Gamma-ray detection via D(γ, n)H neutron counting

The photodisintegration of deuterium (Q = −2.22 MeV) in combination with a neutron counter has been used to detect high-energy γ-rays. The γ-ray source is surrounded by a tank filled with heavy water and the emitted neutrons are counted in 4π geometry with neutron detectors embedded in a graphite moderator. The absolute detection efficiency of a small test system (1.8 1 D₂O) has been determined for E_γ = 2.6–8.1 MeV and found to agree well with calculations (e.g., 8.0 × 10⁻⁵ at E_γ = 4.4 MeV). It is proposed that the system should be useful for the study of capture reactions induced on highly radioactive targets. Improvements in detection efficiency as well as limitations in data analyses are discussed.     

Sensitivity of coded mask telescopes

Simple formulas are often used to estimate the sensitivity of coded mask x-ray or gamma-ray telescopes, but these are strictly applicable only if a number of basic assumptions are met. Complications arise, for example, if a grid structure is used to support the mask elements, if the detector spatial resolution is not good enough to completely resolve all the detail in the shadow of the mask, or if any of a number of other simplifying conditions are not fulfilled. We derive more general expressions for the Poisson-noise-limited sensitivity of astronomical telescopes using the coded mask technique, noting explicitly in what circumstances they are applicable. The emphasis is on using nomenclature and techniques that result in simple and revealing results. Where no convenient expression is available a procedure is given that allows the calculation of the sensitivity. We consider certain aspects of the optimization of the design of a coded mask telescope and show that when the detector spatial resolution and the mask to detector separation are fixed, the best source location accuracy is obtained when the mask elements are equal in size to the detector pixels.     

Background events in the energy region below 4 MeV detected with a 208 cm3 intrinsic germanium detector: I. Characteristic γ-radiation

A detailed study of the events recorded with a 208 cm³ intrinsic germanium γ-ray detector produced by interactions of natural environmental radiation was undertaken. In the first part of this work, the data on events from characteristic radioactivity are analysed to optimise the shielding requirements for reducing the γ-ray background as seen by the detector. A thickness of 15 cm of Pb all around the detector in the laboratory location, or only 5 cm in the underground location at the mine, is found to be sufficient to attenuate the event rates from environmental characteristic γ-rays to less than the event rates from intrinsic contamination of the detector components in the detector assembly. The salt mine is found to be a natural environment low in γ-ray background.     

The 2002 IAEA test spectra for low-level γ-ray spectrometry software

Test spectra for low-level γ-ray spectrometry were acquired and made available to the general public at www.iri.tudelft.nl/~rc/fmr/iaea2002. As opposed to the 1995 test spectra, where reference values were made available only for the peak energies and areas, the new set of test spectra was acquired with certified sources, so that the reference values are radionuclide activities. Two well-defined detection geometries were employed: a 500 ml Marinelli beaker on a 33% relative efficiency HPGe detector; and a 100 ml pillbox on a 96% HPGe detector. The complete set addresses various issues that are especially important in low-level gamma-ray spectrometry, i.e. determination of efficiency curves in the presence of coincidence summing, differences in source geometry and density between standard and sample, poor statistics, shielding of background by the sample, use of low X- or γ-ray energies and the assumption of secular equilibrium of the natural radionuclides. The set was used in an IAEA intercomparison of software packages in December, 2002, reported on in a separate paper.     

Muon and neutron-induced background in gamma-ray spectrometry

A low-background γ-ray spectrometer for environmental surveillance, emergency response, and environmental health research has been constructed. It consists of a 132% efficient HPGe detector, graded lead shielding, and muon-rejection shielding, as well as external steel and building shielding. Detailed operation of the γ spectrometer and all of its components is described. Cosmic-ray muon and cosmic-neutron-induced background was accurately measured, and is discussed. We have achieved an overall background reduction by a factor of 9436 relative to the ambient level. Our integrated background rate in the γ energy range of 50-2700 keV was measured as 2.3 counts per min, corresponding to 15 counts ks⁻¹ kg⁻¹ Ge. Future directions in low-background γ spectroscopy at our facility are described.     

A current mode detector array for -ray asymmetry measurements

We have built a CsI(Tl) γ-ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV γ-rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion–nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode γ-ray detection with vacuum photodiodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of γ-rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of 10^-9 in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.     

