3He Proportional Counter Research Articles

Experimental study of neutron emission characteristics in a compact sub-kilojoule range miniature plasma focus device

In order to gain insight into the possible neutron production mechanism in sub-kilojoule miniature plasma foci, the anisotropy in neutron and hard x-ray emissions from a newly developed fast miniature plasma focus device (2.4 µF, 27 nH, T/4 ∼ 400 ns, 12–15 kV, 170–270 J, total mass ∼25 kg) is investigated. The system produces an average neutron yield of (1.2±0.2) × 106 neutrons/shot into 4π sr at ∼80 kA peak discharge current and 5.5 mbar deuterium operation, as measured by a 3He proportional counter. Relative measurements of radiation yields, to gauge the emission anisotropy, have been performed using a pair of fast scintillator–photomultiplier detectors placed along the axial (0°) and radial (90°) directions. The average forward to radial neutron yield anisotropy is found to be 1.3±0.2. The average peak neutron energy for the axial and radial directions is estimated to be (2.9±0.3) MeV and (2.6±0.1) MeV, respectively. The observed higher fluence along the anode axis and the coincidence between occurrence of maximum neutron yield and anisotropy at 5.5 mbar deuterium filling gas pressure, suggest that the neutron production mechanism in the sub-kilojoule range miniature plasma focus device FMPF-1 may be predominantly beam–target in nature.

Iterative unfolding for Bonner sphere spectrometers using the MSANDB code – Sensitivity analysis and dose calculation

Since many decades, Bonner sphere spectrometers (BSSs) are routinely used for assessment of neutron spectra over a wide energy range from some meV to GeV. Typically, a spectrometer consists of a neutron detector sensitive to thermal neutrons located inside moderating polyethylene (PE) spheres of various sizes. Based on the response functions of the detector/sphere systems that must be calculated by Monte Carlo codes, an unfolding procedure is applied to deduce the present neutron spectrum from the count rates of the detectors. To start the unfolding procedure, a guess spectrum is required that includes some prior information on the physics of the investigated neutron spectrum, and that is iteratively modified to match the measured count rates. For the present investigation, a BSS-system consisting of an 3He proportional counter and seventeen spheres containing PE or a combination of PE and lead is used. The system is used for example to measure secondary neutrons from cosmic radiation at mountain altitudes, or from high-energy accelerators outside the radiation shielding. A systematic study was performed to quantify the influence of the chosen guess spectrum and the number of iteration steps on the unfolded neutron spectrum, and on integral quantities deduced such as total neutron fluence or ambient dose equivalent. It turned out that none of these changes resulted in dose quantities that were more than a few percent different to those deduced when the optimised start spectrum was used. Similarly, use of the two different response matrices available for our BSS system provided similar values for the ambient dose equivalent.

On accelerator-based neutron sources and neutron field characterization with low energy neutron spectrometer based on position sensitive 3He counter

The development of new neutron sources for BNCT applications, based on particle accelerators is currently underway all over the world. Though nuclear reactors were used for a long time as the only neutron source available having the requested flux levels, the accelerator-based ones have recently been investigated on the other hand due to its easy-to-use and acceptable performances. However, when using an accelerator, various secondary particles would be emitted which forms a troublesome background. Moreover, the neutrons produced have usually an energy spectrum somewhat different from the requested one and thus should be largely moderated. An additional issue to be taken into account is the patient positioning, which should be close to the neutron source, in order to take advantage of a neutron flux level high enough to limit the BNCT treatment time within 1 h. This implies that, inside a relatively narrow space, neutrons should be moderated, while unnecessary secondary particles should be shielded. Considering that a background-free neutron field from an accelerator-driven neutron source dedicated to BNCT application is generally difficult to be provided, the characterization of such a neutron field will have to be clearly assessed. In the present study, a low energy neutron spectrometer has been thus designed and is now being developed to measure the accelerator-based neutron source performance. The presently proposed spectrometer is based on a 3He proportional counter, which is 50 cm long and 5 cm in diameter, with a gas pressure of 0.5 MPa. It is quite unique that the spectrometer is set up in parallel with the incident neutron beam and a reaction depth distribution is measured by it as a position sensitive detector. Recently, a prototype detector has been developed and the signal test is now underway. In this paper, the feature of the accelerator-based neutron sources is outlined and importance of neutron field characterization is discussed. And the developed new low energy neutron spectrometer for the characterization is detailed.

