A NaI(T1) scintillator used to measure neutron fluence at about 14 MeV at low intensities

Translated from Izmeritel'naya Tekhnika, No. 2, p. 50, February, 1990.     

Energy measurement of 50 MeV proton beam with a NaI(Tl) scintillator

Energy measurement of 50 MeV proton beam produced on the AVF MC-50 Cyclotron was conducted using a detector telescope with a NaI(Tl) scintillator as an E counter. Protons of various energies, elastically and inelastically scattered from the ¹²C target nucleus were measured at four different angles of 35°, 40°, 50° and 55°. We applied the chi-square method to determine the beam energy, which showed a well defined minimum chi-square corresponding to a beam energy of 49.6 ± 2.3 MeV at the 68% confidence level. Also the light output response of NaI(Tl) to proton energies between 31 and 44 MeV is linear within 0.5 MeV and is in good accord with the recent result of Romero et al. [Nucl. Instr. and Meth. A 301 (1991) 241].     

Structural characteristics and optical properties of vestan (polyester) fibres

Refractive indices and birefringence for skin and core changes with strain, produced by different stresses in undrawn vestan fibres, were measured interferometrically. Applications were carried out using multiple-beam Fizeau fringes in transmission to determine the Cauchy's constants and dispersive coefficient for the fibre layers. The resulting data were used to calculate the polarizability per unit volume for each layer. The scanning electron microscope (SEM), optical microscope and He-Ne laser beam were applied to estimate the geometrical parameters of the fibre cross-section. A comparative study between the three methods used for measuring the fibre diameter was made. The effect of temperature on the refractive index and birefringence for each fibre layer has been also investigated. The relationship between temperature and birefringence of the fibres was studied and the thermal coefficient of the refractive index was determined. An empirical formula is suggested to relate temperature and birefringence. Illustrations are given using microinterferograms.     

Charged particles identification with a CsI(Tl) scintillator

A CsI(Tl) scintillator with two light decay components is used to detect and identify p, d, t, ³He and α particles with a low energy threshold. Besides, the addition of a thin plastic scintillator in front of the CsI(Tl) crystal allows charge identification for ions with Z up to 19.     

Photodiode readout and pulse shape analysis of CsI(Tl) scintillator signals

The light output performance of CsI(Tl) scintillators coupled directly or via wavelength shifters to a photodiode is investigated. Using a photodiode readout system, the zero-crossing technique of pulse shape analysis has been employed to identify gammas, neutrons and charged particles (p, d, t, α) with a low energy threshold.     

Ce3+激活的锂-6玻璃闪烁体发光及核物理性能的研究

选取Ce~(3+)为激活剂离子,锂-6和硼-10为靶核核素分别研制了可用于热中子探测的玻璃闪烁体,并系统研究了其发光性能及部分核物理性能.研究结果表明,Ce~(3+)激活的锂-6玻璃闪烁体具有较长的发射波长(390nm),较短的衰减时间(46.9ns)以及良好的γ射线甄别性能和温度效应,是探测热中子优良的玻璃基质材料.靶核核素与激活剂离子在锂-6玻璃闪烁体中均有一较佳的浓度范围,浓度过大将产生不同程度的淬灭效应.     

Modularized NaI(Tl) detectors in a half barrel configuration

We have constructed a modularized array of NaI(Tl) detectors in a half barrel configuration that is about 75 cm long with an inner radius of 25 cm. 96 NaI modules are surrounded by a layer of 48 modules of scintillating glass which have similar dimensions to the NaI modules. Each NaI crystal was shaped as a trapezoidal pyramid and encapsulated in a 1 mm thick aluminium container with dimensions of (43 × 94) × (110 × 94) × 377 mm³. The construction and performance are described along with the gain calibration and monitoring, the beam position sensitivity, the scattering between modules, etc. The fwhm energy resolution of a 5 × 5 array of NaI modules varied from 6.5% for incident electrons at 100 MeV to 3.6% at 1000 MeV     

Optimal coupling of light from a cylindrical scintillator into an optical fiber

Radiation dose measurements based on scintillator detection are conveniently made by coupling the light from the scintillator into an optical fiber. The low light levels involved typically require sensitive photodetectors, so it is advantageous to increase the available signal by optimizing the optical coupling efficiency between the scintillator and optical fiber. We model this process using geometric optics and finite-element ray tracing to determine the features that maximize the amount of light coupled to an optical fiber from a cylindrical scintillator. We also address whether the coupling can be improved by using an intermediate optical element such as a lens, and we provide a means for calculating its required optical properties for a given geometry.     

