Spherical NaI Research Articles

Efficiency calibration and coincidence summing correction for a NaI(Tl) spherical detector

Spherical NaI(Tl) detectors are used in gamma-ray spectrometry, where the gamma emissions come from the nuclei with energies in the range from a few keV up to 10 MeV. A spherical detector is aimed to give a good response to photons, which depends on their direction of travel concerning the detector center. Some distortions in the response of a gamma-ray detector with a different geometry can occur because of the non-uniform position of the source from the detector surface. The present work describes the calibration of a NaI(Tl) spherical detector using both an experimental technique and a numerical simulation method (NSM). The NSM is based on an efficiency transfer method (ETM, calculating the effective solid angle, the total efficiency, and the full-energy peak efficiency). Besides, there is a high probability for a source-to-detector distance less than 15 cm to have pulse coincidence summing (CS), which may occur when two successive photons of different energies from the same source are detected within a very short response time. Therefore, γ-γ ray CS factors are calculated numerically for a152Eu radioactive cylindrical source. The CS factors obtained are applied to correct the measured efficiency values for the radioactive volumetric source at different energies. The results show a good agreement between the NSM and the experimental values (after correction with the CS factors).

Upgrade of the SND electromagnetic calorimeter

Current status and last results of the Spherical Neutral Detector (SND) calorimeter upgrade are described. The main part of SND detector is a three-layer spherical NaI(Tl) electromagnetic calorimeter (EMC). The total thickness of NaI(Tl) in the calorimeter is 35 cm. The vacuum phototriodes (VPT) are used as photosensitive devices for the calorimeter counters. The quantum efficiency of VPT is about 15%, the average gain is 10, and the light collection efficiency is about 10%. New spectrometric channel of EMC was designed and implemented. It consists of a NaI(Tl) crystal, a vacuum phototriode, a charge-sensitive preamplifier a shaper and digitizing modules. The digitization is performed by 6 ADCs AD9228BCPZ-40 (4 channel, 12 bits, 40 MBPS) and the data go in serial form to the SoC Xilinx Zynq-7000. This SoC is basically a combination of FPGA programmable logic and ARM processor. The special algorithm for signal waveform processing was developed to determine time of flight and amplitude with high time (1 ns) and amplitude (250 keV) resolution. The algorithm is based on the invariability of the signal waveform. The waveform calibration procedures are developed and implemented. Successful data taking run was performed with upgraded EMC in 2019.

A hybrid analytical-numerical method for efficiency calculations of spherical scintillation NaI(Tl) detectors and arbitrarily located point sources

The present work is essentially concerned to introduce a hybrid analytical-numerical method to calculate the geometrical, the total, and the full-energy peak efficiency of the spherical NaI(Tl) detector for using isotropic radiating g-ray point sources. In addition, it calculates the average path lengths travelled by the photon inside the detector active medium, in order to study the characteristics of the source-to-detector configurations. This method was depended mainly on the calculation of the solid angle subtended by the radioactive point sources, which were situated at various locations from the detector surface and the photons path lengths through the detector active medium. Also, taking into account the attenuation coefficients of all the absorbers between the source and the detector material itself. The obtained results were compared against one data set published to show the method's possibility in the calibration process.

Monte Carlo calculation of response functions to gamma-ray point sources for a spherical NaI(Tl) detector

Response functions of a 7.62cm-diameter spherical NaI(Tl) detector to gamma-ray point sources in the energy range up to 1.5MeV were calculated by means of the Monte Carlo method using PENELOPE-2006 (Salvat et al., 2006). The detector materials and dimensions were modeled realistically. The calculated response functions agreed well with the experimental spectra.

Construction of Response Matrices for Various Cylindrical and Spherical NaI(Tl) Scintillation Detectors for Gamma Rays and the Test Results

Construction of Response Matrices for Various Cylindrical and Spherical NaI(Tl) Scintillation Detectors for Gamma Rays and the Test Results

各種形状寸法のＮａＩ（Ｔｌ）シンチレーション検出器のγ線応答関数の整備と試験

Gamma ray spectra analyses using unfolding techniques and dose evaluation are performed widely in the fields of environmental radiation monitoring and related environmental research. But generally the sizes and shapes of NaI(Tl) scintillation detectors of available response functions are limited: usually 3φ×3 cylindrical or 3φ spherical type, the energy range is roughly up to 3 MeV. In this paper, the response functions of eight types of cylindrical and spherical NaI(Tl) scintillation detectors (1φ×1, 2φ×2, 3φ×3, 4φ×4, 5φ×4 cylindrical, and 2φ, 3φ, 5φ spherical) were determined. The energy ranges were extended to 10 MeV maximum. Some response functions reported in the literature were unfolded using our response matrix for comparison, and the relative characteristics were discussed.

