10 +/ (400 MeV/amu spread to 4 cm). The measurements were made with a solid-walled 1.27-cm-inner-diameter spherical tissue-equivalent proportional counter filled with gas to an equivalent tissue thickness of 2 ..mu..m. The values of the dose-averaged lineal energy, anti y/sub D/, in the plateau ranged from 6 (He/sup 2 +/) to 46 keV/..mu..m (Ne/sup 10 +/). In the spread Bragg-peak region the anti y/sub D/S were higher and varied considerably across a given peak: 10 to 37 (He/sup 2 +/); 40 to 115 (C/sup 6 +/); and 110 to 265 keV/..mu..m (Ne/sup 10 +/). Depth-dose curves were measured for all beams with the proportional counter operated at low gain in order to define unambiguously the depths relative to the Bragg peak at which the microdosimetric spectra were obtained.

Quality factor calculations for neutron spectra below 4 MeV.

Abstract A method is described for computing the distribution of absorbed dose, D(L), as a function of linear energy transfer, L, for any neutron spectrum with energies below 4 MeV. The results are used to determine the average quality factor for two distinctly different neutron spectra using the ICRP recommended values of the quality factor, Q(L). A comparison is made between the calculations and measurements of D(L) using a spherical tissue equivalent proportional counter. Heavy ion recoil contributions to the average quality factor are examined in detail.

Measurement of the Neutron and Gamma-Ray Spectra Originating from a 14-MeV Neutron Source in Liquid Nitrogen and Liquid Air

An experiment to measure the radiation transport originating from a 14-MeV neutron source in liquid nitrogen and liquid air is presented. Neutron and gamma-ray spectra were measured with a proton-recoil NE 213 scintillator and with four spherical proportional counters in a tank filled with liquid nitrogen or liquid air. The neutron spectra cover the energy range of 20 keV to 18 MeV. The source-detector separation varies in the liquid medium between 60 and 240 cm. The experimental setup is briefly described and the errors are estimated.

Neutron scattering and energy deposition spectra.

The influence of neutron scattering in the wall of a spherical proportional counter on the energy deposition spectra and the absorbed dose is investigated. Event probabilities, and frequency and dose-averaged deposited energies are calculated with and without scattering contribution and compared. The change of absorbed dose due to attenuation of the primary neutron flux is also evaluated.

Radiation Quality of Californium-252

The radiation quality of 252Cf has been determined in a tissue-equivalentphantom for simulated sample sizes of 1 and 2 μm. The measurements were made with a2.54 cm spherical proportional counter of tissue-equivalent plastic with a source-to-counter distance of 7.8 cm. Event spectra have been obtained for lineal energydensities between 0.04 and 103 keV μm-1. The distributions in both lineal energydensity and LET are presented. The dose mean of the distribution in linealenergy density is 38 keV μm-1 for the 1 μm site and 34 keV μm-1 for the 2 μm site.Correspondingly, the dose mean values of the distribution in LET are 34 and30 keV μm-1 respectively. A mean quality factor of about six for 252Cf is obtained forthe geometry used here. The absorbed dose rate per unit source mass was found to be11 rad s-1 g-1, in substantial agreement with most previous measurements, and theratio of absorbed dose due to protons and other heavy particles to that of electrons isapproximately 0.9.

System for 14Co 2 radioassay in the presence of a high background radiation

The detection system consist of a 30 cm 3 spherical proportional counter to measure the beta rays from 14C labelled gas. This counter is built inside a plastic scintillator. Pulses from the photo-multiplier tube which views this scintillator and from the wire anode of the proportional counter are amplified and fed to an anti-coincidence circuit which passes the proportional counter pulses only when no pulse occurs in the scintillator. The purpose of the scintillator and anti-coincidence circuitry is to reduce sensitivity to background radiation. Rejection factors of as high as 80 have been achieved for gamma radiation. Under optimum operating conditions, the detection system achieves a figure of merit of 6.2 in the presence of a SNAP-19 unit and 27 in the laboratory environment. This allows lower limits of detectability of 7 pCi and 1.4 pCi of 14CO 2, respectively, at a confidence level of 0.999999. The system has been designed for possible use on a Mars mission.