A modular NaI(Tl) detector for intermediate energy photons

We describe the properties of a detector array made up of 64 NaI(Tl) 406 × 63 × 63 mm³ modules, used as an intermediate energy photon spectrometer. We obtain an energy resolution of 6% FWHM at 129 MeV, a time resolution of 1 ns FWHM and a resolution of 48 mm FWHM for the location of the impact point on the front face of the detector. The modularity allows to some extent a discrimination between photons and neutrons. We also present the response of the detector to 69 MeV neutrons.     

Comparison of PVT and NaI(Tl) scintillators for vehicle portal monitor applications

The demand for radiation portal monitor (RPM) systems has increased, and their capabilities are being further scrutinized as they are being applied to the task of detecting nuclear weapons, special nuclear material, and radiation dispersal device materials that could appear at borders. The requirements and constraints on RPM systems deployed at high-volume border crossings are significantly different from those at weapons facilities or steel recycling plants, where RPMs have been historically employed. In this new homeland security application, RPM systems must rapidly detect localized sources of radiation with a very high detection probability and low false-alarm rate, while screening all of the traffic without impeding the flow of commerce. In light of this new Department of Homeland Security application, the capabilities of two popular gamma-ray-detector materials as applied to these needs are re-examined. Both experimental data and computer simulations, together with practical deployment experience, are used to assess currently available polyvinyltoluene and NaI(Tl) gamma-ray detectors for border applications.     

Investigations on the photoelectron conversion efficiency of NaI(Tl) counters

The photoelectron conversion efficiency of NaI(Tl) detectors has been measured by counting single photoelectron pulses of the detector output pulse train on a very fast storage oscilloscope. This result is compared with those obtained by the measurement of the achievable time resolution using the first photoelectron triggering technique.     

Experiences with area specific spectrum stripping of NaI(Tl) gamma spectra

Processing of airborne and carborne gamma-ray spectra (AGS and CGS) often includes the stripping (elimination) of the signals from natural radioactivity. Hereby the net result becomes the signals from man-made radioactivity or other radiation anomalies. The parameters needed for spectrum stripping are dependent on detector size and quality as well as on the energy windows. In addition they depend on the environmental geometry including the vehicle carrying the detector. For AGS the altitude also influences the parameters. In general the stripping parameters are determined from tedious laboratory or field measurements with known sources of natural radioactivity. Stripping parameters may, however, often be calculated from the actual survey data or from data from a similar area. Both post-processing and real-time processing are possible. The technique is useful for gamma source search, for detection of radiation anomalies and for mapping of contamination levels. The use of the technique is illustrated with field exercise data.     

Effects of packaging SrI2(Eu) scintillator crystals

Recent renewed emphasis placed on gamma-ray detectors for national security purposes has motivated researchers to identify and develop new scintillator materials capable of high energy resolution and growable to large sizes. We have discovered that SrI₂(Eu) has many desirable properties for gamma-ray detection and spectroscopy, including high light yield of ∼90,000 photons/MeV and excellent light yield proportionality. We have measured <2.7% FWHM at 662 keV with small detectors (<1 cm³) in direct contact with a photomultiplier tube, and ∼3% resolution at 662 keV is obtained for 1 in.³ crystals. Due to the hygroscopic nature of SrI₂(Eu), similar to NaI(Tl), proper packaging is required for field use. This work describes a systematic study performed to determine the key factors in the packaging process to optimize performance. These factors include proper polishing of the surface, the geometry of the crystal, reflector materials and windows. A technique based on use of a collimated ¹³⁷Cs source was developed to examine light collection uniformity. Employing this technique, we found that when the crystal is packaged properly, the variation in the pulse height at 662 keV from events near the bottom of the crystal compared to those near the top of the crystal could be reduced to <1%. This paper describes the design and engineering of our detector package in order to improve energy resolution of 1 in.³-scale SrI₂(Eu) crystals.     

