Fast Neutron Measurements Research Articles

Fast-neutron spectrum measurement by a stilbene scintillation spectrometer

A stilbene scintillation spectrometer for continuous fast-neutron spectra measurement is described in this paper. Discrimination of gamma rays is achieved by utilizing differences in the shapes of scintillation pulses produced by recoiled protons and electrons. In addition, results of measurements of fast-neutron spectra of the reactor beam are given.

Measurement of fast neutrons produced by high-energy X-ray beams of medical electron accelerators

The fast neutron contamination associated with the 25 MV X-ray beam of a clinical linear accelerator and with the 19 MV X-ray beam of a betatron has been measured at the patient treatment location, using both fission fragment track detectors and silicon diode dosemeters. Measurements are made of the neutron fluence and dose, both in and out of the primary photon beam, at distances up to 60 cm from the central axis. Neutron distributions are similar for the linac and the betatron, with approximately ten times greater neutron dose in the X-ray field than at 20 cm outside it.

The 1H( n,n) 1H scattering observables required for high-precision fast-neutron measurements

The purpose of this paper is to provide the best possible values of the free n-p scatteringtotal cross sections, differential cross sections, and polarization, for neutron energies between 100 keV and 30 MeV. These are extremely important numbers because they are used as standards for most fast-neutron cross-section measurements above 2 MeV and are frequently used at much lower energies. The relevant n-p and p-p data, much of which has not been used in previous evaluations of the n-p cross sections, will be discussed. It is shown that the differential cross sections below 10 MeV do not possess the isotropy frequently assumed. In fact, the cross-section ratios, σ(180°)/σ(90°), are approximately 1.023 at 7 MeV, 1.011 at 3 MeV, and 1.004 at 1 MeV. These ratios, minus 1.000, are higher than the values predicted by the Gammel formula by factors of about 2 at 7 MeV, 5 at 3 MeV, 8 at 2 MeV, and 18 at 1 MeV. In addition, it will be shown that the shape of the differential cross section is far from symmetric about 90°, the 0° cross section, in fact, being less than the 180° cross section for all energies below 30 MeV. The cross sections given here differ significantly from previous estimates based only upon n-p data up to 30 MeV. The present results are derived from calculations using phase shifts obtained by the Yale N-N Interaction Group. Comparisons have been made with the results obtained from calculations employing the Lawrence Radiation Laboratory (LRL) and Dubna phase shifts. In all cases, the general features of large anisotropy and nonsymmetry are verified.

Beam Plug for Radiation Shielding Penetrations

A beam line plug has been designed to fill the cylindrical hole formed by beam lines which pass through radiation shielding walls. The plug consists of multiple steel discs held together along their cylindrical axis by a steel cable forming a flexible snake-like structure. Other disc materials including laminates may be used to solve a variety of radiation shielding problems. When the beam line is to be used, the plug is pneumatically withdrawn into a vertical extension of vacuum line that is connected through a 90° bend to the beam line. Upon release, the plug is reinserted by pneumatic action. The beam plugs being used at the Brookhaven Three-stage MP Tandem Van de Graaff Facility are 7.6 cm diameter and provide 80 cm equivalent thickness of steel to fill penetrations through a 120 cm thick concrete wall. Fast-neutron measurements at the shielding wall surface show that the beam plug is indistinguishable from concrete shielding. A description of the beam plug mechanism, failsafe features, test results, and performance data is presented.

Fast-Neutron Spectrum and Dose-Rate Measurements in Water and Aluminum-Water Laminations

The fast-neutron spectrum and dose rate were measured at various distances from a point fission-neutron source in water and in two aluminum and water mixtures using seven threshold reaction detecto...