Si(Li)NaI(Tl) sandwich detector array for measurements of trace radionuclides in soil samples

An ultra-sensitive X/γ-ray detector system for assaying trace radioactivity in actinide contaminated soil and ash samples has been developed. The new system consists of an array of 6 large Si(Li) X-ray detectors sensitive on both faces and mounted on edge in a paddle-shaped cryostat with a 14 cm diameter Be window on each side. The paddle, with a sample of the soil placed at each window, is sandwiched between 2 large NaI(Tl) scintillators which suppress the γ background. With X-rays being measured simultaneously from soil in 2 sample holders and background reduced by 50% using anticoincidence, the sensitivity of this detector is 4 times higher than that of conventionally mounted Si(Li) detectors. A soil sample containing 50 pCi/g ²³⁹Pu was measured in 5 min with an uncertainty of < 20% and a sample containing 7 pCi/g was measured in 1 h. With fwhm resolution of 400 eV at 17 keV, the ULβ₁ and NpL_β1 X-ray peaks are resolved thus permitting measurement of trace Pu in the presence of ²⁴¹Am. This is the most sensitive and selective detector known for nondestructive assay of radioactivity in soil and other samples.     

B对Pb-Mg-Al-B屏蔽功能材料的显微组织与屏蔽性能的影响

利用立式高频感应炉制备出铸造Pb-Mg-Al-B屏蔽材料,并对其显微组织和屏蔽性能进行了研究。结果表明,随着B的添加,材料组织中AlB2颗粒增多;室温抗拉强度可达105MPa,布氏硬度为160;厚度为20mm、B含量为2.0%(质量分数)的Pb-Mg-Al-B屏蔽材料对能量为250、118和65keV的X射线屏蔽率分别为90.29%、99.22%和97.9%,对γ射线的屏蔽率达到49.75%(137Cs源)和34.21%(60Co源),对中子的屏蔽率高达92.7%,说明屏蔽材料具有强度高、X(γ)射线与中子屏蔽性能优异,具备结构-功能(屏蔽)一体化的特点。     

Coded source neutron imaging with a MURA mask

In coded source neutron imaging the single aperture commonly used in neutron radiography is replaced with a coded mask. Using a coded source can improve the neutron flux at the sample plane when a very high L/D ratio is needed. The coded source imaging is a possible way to reduce the exposure time to get a neutron image with very high L/D ratio. A 17×17 modified uniformly redundant array coded source was tested in this work. There are 144 holes of 0.8 mm diameter on the coded source. The neutron flux from the coded source is as high as from a single 9.6 mm aperture, while its effective L/D is the same as in the case of a 0.8 mm aperture. The Richardson-Lucy maximum likelihood algorithm was used for image reconstruction. Compared to an in-line phase contrast neutron image taken with a 1 mm aperture, it takes much less time for the coded source to get an image of similar quality.     

An on-line tritium-in-water monitor

The paper describes the development and operation of a continuous on-line tritium-in-water monitor for the detection of heavy water leaks into the secondary coolant light water of a heavy water power reactor. The heart of the instrument is its plastic scintillator sponge detector, made from 5 μm thick plastic scintillator films. The sponge weighs only about 1 g and is in the form of disc of 48 mm diameter and 8 mm thickness. The total surface area of the films is about 3000 cm². In the coincidence mode of counting, the detector gives 1000 cps for the passage of 3.7 × 10⁴ Bq/cm³ (1 μCi/cm³) of tritiated water. The background in 6 cm thick lead shielding in the laboratory is 0.2 cps, and inside the reactor building it is below 1 cps. The monitor presently scans 18 sample lines in sequence for 5 min each and gives a printout for the activity in each line.     

A Novel Particle/Photon Detector Based on a Superconducting Proximity Array of Nanodots

The current frontiers in the investigation of high-energy particles demand for new detection methods. Higher sensitivity to low-energy deposition, high-energy resolution to identify events and improve the background rejection, and large detector masses have to be developed to detect even an individual particle that weakly interacts with ordinary matter. Here, we will describe the concept and the layout of a novel superconducting proximity array which show dynamic vortex Mott insulator to metal transitions, as an ultra-sensitive compact radiation-particle detector.     

In situ prompt gamma-ray activation analysis of water pollutants using a shallow 252Cf-HPGe probe

A shallow ²⁵²Cf-HPGe probe used for in situ prompt γ-ray activation of water pollutants is described. A 2.7 μg ²⁵²Cf neutron source and a 10% HPGe detector are inserted into a waterproof stainless steel probe, which is designed to be submerged and recovered in field operation. A laboratory test is performed to obtain the neutron flux distribution and prompt γ-ray contribution to the HPGe detector counts from around the submerged probe. The concentrations of toxic cadmium and chlorine in water are determined in the prompt γ-ray spectrum. The detection limit of industrial pollutants and some improvements of the current design are discussed.     

