BF3 Counter Research Articles

Application of Different Orthonormal Expansion Methods to Neutron Measurements. I. Analysis of Spectra

This is the first of three papers. The following ones are entitled “II. Estimation of dose equivalent rates,” and “III. Actual measurements of spectra and dose equivalent rates,” respectively. Approximate neutron spectra can be obtained from the counting rates of BF3 proportional counters. The BF3 counters are covered with paraffin of different thicknesses. In the present work, the use of the meaningful orthonormal expansion methods is possible even when sensitivities of only modearte precision are used. The orthonormal sets of functions are constructed from the expected spectra as well as from the sensitivities of the detectors. The calculations using a digital computer show that the method employing the expected spectra is useful particularly when the number of employed detectors is small.

Application of Different Orthonormal Expansion Methods to Neutron Measurements. III. Actual Measurements of Spectra and Dose Equivalent Rates.

Neutron spectra and dose equivalent rates were measured with a BF3 proportional counter covered with two, three or five of 0-, 3.5-, 4.5-, 6.5-, 8.5- and 14.0-cm-thickness paraffin moderators. The dose equivalent rates were also measured with a “Remcounter” having two detectors, one of which consists of a BF3 counter centered in 30-cm-diameter paraffin cylinder and the other of three BF3 counters embeded at intervals of 120 degrees along a coaxial 16-cm-diameter cylinder in the same paraffin moderator. The obtained dose equivalent rates for neutrons from a RaD-Be source did not differ by more than 22 per cent from the theoretical values given in the paper II, even when the measured spectra had large negative values at some energy.

Cosmic-ray-produced neutrons at ground level: Neutron production rate and flux distribution

The absolute rates of cosmic-ray neutron production and neutron flux distribution on the ground were determined at sea level and mountain altitude at a geomagnetic latitude λ = 44°N in 1964. The thermal neutron flux was measured with a well-calibrated BF3 counter; a Maxwellian energy distribution with a shifted neutron temperature was assumed. By using two differently moderated BF3 counters, the fast-neutron flux was determined in the energy range 0.4 ev to 10 Mev. The neutron fluxes were also estimated from the measured production rate, and they are in good agreement with the measured fluxes; possible occurrence of air-ground boundary effects on the neutron flux distribution was considered. Anisotropy of the thermal neutron flux on the ground was experimentally demonstrated, and the angular distribution was well filled by the first two terms of a spherical-harmonics expansion. The air-ground boundary effects are discussed on the basis of experimental results.

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.

BF3 Counters for Use in Field Instrumentation

A number of different neutron detectors employing arrays of BF3 counters in hydrogenous moderators have been constructed and tested in the field. Some of the problems of obtaining BF3 counters with matching mechanical and electrical characteristics from commercial manufacturers are briefly reviewed. Several unexpected problems which arise when BF3counters are not used in a controled environment are discussed. A transient sensitivity to gamma rays in some counter designs immediately after the application of high voltage is noted and explained. A technique for reducing spurious counts due to humidity effects by coating the ceramic high voltage insulators with Teflon is described. Spurious counts due to inverting and vibrating the BF3 counters are examined for different sizes of counters.

Neutron Flux and Energy Measurements

The problem of neutron energy measurement is considered in the energy ranges, thermal (< 0·5 eV), intermediate (0·5 eV-10 keV), fast (10 keV-15 MeV) and relativistic (> 15 MeV), together with the general problem of neutron flux measurement. Sensitivities and resolutions obtainable are given for a variety of methods. It is concluded that for thermal and intermediate energy measurements in radiological applications it is best to use BF3 counters, fission chambers, moderated detectors, together with gold or manganese foils, in conjunction with the Westcott flux convention; at higher energies methods based on proton recoil detectors are preferred for high sensitivity and accuracy, while threshold detectors are suggested if multichannel pulse-height analysers and computers are not available, or if there is a limitation on the size of the detectors.

Large area Li 6I(Eu) detector bank for slow neutron time-of-flight measurements

For slow neutron time-of-flight experiments a large surface detector bank was constructed which uses 20 Li 6I(Eu) crystals, 2mm thick and 3″ in diameter, mounted on 20 phototubes. After a general consideration of the properties of LiI(Eu) scintillators, the construction of the bank is described. Some typical performances are presented. Neutron counting efficiency of the bank is better than that of the normally used BF 3-counters and follows a simple 1 v - law . Thus corrections for efficiency become low and easily calculable. Unfortunately this is at the price of a non-negligible gamma-sensitivity. For short flight-times the improvement in time resolution by a factor of about 50 compared with equivalent BF 3-counters may be interesting.