An improved spherical dose equivalent neutron detector

Improvements have been made to the spherical neutron detector developed earlier. The scintillation detector used in the earlier spherical moderator assembly has been replaced by a small spherical proportional counter, filled with 3He gas, and this improves the gamma rejection properties, particularly at the higher gamma energies ( E γ 3.5 MeV) where the earlier detector was particularly sensitive. The new detector has a neutron/gamma ratio better than 1000:1 for a range of gamma energies from 0 to at least 7 MeV. The neutron/gamma ratio of all neutron detectors employing polythylene (or paraffin wax) as a moderator is shown to be limited by the onset of (γ,n) reactions in the moderator at high gamma energies. The neutron sensitivity and the neutron energy response of the complete detector assembly have also been improved so that with a calibration figure of 8 cps = 2.5 mrem/h the detector matches the ICRP (1963) recommendations within a factor of 2 (+100%−50%) from 0.025 eV to 7 MeV. The total weight is unchanged at 5 kg (11 lbs.). The new detector is used in the Harwell monitor 0075 which has a total weight, including batteries, of 6.6 kg.

A high resolution spherical proportional counter

A compact proportional counter has been constructed with a spherical cathode and a thin wire anode along a diameter of the sphere. The electric field is made constant along the length of the anode by balancing the field variations due to the spherical cathode against those resulting from the presence of the wire supports. The resolution due to residual field distortion varies with gas multiplication and is 4.3% at a multiplication of 7 and 9.6% at a multiplication of 10 000. The response of the counter is not dependent on the direction of the radiation field to which it is exposed.

The LET Distribution of the Recoil Proton Dose from DD and DT Neutrons

The proton recoil absorbed dose from fast neutrons, measured at depth in a tissue-equivalent phantom using a Bragg-Gray type detector, is expressed as a function of the linear energy transfer, LET.The experimental distribution of dose with LET is compared with the theoretical distributions predicted by Boag for 2·5 MeV and 14·1 MeV neutrons, and by Snyder for 2·5 MeV neutrons. A comparison is made between the technique described in this report, by which the LET distribution of dose can be determined at any point in the phantom, and the spherical proportional counter method of Rossi and Rosenzweig. The experimental curves show excellent agreement with those predicted by Boag. There is also good agreement with Snyder's curves near the front face of the phantom, but significant differences in the centre and towards the back of the phantom.

Radiation protection surveys at heavy-particle accelerators operating at energies beyond several hundred million electron-volts.

The RBE dose in the environs of two high energy accelerators has been evaluated utilizing tissue equivalent ionization chambers and spherical proportional counters designed for the measurement of absorbed dose as a function of LET. The method is generally applicable whenever mixed radiation fields are encountered.

Determination of the Quality of the Absorbed Dose Delivered by Monoenergetic Neutrons

In order to allow a more precise characterization of the quality of ionizing radiation, a spherical proportional counter has been developed at this laboratory. It was shown (1) that because of its shape the pulse-height spectrum obtained with this counter could under certain conditions be subjected to a mathematical analysis in order to obtain the distribution of the dose in the LET (linear energy transfer) of the ionizing particles. Actual measurements on Pu-Be and Po-B neutron sources and on 3-Mev neutrons from a deuterium (d, n) reaction were performed and reported (2). At that time a comparison was made with calculations by Boag (3) based on the LET versus energy curve for protons in water. Some differences between the calculated and observed spectra were found and possible causes discussed. Subsequent measurements indicated that the differences could not be completely explained on the basis of experimental error and that the measured spectrum must differ from the calculated one because of physical factors that are more fundamental. The calculated spectrum uses the concept of the LET of a particle at a point. This is a useful abstraction which can never be quite approached in a physical measurement. The gas volume of the counter must take a finite sample from the track of the ionizing particle. If the sample is very small there will be large statistical fluctuations in the energy loss of the particle owing to the discontinuous nature of the collision process and the possible loss of energetic 5-rays to the counter wall. This effect will be minimized by making the sample larger, but in this case the ionizing particle loses so much energy that it can no longer be characterized by a single LET. It has already been pointed out (4) that the counter measurement presents a picture which resembles more closely events that take place in a sphere of tissue under irradiation. The present work is a study of the pulse-height spectra and resultant dose distributions for monoenergetic neutrons as a function of neutron energy and also sample size. The neutrons were produced at a Van de Graaff generator at the follow