Pulse Shape Analysis with Scintillating Bolometers

Among the detectors used for rare event searches, such as neutrinoless Double Beta Decay (0νDBD) and Dark Matter experiments, bolometers are very promising because of their favorable properties (excellent energy resolution, high detector efficiency, a wide choice of different materials used as absorber, …). However, up to now, the actual interesting possibility to identify the interacting particle, and thus to greatly reduce the background, can be fulfilled only with a double read-out (i.e. the simultaneous and independent read out of heat and scintillation light or heat and ionization). This double read-out could greatly complicate the assembly of a huge, multi-detector array, such as CUORE and EURECA. The possibility to recognize the interacting particle through the shape of the thermal pulse is then clearly a very interesting opportunity. While detailed analyses of the signal time development in purely thermal detectors have not produced so far interesting results, similar analyses on macro-bolometers (～10–500 g) built with scintillating crystals showed that it is possible to distinguish between an electron or γ-ray and an α particle interaction (i.e. the main source of background for 0νDBD experiments based on the bolometric technique). Results on pulse shape analysis of a CaMoO₄ crystal operated as bolometer are reported as an example. An explanation of this behavior, based on the energy partition in the heat and scintillation channels, is also presented.     

Neutron sources for in-situ planetary science applications

There are a number of future European Space Agency (ESA) and NASA planetary science missions that are in the planning or initial study phases, where the scientific objectives include determining the surface composition, measuring planetary surface heat flow and constraining planetary chronology. The University of Leicester is developing instrumentation for geophysical applications that include γ-ray spectroscopy, γ-ray densitometry and radiometric dating. This paper describes the modelling of a geophysical package, with the Monte Carlo code MCNPX, in order to determine the impact that a neutron source would have on in-situ composition measurements, radiometric dating and, in particular, trace element detection. The suitability of 2.54×2.54 cm LaBr₃(Ce) detectors in the geophysical package for in-situ missions was examined. ²⁵²Cf, Am–Be and Pu–Be neutron sources were compared in a trade-off study to determine mission suitability, potential for thermal and electric power production, mass and shielding requirements. This study is linked to a parallel examination of the suitability of radioisotope thermal generators for in-situ planetary science applications. The aim of the modelling was to optimise the source type and detector geometry in order to measure the elemental peaks of interest with a precision of 10% or better based on the Poisson statistics of the detected counts above background.     

A CdTe detector with a Gd converter for thermal neutron detection

A CdTe detector with a Gd converter has been developed and investigated as a neutron detector for neutron imaging. The fabricated Gd/CdTe detector with the 25 μm thick Gd was designed on the basis of simulation results of thermal neutron detection efficiency and spatial resolution. The energy resolution of the Gd/CdTe detector is less than 4 keV, which is enough to discriminate neutron capture gamma rays from background gamma emission. The Gd/CdTe detector shows the detection of neutron capture gamma ray emission in the ¹⁵⁵Gd(n, γ)¹⁵⁶Gd, ¹⁵⁷Gd(n, γ)¹⁵⁸Gd and ¹¹³Cd(n, γ)¹¹⁴Cd reactions and characteristic X-ray emissions due to conversion-electrons generated inside the Gd film. The observed efficient thermal neutron detection with the Gd/CdTe detector shows its promise in neutron radiography application.     

Commissioning of the NPDGamma Detector Array: Counting Statistics in Current Mode Operation and Parity Violation in the Capture of Cold Neutrons on B4C and 27Al

The NPDGamma γ-ray detector has been built to measure, with high accuracy, the size of the small parity-violating asymmetry in the angular distribution of gamma rays from the capture of polarized cold neutrons by protons. The high cold neutron flux at the Los Alamos Neutron Scattering Center (LANSCE) spallation neutron source and control of systematic errors require the use of current mode detection with vacuum photodiodes and low-noise solid-state preamplifiers. We show that the detector array operates at counting statistics and that the asymmetries due to B₄C and ²⁷Al are zero to with- in 2 × 10⁻⁶ and 7 × 10⁻⁷, respectively. Boron and aluminum are used throughout the experiment. The results presented here are preliminary.     