Energy Response of BF3 Counter Using Paraffin Moderators

Neutron energy response of a BF3 counter using paraffin moderator was obtained in the energy range from 2 eV to 1 KeV. Neutron energy was determined by the time-of-flight method. The counter was covered with a cadmium sheath to eliminate the thermal neutron diffusing from the moderator. The results with BF3 counters with thinner moderator were consistent with Fräki's calculation while those with thicker moderator were not. This inconsistence may be due to the fact that the mean energy of neutrons passing through cadmium sheath decreases in the case of thicker moderator and that the counting efficiency of the counter varies according to the 1/v law of the 10B(n, α) reaction.

Current-Sensitive Nuclear Instrument

The radiation detector is by nature a current generator of high output impedance. A current sensitive amplifier of low input and high output impedance can be devised for incorporation in a nuclear electronic system, providing a matched impedance at the terminal of the detector cable. The amplifier output is given by i×Ai (i: detector current output pulse, Ai: current amp. gain), when the amplifier rise time (t r ) is faster than the current pulse rise time. Meanwhile, when t r , is longer than the current pulse width, the output should be defined by Q/t r ×Ai (Q: detector output charge) in amp/coulomb units. The rise time of fission counters, BF3 counters, PR gas-flow proportional counters and inorganic scintillators with conventional photomultipliers is short and is even shorter in the case of fast photomultipliers combined with organic scintillators and semi-conductor detectors. Thus marked improvement in signal-to-noise ratio can be expected from a current-sensitive system, and this has been proved even with a slow amplifier (t r =200 ns), not to mention what can be expected with a faster amplifier (t r <20 ns). G-M tubes are inherently, slow, and combination with slow amplifiers does not distort the current pulse shape. Current pulse transmission via. cable is more rational and easier than the conventional voltage pulse transmission. In the present study, G-M tube current pulses were successfully sent over a distance of 1 km without any preamplifier or pulse transformer.

Neutron albedo measurements on polar orbiting satellites

An experiment to measure the earth's neutron albedo was flown on three polar orbiting Agena satellites in 1962. The detector, a BF3 counter within a moderator, was surrounded by a ring of charged-particle proportional counters to discriminate against neutrons produced by protons in the detector assembly. The results show an albedo flux of 0.085±0.03 n/cm2 sec at the equator at 300-km altitude, in agreement with the rocket measurements of Bame et al. and the calculations of Lingenfelter. However, at high latitudes we find 0.24±0.06 n/cm2 sec, a factor of 4 below the calculations of Lingenfelter. The appendix outlines calculations that have been used to estimate the neutron production by primary cosmic-ray protons on the detector assembly and the vehicle itself. These calculations are compared with the experimental measurements of production in the detector assembly.

A Paraffin-Moderated 4π-Neutron Detector

A flat response 4π-neutron detector has been constructed by arranging BF3 counters in multiple concentric rings in a cylindrical paraffin moderator. The sensitivity is flat within ±5% for neutrons between 50 keV and 700 KeV. The absolute efficiency is 2.24% for 50 to 700 keV neutrons and 1.76 % for RaD–Be neutrons in the standard counter arrangement. This efficiency can be increased by mounting more BF3 counters but the relation is not linear because of the mutual interference effect in BF3 counters. The dependence of the counting efficiency on the angular distribution of neutron source was investigated by measurements of thermal neutron distributions in the moderator, and it has proved to be quite small (<1.5%) even for the T3(p, n) He3 reaction in its threshold region.

Calculation of flat-response fast-neutron detection systems employing finite size BF 3-counters

Making the assumption that φ the rate at which neutrons are emitted from a source, is proportional to the volume integral of the slow neutron density throughout the moderator, one can calculate a weighting function ⨍( r) such that ∫ moderator ⨍( r)I( r) dV is proportional to φ where I( r) is the count-rate of a finite size BF 3-counter at position coordinate r . The weighting function ⨍( r) is independent of the energy spectrum of the source neutrons. The calculation is demonstrated by a sample calculation of a flat-response fast neutron detection system.

Very Large Boron Trifluoride Proportional Counters

More than 50 very large BF3 counters, each approximately 6 ft (180 cm) long and 6 in. (15 cm) in diameter have been made for cosmic-ray monitors. During the fabrication of these counters, the characteristics at varying gas pressures have been studied and the alpha backgrounds of the various metals used for cathodes have been determined. The counters have excellent characteristics at pressures up to 30 cm Hg and perfectly acceptable characteristics to 60 cm Hg pressure. With anode diameter 0.008 in. (0.2 mm) and gas amplification 35, the operating voltage for a filling of 20 cm Hg pressure of pure BF3 is 2800 V; at 30 cm Hg it is 3400 V. Counters with copper cathodes have backgrounds due to residual alpha contamination of about 2 counts/min, and with stainless steel cathodes, about 5 counts/min. Using a BF3–Ar filling a value of 6.30±0.05% has been obtained for the branching ratio of the B10(n,α)Li7 reaction.