Improved Fabrication Technique for Microstructured Solid-State Neutron Detectors

AbstractMicrostructured semiconductor neutron detectors have superior efficiency performance over thin-film coated planar semiconductor detectors. The microstructured detectors have patterns deeply etched into the semiconductor substrates subsequently backfilled with neutron reactive materials. The detectors operate as pn junction diodes. Two variations of the diodes have been fabricated, which either have a rectifying pn junction selectively formed around the etched microstructures or have pn junctions conformally diffused inside the microstructures. The devices with the pn junctions formed in the perforations have lower leakage currents and better signal formation than the devices with selective pn junctions around the etched patterns. Further, pulse height spectra from conformally diffused detectors have the main features predicted by theoretical models, whereas pulse height spectra from the selectively diffused detectors generally do not show these features. The improved performance of the conformal devices is attributed to stronger and more uniform electric fields in the detector active region. Also, system noise, which is directly related to leakage current, has been dramatically reduced as a result of the conformal diffusion fabrication technique. A sinusoidal patterned device with 100 μm deep perforations backfilled with 6LiF was determined to have 11.9 ± 0.078% intrinsic detection efficiency for 0.0253 eV neutrons, as calibrated with thin-film planar semiconductor devices and a 3He proportional counter.

Monte Carlo for the NCD phase of the Sudbury Neutrino Observatory

In the third phase of the Sudbury Neutrino Observatory (SNO) experiment an array of 3He proportional counters was added deployed in the heavy-water volume of the SNO detector. This Neutral-Current Detection (NCD) Array detected the neutrons from the neutral-current interaction of 8B solar neutrinos with deuterium. Before we can determine the neutrino flux we must separate the neutron-capture pulses from pulses due to alpha particles and instrumental backgrounds. We have created a unique, detailed simulation of the current pulses from the proportional counters that includes energy straggling, ion drift, electron diffusion, space charge, and electronics effects. We have conducted extensive studies to determine the accuracy of the simulation. In the solar neutrino analysis the NCD Monte Carlo is used to determine the energy spectrum of the alpha background, as well as the applicable systematic effects. In the near future it will be used to fit the data pulses to separate neutron-capture and alpha pulses. With this pulse-shape analysis method the differences between pulse characteristics can be associated directly with the physical mechanisms of track formation and charge motion in the counter gas.

Demonstration of neutron production in a table-top pinch plasma focus device operating at only tens of joules

Neutron emission from a deuterium plasma pinch generated in a very small plasma focus (6 mm anode diameter) operating at only tens of joules is presented. A maximum current of 50 kA is achieved 140 ns after the beginning of the discharge, when the device is charged at 50 J (160 nF capacitor bank, 38 nH, 20–30 kV, 32–72 J). Although the stored energy is very low, the estimated energy density in the plasma and the energy per particle in the plasma are of the same order as in higher energy devices. The dependence of the neutron yield on the filling pressure of deuterium was obtained for discharges with 50 and 67 J stored in the capacitor bank. Neutrons were measured by means of a system based on a 3He proportional counter in current mode. The average neutron yield for 50 J discharges at 6 mbar was (1.2 ± 0.5) × 104 neutrons per shot, and (3.6 ± 1.6) × 104 for 67 J discharges at 9 mbar. The maximum energy of the neutrons was (2.7 ± 1.8) MeV. Possible applications related to substance detection and others are discussed.

Independent Measurement of the Total ActiveB8Solar Neutrino Flux Using an Array ofHe3Proportional Counters at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) used an array of 3He proportional counters to measure the rate of neutral-current interactions in heavy water and precisely determined the total active (nu_x) 8B solar neutrino flux. This technique is independent of previous methods employed by SNO. The total flux is found to be 5.54_-0.31;+0.33(stat)-0.34+0.36(syst)x10(6) cm(-2) s(-1), in agreement with previous measurements and standard solar models. A global analysis of solar and reactor neutrino results yields Deltam2=7.59_-0.21;+0.19x10(-5) eV2 and theta=34.4_-1.2;+1.3 degrees. The uncertainty on the mixing angle has been reduced from SNO's previous results.

Compact sub-kilojoule range fast miniature plasma focus as portable neutron source

As a first step toward the development of a compact and portable, quasi-continuous pulsed neutron source, we have demonstrated a ‘fast miniature plasma focus (PF) device’ as a compact nuclear fusion apparatus. The system operates with deuterium gas and produces an average neutron yield (Y90°) of (1 ± 0.27) × 104 neutrons/shot at ∼70 kA peak discharge current. In the range of 1.5–4 mbar, a distinct and sharp dip in the current derivative signal indicates a strong pinching action with subsequent emission of hard x-rays followed by a neutron pulse. The yield and the time history of the neutrons were measured by both active and passive detection techniques (such as a 3He proportional counter, NE102A plastic scintillator and CR-39 SSNTDs). The overall dimensions of the apparatus, which includes a capacitor bank, sparkgap switch and the focus chamber, are 0.2 m × 0.2 m × 0.5 m and the total mass of the system is ∼25 kg. The scope of this paper is to evaluate/demonstrate the potential of such a fast miniature PF device as a compact and portable fusion apparatus producing neutrons while operating at relatively low energy.

