Boron Filters Research Articles

Evaluation of the spectrometric and dose characteristics of neutron fields inside the Russian segment of the ISS by fission detectors

The results of measuring the dose and the energy spectrum of neutrons inside the Russian segment of the International Space Station (ISS) from March 21 until November 10, 2002 are presented. Statistically reliable results of measurement are obtained by using thorium- and uranium-based fission detectors with cadmium and boron filters. The kits of the detectors with filters have been arranged in three compartments within assembled passive detectors in the BRADOS space experiment. The ambient dose rate H* = 139 μSv day and an energy spectrum of neutrons in the range of 10–2–104 MeV is obtained as average for the ISS compartments and is compared with the measurements carried out inside the compartments of the MIR space station. Recommendations on how to improve the procedure for using the fission detectors to measure the characteristics of neutron fields inside the compartments of space stations are formulated.

Energy dependence of neutron damage in silicon bipolar transistors

2N2222A transistors were exposed in neutron fields ranging in energy from thermal to 14 MeV. Spectrum characterization data are reported for the three fast reactors used in hardness testing. The neutron energy dependence of transistor damage response was measured and compared with predictions based on ASTM standards and recent compilations of the silicon kerma factors calculated using NJOY. For all the environments considered, the hardness parameters calculated using the NJOY displacement kerma factors are significantly smaller than those calculated using the ASTM standard values, using the same value of 95 MeV-mbarn for the kerma factor at 1 MeV. The NJOY calculations actually show that this value is too high, so that if the standard is revised the 1-MeV value should also change. The spectrum evaluations for the fast reactors improve the agreement between the observed and predict hardness ratios, provided consistent methods are used for the unfolding. The thermal reactors and the moderated californium source show consistent results only when thermal and epithermal neutrons are removed by boron filters. The ASTM standards should be revised to include this precaution as well. >

Advantages of boron filters in instrumental epithermal neutron activation analysis of biological materials

Boron carbide (B4C) and boron nitride (BN) irradiation filters have been constructed and used for epithermal neutron activation analysis of biological materials. The major advantage offered by these filters in this application is the substantial reduction of interfering matrix activities. Consequently some trace elements of interest can be determined instrumentally with minimum delay. The filters are particulary beneficial if utilized in short reactor irradiations for rapid analysis of biological materials.

Detection limits in epithermal neutron activation analysis of geological material

The improvement of detection limits for trace elements in geological samples by epithermal neutron activation analysis is examined. The relative merits of cadmium, boron and composite cadmium+boron filters are compared for trace elements Ni, As, Pd, Cd, Sb, W, Ir, Pt and Au, and interfering elements Na, K, Sc, Cr, Fe, Co and Cu. A boron filter gives optimum sensitivity for the trace elements based on interference from46Sc, but the detection limits are only improved 2–5 times. Ma imum possible improvement, which is shown by Ni, gives sensitivities 5 times better under cadmium and 15 times under boron.

Measurement of low concentrations of hydrogen in small samples by intermediate neutron moderation

An improved method based on the moderation of intermediate neutrons for the measurement of hydrogen in small samples is described. With the aid of boron and cadmium filters, a space shielded from slow neutrons is set up close to an isotopic neutron source shrouded by water moderator. A BF3 proportional counter enclosed with a sample cell is placed in this space. The neutron count rate of the counter increases when a hydrogen-containing material is introduced into the cell, due to the moderation of intermediate neutrons passing through the filters. With a 1.3 μg252Cf neutron source, the lower limit of hydrogen detection for 200 ml samples in 10 min count time is 0.01 wt.%. This method is suitable for measuring the H2O content of heavy water.

New Methods for Measuring Neutron Spectra with Energy from 0.4 eV to 10 MeV by Track and Activation Detectors

Additional possibilities in achieving high-accuracy measurements of continuous neutron spectra by track and activation detectors have been found. An analysis of energy characteristics of detectors containing /sup 235/U in boron filters of different thicknesses shows that they can be used successfully for measuring intermediate neutron spectra. Application of detectors with /sup 231/Pa and /sup 236/U considerably increases the accuracy of fast-neutron measurements. The problems associated with increasing the accuracy measurements resulted in the necessity of studying a method for minimizing a directed divergence to use it for unfolding the neutron spectra with energy from 0.4 eV to 10 MeV. A suggested variation of the method makes it possible to decrease the influence of experimental errors and composition of the detector set on the spectrum unfolding errors. The neutron spectra were measured at the Vinca, IBR JINR, and IRT-1000 reactors.

Measurement of Resonance Activation Integral of55Mn

The resonance activation integral of 55Mn was measured relatively to that of 197Au by absolute counting on 56Mn and 198Au: 56Mn was measured with a Ge(Li) counter whose detecting efficiency for γ-rays had been previously determined by a set of IAEA calibrated γ-sources, while 198Au was measured with a 4πβ-γ coincidence counter. The neutron spectrum was determined by p 1 calculation and was verified by time-of-flight method. The ratio I 1/v/I res was obtained experimentally making use of boron filters of three different thicknesses. The results were I total= 14.1±0.6 barn and I 1/v/I res =0.57±0.06, for a cutoff energy of 0. 63 eV.

Measurement of Resonance Activation Integral of 55Mn

The resonance activation integral of 55Mn was measured relatively to that of 197Au by absolute counting on 56Mn and 198Au: 56Mn was measured with a Ge(Li) counter whose detecting efficiency for γ-rays had been previously determined by a set of IAEA calibrated γ-sources, while 198Au was measured with a 4πβ-γ coincidence counter. The neutron spectrum was deter-mined by P1 calculation and was verified by time-of-fiight method.The ratio Il/υ/Ires was obtained experimentallymaking use of boron filters of three different thicknesses.The results were Itotal=14.1±0.6 barn and Il/υ/Ires=0.57±0.06, for a cutoff energy of 0.63 eV.

The determination of backgrounds for neutron time-of-flight spectrometers

A technique for determining the time-dependent backgrounds associated with low-energy neutron time-of-flight spectrometers has been developed. Mathematical equations are presented showing how the backgrounds can be determined from transmission measurements through boron filters. A detector-scattered background, which consists of neutrons scattered by the detector, somewhat moderated by the surroundings and subsequently detected, is also discussed. Data from neutron transmission measurements of water, carbon and cobalt are presented which demonstrate background are being measured adequately.

Effective Cutoff Energies for Boron, Cadmium, Gadolinium, and Samarium Filters

Effective Cutoff Energies for Boron, Cadmium, Gadolinium, and Samarium Filters

Effective Cadmium Cutoff Energies

Effective cutoff energies for point 1/υ absorbers inside of spherical and cylindrical cadmium filters have been calculated for thermal reactor neutrons. The neutron spectrum was assumed to consist of a Maxwellian plus a 1/E component, and the parameters varied were the thickness of filter, the Maxwellian temperature, and the Maxwellian to 1/E flux ratio. Because of the sensitivity of the effective cutoff to Maxwellian flux parameters for thin filters it is recommended that filter thicknesses of about 40 mils be used. Forty-mil filters show effective cutoffs at about 0.50 to 0.55 ev for temperatures up to about 500°A (or about 0.045 ev). Effective cutoff energies for boron filters were also calculated for purposes of comparison.

Angular Distribution of Neutrons Scattered by the 2.80-kev Resonance in Sodium

The angular distribution of neutrons scattered by the 2.80-kev resonance in sodium has been measured with a pile beam and various combinations of boron and sodium filters. This distribution is found to be isotropic to within 10%.