A Monte Carlo simulation applied to hexagonal NaI(Tl) detectors

A computer code using Monte Carlo techniques is applied to hexagonal NaI(Tl) detectors, collimated and shielded with lead in a single detector operation or configured in a total energy spectrometer. Comparisons are made with experimental data.     

Connection cube modules for optical backplanes and fiber networks

A modular free-space optical system, called the connection cube, for connecting arrays of electro-optic transceivers and fiber-array connectors is presented. The connection cube module provides bidirectional data transfer between four processing nodes on a cube face and can be used as a basic building block for optical backplanes and interconnect networks. An experimental system for connecting four processing nodes is presented and used to examine alignment and packaging issues. An analysis of the dimensional requirements and scaling capability for systems based on this module is conducted. This analysis shows that, when the connection cube module is adapted to vertical-cavity surface-emitting-laser-based point-to-point fiber-array links currently under development, it can connect up to 14 processing nodes with an aggregate data transfer capacity of 112 Gbits/s with 19.6-W power consumption.     

Pileup Correction Algorithms for Very-High-Count-Rate Gamma-Ray Spectrometry With NaI(Tl) Detectors

In this paper, we propose algorithms that are suitable for gamma-ray spectrometric systems with NaI(Tl) detector that support pileup correction at extremely high count rates of $4 cdot 10^{6} hbox{pulses/s}$. The following two algorithms are presented: 1) an algorithm based on modified phase-only correlation (MPOC) for the detection of the beginning of pulses and maximum likelihood estimation (MLE) for the estimation of the pulses' amplitudes and 2) an algorithm based on the modified pulse clipping (MPC) method. Simulation results have shown that the systems based on both MPC and MPOC–MLE are capable of reducing pileup effects and improving the resolution of the energy spectrum at very high count rates. MPOC–MLE can support extremely high count rates while keeping good energy resolution. The resolution of $^{60}hbox{Co}$ at $4 cdot 10^{6} hbox{pulses/s}$ is 7.6%, with only 14% of lost pulses from the channels of interest.      

Improvement on the light yield of a high-Z inorganic scintillator GSO(Ce)

Cerium-doped gadolinium silicic dioxide crystal, GSO(Ce), is a high-Z non-hydroscopic scintillator that gives higher light yield than BGO, and can potentially replace NaI(Tl), CsI(Tl) and BGO in many applications. Its production cost, however, has been substantially higher than any of them, while its energy resolution has been worse than that of NaI(Tl) or CsI(Tl). The merit did not overcome these deficiencies except in limited applications.

We developed a low background phoswich counter (the well-type phoswich counter) for the Hard X-ray Detector of the Astro-E project based on GSO scintillator. In the developmental work, we have succeeded in improving the light yield of GSO(Ce) by 40–50%. For energies above 500 keV, a large GSO(Ce) crystal (4.5 cm×4.5φ cm) now gives energy resolution comparable to or better than the best NaI(Tl) when read out with a phototube. With a small GSO(Ce) crystal (5×5×5 mm³) and a photodiode, an energy resolution comparable to or better than the best CsI(Tl) has been obtained. With this improved performance, we find that the much higher photopeak efficiency and the shorter scintillation decay time of GSO(Ce) offsets its higher cost for many applications.

We summarize our past developmental work to decrease radioactive contamination and to increase light yield of GSO(Ce) for astronomical hard X-ray detection. Included also are measurements done after the unsuccessful launch of the Astro-E mission. The work is still continuing for the remake version of Astro-E Hard X-ray Detector.