中性子温度測定用ガドリニウム・フィルタの研究

For the study of the low-energy neutron spectrum by the integral method, it is important to develop sensitive neutron temperature detectors. The non-l/ nu property of the effective cross sections of Pu²³⁹ or Lu¹⁷⁶ has been used for this purpose. If the non-l/ nu property of the neutron detector is varied artificially, a very useful technique for the study would be obtained. Gadolinium has a large absorption cross section in the low energy region; the cross section decreases rapidly with increase of neutron energy. Thus, the neutrons filtered through a Gd foil will be devoid of the low energy part in the spectrum. It would be sharply cut off at the energy of 0.1 to 0.5 ev. With this sharp cut-off filter, the shift of Maxwellian distribution of thermal neutrons would be detected very precisely. Two BF₃ counters were located at the same geometries. One of them was bare and the other was covered with the Gd filter. The ratio of their count rates will be defined as the Gd ratio and is very sensitive to the neutron temperature. The neutron temperatures were varied by re-thermalization in a water tank at different temperatures. The pure Maxwellian thermal neutrons were introducedmore » into the tank from the thermal column of the JRR-l reactor. The temperature of the tank was varied very slowly between the range of 15 to 85 deg C. The resolving power of the neutron temperature detector was found to be 2 deg C. (auth)« less

Strahlenschutzmessungen an verschiedenen europäischen Betatronstationen

In connection with comparative measurements on dosimeters used in therapy at various European betatrons the distribution was measured also of the unwanted bremsstrahlung and of fast neutrons. As bremsstrahlung detector use was made of a portable cylindrical ionization chamber. The results show that the distribution of the unwanted bremsstrahlung depends on the construction of the betatron. The measurement of the fast neutrons produced by nuclear photoeffects was carried out on a paraffin double moderator by means of a BF3 counter. In this way it was possible to determine either the neutron dose rate or flux density and so to establish the mean neutron energy. The distribution of the fast neutrons is determined by the shape and dimensions of the therapy room. The measurements thus provide important clues for a suitable construction of therapy rooms. (auth)

Scintillator for Thermal Neutrons using Li6F and ZnS (Ag)

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On the “radius effect” in large BF3 counter tubes

A direct measurement was made of the radius effect in large-diameter (5 to 6 cm) BF3 tubes. The number of counts per minute was measured as a function of the distance from the center of the hole in the Cd screen to the anode wire. The results showed that up to a distance of 20 mm from the anode wire the efficiency of these counters is virtually constant.

水中における高速中性子の減速

The migration area of thermal neutrons and the age of In-resonance neutrons in water have been obtained with T-d neutrons. Taking into account the angular dependence of the emitted neutron energy and yield in the T-d reaction, the average source energy was taken to be 14.2 MeV. By means of “Cd difference” method and a small (13mmφ×50mm) BF3 counter, the spatial distribution of thermal neutrons in a large water tank was measured at the 0°, ±45° and ±80° direction along the radial distance, from the source to the point of 110cm. Then, the migration area M2 was obtained from the spatial distribution of thermal neutrons in each direction. The spatial distribution of In-resonance neutrons also was measured by In foils with Cd covers to the point of 55cm at the same directions. And the neutron age τln of In-resonance energy (1.44 eV) was obtained, using the same relaxation length in large distance as the thermal neutron distribution. The values of migration area M2 and neutron age τln are determined as 176±5cm2 and 167±8cm2, respectively. The slowing down length Ls (=τln1/2) to In-resonance energy is 12.9±0.3cm, and in agreement with Volkin's curve, which was calculatad assuming the anisotropic nature of the elastic scattering by oxygen.

New Goniometers for Single-Crystal Neutron Diffraction Studies

A new type of goniometer has been designed and constructed for neutron diffraction analyses, and particularly for single-crystal studies. The neutron detecting system employs a scintillation crystal and photomultiplier, or a miniature high-pressure BF3 counter; and the detector and diffracting crystal are located within a cylindrical shielded region, in such a manner that the moving parts are entirely free of heavy shielding elements. When a miniaturized BF3 counter is used, no shielding except B4C in plastic immediately surrounding the counter is required. Rotations are accomplished by servo-controlled motors, with selsyn indicators coupled to an external servo system. This servo control is designed to permit future introduction of automatic crystal and detector setting. The system is small and inexpensive, and has the particular advantage of readily permitting three-dimensional diffraction measurements.