System for measurement of low yield neutron pulses from D–D fusion reactions based upon a 3He proportional counter

A conventional neutron detection technique was adapted to measure low neutron yields from D–D fusion pulses. This method uses a 3He proportional counter surrounded by a paraffin moderator. Electric signals generated in the 3He tube are fed into a preamplifier. The output of the preamplifier is directly connected to a digital oscilloscope. The time-integrated signals represent the charge generated in the 3He tube which is proportional to the total neutron yield. The integration time is determined by the preamplifier and moderator characteristics within some hundreds of microseconds. No meaningful neutron background was detected during this time window. The system, previously calibrated, was used to measure the neutron yield (<106 neutron/pulse) generated in a fast and very small plasma focus device designed to operate with energies of tens of Joules. Neutron yields as low as 103 neutrons per pulse were measured.

Design and characterisation of a pulsed neutron interrogation facility

The Joint Research Centre recently obtained a license to operate a new experimental device intended for research in the field of nuclear safeguards. The research projects currently being planned for the new device includes mass determination of fissile materials in matrices and detection of contraband non-nuclear materials. The device incorporates a commercial pulsed neutron generator and a large graphite mantle surrounding the sample cavity. In this configuration, a relatively high thermal neutron flux with a long lifetime is achieved inside the sample cavity. By pulsing the neutron generator, a sample may be interrogated by a pure thermal neutron flux during repeated time periods. The paper reports on the design of the new device and the pulsed fast and thermal neutron source. The thermal neutron flux caused by the neutron generator and the graphite structure has been characterised by foil activation, fission chamber and (3)He proportional counter measurements.

Measurements of fast neutrons above ground and in 47 m depth with a 3He proportional counter

Neutrons contribute to the background of low radioactivity measurements, especially with detectors in large lead shields above ground, and to a lower extent in underground laboratories of medium depth. We measured the natural neutron flux in the energy region of fission by commercial 3He filled proportional counters at different places above ground and in an underground laboratory, which is covered by 47 m of rock, and demonstrated the drastic reduction of the neutron flux in the underground and the influence of the materials of the environment.

Status of the SNO experiment

The Sudbury Neutrino Observatory (SNO) is a 1-kt heavy water Cherenkov detector sensitive to the flavor content of the 8B neutrinos originating in the Sun. The analysis of the second phase, in which salt (NaCl) was added to the heavy water in order to increase the cross section for neutrons and therefore enhance the sensitivity to solar neutrinos, is completed. Results from 391 d of data (June 2001 until September 2003) are summarized and constraints on the neutrino mixing parameters are given. The third phase of operation has started in which 3He proportional counters have been deployed inside the D2O. These neutral-current detectors will perform a systematically independent measurement of the Solar-neutrino flux on a event-by-event basis. SNO finishes data taking at the end of 2006 and the heavy water will be removed. A new experiment using liquid scintillator to measure the pep solar neutrinos and geoneutrinos is proposed and will be described briefly.

A Monte Carlo design of a neutron dose-equivalent survey meter based on a set of 3He proportional counters

In order to improve the energy dependence of the Rem-meter's response characteristics, a device was studied which uses a spherical moderator and two types of proportional counters, a spherical counter located at the centre of the moderator and six cylindrical counters located close to the moderator surface. The energy and angular dependence of the neutron-energy response of the device in the neutron-energy range of 2.5 × 10 - 8 – 20 MeV was calculated using Monte Carlo simulations. An evaluation method of ambient dose equivalent was investigated.

Neutron beam test of multi-grid-type microstrip gas chamber

Multi-grid-type microstrip gas chambers (M-MSGCs) are being developed for the next-generation pulsed neutron source. Two new concepts, a global–local-grouping (GLG) method and a graded cathode pattern readout method, were applied to the M-MSGC design for realizing higher counting rate than traditional 3He proportional counters. One-dimensional detectors with 700 mm-long test plates were fabricated and tested with X-ray and neutron beams, which demonstrated position detection capability based on these concepts.

Studies on the response of 3He and 4He proportional counters to monoenergetic fast neutrons

Two helium filled proportional counters ( 3He and 4He) were studied to establish the optimum operating conditions when these counters are used for fast neutron measurements, as well as to examine the linearity of the pulse height with neutron energy. The detectors were irradiated with mono-energetic neutrons in the energy range of 230 keV–22 MeV, produced via 7Li(n,p) 7Be, 2H(d,n) 3He and 3H(d,n) 4He reactions in a Tandem Van de Graff accelerator. The gamma ray contribution to the obtained pulse height distribution and the resolution of the counters as a function of shaping time constant and applied high voltage were studied.