Cosmic-ray fast-neutron flux measurements in the atmosphere at various latitudes

We have extended in time our series of balloon flights, made at several latitudes between Hyderabad, India, and Ft. Churchill, Manitoba, to altitudes close to the top of the atmosphere. In these flights the neutrons generated by the cosmic radiation in the energy interval between 1 and 10 MeV are measured. The first set of measurements was made during the period of the minimum of solar activity, and the more recent flights carry the work into the start of the next solar cycle. A decrease in intensity at high elevations with the onset of the present solar cycle has been noted. Further data were also obtained on an airplane flight around the world over both poles, thus covering the full range of latitudes at two opposite longitudes. The relationship between the observed neutron spectrum and that derived by the use of a neutron transport code will be discussed. We shall also discuss other factors emerging from this analysis, including the numbers for radiocarbon production and the leakage flux.

Potentiometric micro determination of the sulphate ion

In the measurement of fast neutrons, the amount of sulphur dispersed in the special films used is important. To determine their sulphur content a potentiometric titration has been studied in which sulphate ions, obtained by oxidation, are titrated with standard lead nitrate solution. The standard deviation in the titration value for 10-µg amounts of sulphur is lower than 1 per cent. As constant potential is rapidly attained after each addition of titrating solution, an automatic titration is carried out.

Detection Efficiency of Bare and Moderated BF3-Gas-Filled Proportional Counters for Isotropic Neutron Fluxes

An elaborate calibration of BF3-gas-filled proportional counters with and without moderator was made for isotropic neutron fluxes. The effective sensitivity of a hare BF3 counter in a nondiffusing medium was determined by means of a calibrated indium foil. The flux-depression effect in a diffusing medium was experimentally evaluated. Details of the experimental techniques and results are described. For the measurement of fast neutrons, energy dependence of sensitivity of two differently moderated counters was determined in the energy region from 1 eV to 10 MeV. Angular dependence of sensitivity of the moderated counters and its dependence on neutron energy are also described.

Investigation of the High-Energy Acceleration Mode in the Coaxial Gun

The amount of deuterium gas admitted to a short center electrode (1 cm) coaxial plasma gun delineates two regions of operation, depending on whether the gas is localized near the inlet port or allowed to diffuse throughout the gun length. In the former, fast deuterium ions with energy in excess of the applied voltage (V0 = 15 kV) emerge from the gun muzzle whereas in the latter, the accelerated plasma which initially starts at the breech end reaches an axial velocity of ∼2.5 × 107 cm/sec at the muzzle end and forms a high-density plasma focus on the axis in front of the central electrode. Neutron measurements show that this dense plasma is not in contact with the electrode and is most probably stagnated by the high ambient gas pressure downstream. Particle momentum analysis of the fast ion (> V0) distribution and neutron measurements downstream of the gun suggest two accelerated groups of fast ions, (1) a group of ∼1016 particles which diverge from the gun muzzle and (2) a group of ∼1014 particles which stream primarily along the axis with a radial distribution of ∼3 cm width. Multiple probes (Bθ) show an asymmetric angular distribution of the discharge current that varies as a function of time during a given discharge. A sharp break in the total gas current is accompanied by large inductive voltages. In the high-energy mode, inductive voltages up to 7V0 have been recorded at the breech end.

Measurement of the neutron spectrum from radium D-beryllium with nuclear emulsions

This work aims to measure the neutron spectrum from a RaD-Be source with the nuclear emulsion technique. This technique allows spectrum measurements of fast neutrons with a good energy resolution and can be used also for wide and uncollimated sources, such as in the case of «in core» neutron fluxes. The plate chosen is a 200 µm thick Ilford K2, on a glass mount. The method is basedo n the determination of the track lengths of recoil protons from elastic scattering of neutrons on hydrogen nuclei of the emulsion. The differential spectrum obtained has 0.2 MeV channel width. Statistical errors are not larger than 7% for 85% of the channels taken into account in this case.