Development of an inconel self powered neutron detector for in-core reactor monitoring

The paper describes the development and testing of an Inconel600 (2 mm diameter×21 cm long) self-powered neutron detector for in-core neutron monitoring. The detector has 3.5 mm overall diameter and 22 cm length and is integrally coupled to a 12 m long mineral insulated cable. The performance of the detector was compared with cobalt and platinum detectors of similar dimensions. Gamma sensitivity measurements performed at the ⁶⁰Co irradiation facility in 14 MR/h gamma field showed values of −4.4×10⁻¹⁸ A/R/h/cm (−9.3×10⁻²⁴ A/γ/cm²-s/cm), −5.2×10⁻¹⁸ A/R/h/cm (−1.133×10⁻²³ A/γ/cm²-s/cm) and 34×10⁻¹⁸ A/R/h/cm (7.14×10⁻²³ A/γ/cm²-s/cm) for the Inconel, Co and Pt detectors, respectively. The detectors together with a miniature gamma ion chamber and fission chamber were tested in the in-core Apsara Swimming Pool type reactor. The ion chambers were used to estimate the neutron and gamma fields. With an effective neutron cross-section of 4b, the Inconel detector has a total sensitivity of 6×10⁻²³ A/nv/cm while the corresponding sensitivities for the platinum and cobalt detectors were 1.69×10⁻²² and 2.64×10⁻²² A/nv/cm. The linearity of the detector responses at power levels ranging from 100 to 200 kW was within ±5%. The response of the detectors to reactor scram showed that the prompt response of the Inconel detector was 0.95 while it was 0.7 and 0.95 for the platinum and cobalt self-powered detectors, respectively. The detector was also installed in the horizontal flux unit of 540 MW Pressurised Heavy Water Reactor (PHWR). The neutron flux at the detector location was calculated by Triveni code. The detector response was measured from 0.02% to 0.07% of full power and showed good correlation between power level and detector signals. Long-term tests and the dynamic response of the detector to shut down in PHWR are in progress.     

Tritium digital autoradiography with a Medipix2 hybrid silicon pixel detector

We report on the first measurements of ³H beta autoradiography obtained using a room temperature hybrid pixel detector, consisting of the Medipix2 single particle counting read-out chip bump-bonded to a 300 μm thick silicon pixel detector. This system has 256×256 square pixels of 55 μm pitch for a total sensitive area of 14×14 mm². Each pixel contains a double threshold discriminator and a 13-bit counter. Using a detection threshold equivalent to less than 6 keV and a background count rate of 5×10⁻³ counts mm⁻² s⁻¹, with exposures up to several hours, real-time images have been obtained of tritium-labeled solution drops spotted on a thin mylar foil placed in contact with the continuous top electrode of the silicon detector, in open air condition.     

Coded source imaging simulation with visible light

A coded source could increase the neutron flux with high L/D ratio. It may benefit a neutron imaging system with low yield neutron source. Visible light CSI experiments were carried out to test the physical design and reconstruction algorithm. We used a non-mosaic Modified Uniformly Redundant Array (MURA) mask to project the shadow of black/white samples on a screen. A cooled-CCD camera was used to record the image on the screen. Different mask sizes and amplification factors were tested. The correlation, Wiener filter deconvolution and Richardson-Lucy maximum likelihood iteration algorithm were employed to reconstruct the object imaging from the original projection. The results show that CSI can benefit the low flux neutron imaging with high background noise.     

A position-sensitive γ-ray detector for positron annihilation 2D-ACAR based on metal package photomultiplier tubes

A new position-sensitive γ-ray detector to be used in a two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) apparatus has been developed. It consists of 36 compact position-sensitive photomultiplier tubes (PS-PMT: HAMAMATSU R5900-00-C8), a light guide, and 2676 Bi₄Ge₃O₁₂ (BGO) scintillator pieces of size 2.6 mm×2.6 mm×18 mm. A high detection efficiency for 511 keV γ-ray is achieved with the length of BGO scintillators used. The detection area is about 160 mm×160 mm. The 288 anode outputs of the PS-PMTs are wired and connected to resistor chains from which 16 outputs (8 outputs each along the X and Y directions) are taken to identify the incident position of the γ-ray. The spatial resolution is about 3 mm (FWHM). The timing signal taken from the last dynodes of the PS-PMTs gives a timing resolution of 7.7 ns (FWHM) for 511 keV positron annihilation γ-rays.