A detailed study on the neutron contamination for a 10 MeV medical electron accelerator

Neutron contamination from a 10 MeV medical electron accelerator was investigated by applying both calculations and measurements. The FLUKA Monte Carlo code was used for calculating neutron production and transport with an emphasis on the detailed modeling of the accelerator head and the treatment room. It was found that neutrons were produced dominantly from both the primary and the secondary collimators by 10 MV X-rays. Neutrons produced from upper gantry components will be attenuated to some extent by the lower collimators. The FLUKA-calculated results for detector positions located along the maze of the treatment room were compared with measurements of He-3 proportional counters and conventional bubble detectors, and also compared with the estimations predicted by the Kersey method.

Neutron response function for a detector with 3He counters for the 0.39–1.54 MeV neutron energy range

An experimental study was carried out on the neutron response functions of two neutron detector arrays consisting of 39 3He proportional counters with a polyethylene moderator for monoenergetic neutrons within the 0.39–1.54 MeV neutron energy range. Experimental data on the sensitivity of neutron counting to a change in neutron energy and the influence of the thickness of polyethylene moderator were obtained. The experimental efficiency curves were compared with the calculated response functions generated by a neutron transport code.

中性子個人線量計の校正における室内散乱線及びジオメトリ効果による影響

Correction for the influence of room-scattered neutrons in irradiation rooms is essential in the case of calibrating neutron dosemeters. The ISO8529-2 recommends some correction method such as the shadow-cone method and the generalized-fit method for the calibration of neutron ambient dose equivalent (rate) meters. However, the ISO standard does not describe the correction methods for personal neutron dosemeters in detail. The authors investigated the variation of responses of neutron detectors mounted on phantom as a function of source-to-phantom distance, and discussed the applicability of the shadow-cone method and generalized-fit method for calibrating personal neutron dosemeters. The measurements were carried out using 3He and hydrogen-filled proportional counters as surrogates of albedo and recoil-proton type dosemeters, respectively, at different distances ranging from 30cm to 400cm. As a result, it was clarified that both correction methods are applicable for recoil-proton type detectors over any distance. Contrarily, for albedo-type detectors, the variation of response does not follow the curve predicted from the generalized fit method at distances smaller than 70cm. This result strongly suggests that the correction for source-to-phantom geometry effect should be made for calibrating albedo-type detectors at smaller distances.

Polarized neutron reflectometry of a patterned magnetic film with a He3 analyzer and a position-sensitive detector

We have employed a polarized He3 spin filter in conjunction with a position-sensitive detector (PSD) to perform efficient polarization analysis of neutron diffuse reflectivity. This work was carried out on the NG-1 polarized neutron reflectometer at the National Institute of Standards and Technology Center for Neutron Research. We measured the specular and diffuse reflectivity of a patterned magnetic array that has periodic square holes in a Co film. Analysis of the data yielded spin-analyzed two-dimensional Qx−Qz reciprocal space maps for the sample in magnetized and demagnetized states. We compared the measurements obtained with a 3He analyzer and a PSD with those obtained using a conventional supermirror analyzer and a 3He proportional counter. The results are in good agreement. For this experiment, 3He gas was polarized by the spin-exchange optical pumping method and stored in a uniform magnetic field provided by a shielded solenoid. Improved optical pumping using a spectrally narrowed diode laser array yielded an initial 3He polarization of 70% in a 3He cell volume of 280 cm3.

Current status and future directions of position sensitive neutron detectors at ISIS

Position sensitive detectors at the ISIS facility largely comprise of a mixture of 3He proportional counters and fibre-coded scintillation detectors based on ZnS/ 6LiF. The latest developments of these devices at ISIS will be discussed with reference to three ISIS instruments, MERLIN, GEM and ENGIN-X. MERLIN is the new chopper spectrometer now being developed at ISIS and features a large 2D detector array covering more than 20 m 2 with a requirement in position resolution of 25×25 mm 2. Resistive wire detectors are proposed for this detector, but to minimise gaps in the solid angle, detectors with active lengths of 2.9 m are being proposed. Initial results from prototype detectors will be presented. GEM is a general materials diffractometer now nearing completion and ENGIN-X is the engineering instrument, which became operational in November 2002. Detectors for both these instruments use linear arrays of fibre optic-encoded ZnS/ 6Li scintillator, with position resolutions of 5 and 3 mm, respectively. Work has now started on the construction of the ISIS second target station, TS-2. This facility will be tailored to the production of lower-energy neutrons compared with target station 1, with applications predominantly in soft condensed matter, bio-molecular sciences and advanced materials. The challenges this presents for future neutron detector development at ISIS will be reviewed together with some potential lines of detector development.