The use of a phosphorus-polythene mixture for fast neutron measurements

A convenient way of measuring relative fast fluxes in a small reactor core using foils of phosphorus-Polythene mixture is described. Determination of the disintegration rate of these foils involves the measurement of the β-ray self-absorption of the foils as a function of thickness. Accurate disintegration rate measurements after irradiation for 6 hr using 7/16 in. dia. foils enabled fission fluxes of 5 × 105 n/cm 2 sec or more to be measured absolutely to within ±6 per cent and fluxes as low as 5 x 104 n/cm 2 sec to within ±20 per cent when the counter background was 80 c/min. By reducing this background and increasing foil size both of these limits are lowered by about a factor 50. The method compares favourably with methods using the 32S(n,p) 32P threshold reaction.

Nomograph for Time-of-Flight Measurements of Fast Neutrons

The nomograph, when used with a graphical presentation of the data, allows an estimate to be made of the energy of a particular neutron group. The error is usually less than a few percent for the system. (C.J.G.)

Dark-field microscopic illumination for energy measurements of fast neutrons with photographic emulsions

By applying an azimuth diaphragm to the aperture iris of a bright-field condenser and thus producing dark-field illumination we succeeded in rendering a great emulsion volume readily visible, so that only tracks of ±25° azimuthal angle interval might be observable, that is about 28–30% of all the existing tracks.

New techniques for the precision measurement of gamma rays and fast neutrons with chemical dosimeters: George V. Taplin, Atomic Energy Project, University of California, Los Angeles, U.S.A.

New techniques for the precision measurement of gamma rays and fast neutrons with chemical dosimeters: George V. Taplin, Atomic Energy Project, University of California, Los Angeles, U.S.A.

The n unit and energy absorption in tissue.

When fast neutrons were first employed for irradiating biological material more than ten years ago, it was apparent that the choice of a unit was a difficult problem. It was realized at the outset that the best basis of comparison with x- or gamma radiation was energy absorption in tissue. A determination of this quantity, however, was not practicable especially in routine measurements, and for that reason a more convenient unit was needed. In 1942 Aebersold and Lawrence (1) introduced the n unit, which represents the quantity of neutron radiation that will produce the same ionization in a 100-r Victoreen chamber as 1 roentgen of x-radiation. The n unit has since been used extensively to quantitate neutron exposures of biological material, although it suffers from a number of limitations. Among these is the fact that the unit is arbitrary in the sense that the factor relating the response of the ionization chamber to energy absorbed in tissue depends on a number of complex variables, most of which depend on neutron energy. Examples of these are: relative interaction cross sections for neutrons; stopping power, relative to air, of the wall material of the chamber for the recoil nuclei produced; the ratio of energy loss and ionization for these nuclei; and corrections for the fact that the chamber dimensions may be comparable to the range of the ionizing particles. Aebersold and Anslow (2), taking into account these and other factors, came to the conclusion that tissues exposed to 1 n of fast neutrons receive between 1.98 and 2.5 times as much energy as when exposed to 1 r of hard x-rays. Another difficulty lies in the fact that Victoreen chambers are not designed for the measurement of fast neutrons. Consequently, they may show considerable variation in response. This may be due to such factors as difference of wall thickness, variation in the thickness of the interior conducting coating, and inequality of dimensions of the sensitive volume. As a result, two chambers having equal sensitivity to x-rays may exhibit appreciable differences when exposed to neutrons. In the absence of better, readily available, reference standards, these chambers have been used in spite of the above-mentioned drawbacks. It is therefore a matter of interest to obtain further information on the energy absorbed by tissue exposed to 1 n of fast neutrons and on the likely limits of variation of this quantity caused by inherent differences between Victoreen chambers. In the following are reported the results of the calibrations of a number of Victoreen chambers at two neutron sources. While the actual figures apply, of course, only to the actual conditions of this study, they have general applicability, since the sources employed are typical and because the variations in chamber response suggest limits within which the n unit is likely to be a consistent basis for comparison.

Fast neutron measurements with recoil counters

Fast neutron measurements with recoil counters

Fast Neutron Measurements with Recoil Counters

Fast Neutron Measurements with Recoil Counters