A Passive Method for Separating Elevated Photon Dose from Background in the Vicinity of a Nuclear Facility

A simple method for estimating elevated dose in the vicinity of a nuclear facility due to downward radiation arising from the plume discharge and direct or scattered radiations is presented. In eliminating the background this method relies on lead shields and a 'control location'. By using two different types of passive dosemeter, a photoluminescence dosemeter (PLD SC-1) and a thermoluminescence dosemeter (TLD CaSO 4 :Tm), three runs of monthly measurements have been carried out to evaluate their applicability in isolating the elevated dose at locations close to the radioactive waste storage in the Oarai site and the Tokai site of the Japan Atomic Energy Research Institute (JAERI). The results obtained were intercompared with the reference values measured with a 3 diam. spherical NaI(T1) detector applying a function G(E). By correcting the results of the TLD by a correction factor associated with the energy dependence effect, almost all of the results of the three runs were in agreement within the limits of uncertainty. Compared with the reference values, the results based on the PLD readings are closer than those from TLD.

The Absorbed Dose in Air of Photons Generated from Secondary Cosmic Rays at Sea Level at Nagoya, Japan

Investigations have been carried out to determine the absorbed dose in air of photons generated from secondary cosmic radiation at sea level at Nagoya, Japan. To isolate the contribution from cosmic photons, the pulse-height distributions due to µ particles and electrons were eliminated from the observed pulse-height distribution of a measurement with a 3" diam. spherical NaI(Tl) detector. The pulse height due to µ particles and electrons was inferred from the coincidence technique using two types of scintillation detectors with different sensitivities to photons. To obtain the photon fluence rate for further dose calculation, the pulse-height distribution of cosmic photons was unfolded by the iterative method. The mean and its standard deviation of the absorbed dose in air and fluence rate due to cosmic photons calculated from a one year observation are 2.86±0.05 nGy.h-1 and 0.1342±0.0015 photons.cm-2.s-1, respectively. The absorbed dose in air from cosmic photons was 0.5% lower during autumn to winter and 0.6% higher during spring to summer than the mean taken over the year.

３′′φ球形ＮａＩ（Ｔｌ）シンチレーション検出器を用いた屋内宇宙線線量率の簡便測定法

Two kinds of major techniques are usually applied to measurement of the ionizing component of the cosmic ray dose rate. One is a “precise method” by which the cosmic ray dose rate is obtained from the difference between the reading of an ionization chamber and that of a gamma ray detector such as a NaI(T1) scintillation detector. The former detects both gamma ray and cosmic ray, and the latter only gamma rays. The other is the “simplified method” by which the cosmic ray dose rate is obtained from the count rate above 3MeV of the gamma ray detector based on the assumption that the cosmic ray dose rate is proportional to the count rate. It is reasonable to apply the simplified method in the open air near the ground since the cosmic ray spectrum does not change much. However, attenuation of low energy component by building materials must be considered when the simplified method is applied to indoor measurement because the cosmic ray spectrum may be changed.From a series of measurements by the precise method with a spherical air equivalent ionization chamber of 14 liters and a 3″ diameter spherical NaI(T1) scintillation detector under several environmental coditions, proportionality was found between the cosmic ray dose rate and the count rate above 3MeV. The proportional constant was evaluated to be 20±1.4nGy/h/cps. Also, no apparent difference in the proportional constants was found between indoor and outdoor. It is concluded that the simplified method can be applied to indoor measurement as well as outdoor measurement of the cosmic ray dose rate.

富士山における宇宙線強度の高度変化 球形ＮａＩ（Ｔｌ）シンチレータによる電離成分の測定

Variation of cosmic ray intensity with altitude was measured at Mt. Fuji located at about 26°N on geomagnetic latitude on August 1994. The measurement was carried out with a spectrometer equipped with a 76.4 mm (3″) diameter spherical NaI (Tl) scintillator. Analysis was made for absorbed energy spectra above 3 Me V by the peeling-off method, and the cosmic ray intensity was calculated as the exposure rate from the ionizing component of the cosmic rays. The cosmic ray intensity increased with increasing altitude. The maximum (30.7×10-10 C/kg/h) and minimum (8.5×10-10 C/kg/h) values were observed at summit area of Mt. Fuji (3740 m) and near the sea level (about 20 m), respectively. Above 2500 m, the rate of change of cosmic ray intensity with altitude was estimated to be about 0.99×10-12 (C/kg) /h/m. The rate from near the sea level to about 2 500 m was also estimated to be about 0.39×10-12 (C/kg) /h/m.

ＮａＩ（Ｔｌ）シンチレーションスペクトロメータによる１センチメートル線量当量率の評価

A method for evaluating 1 cm dose equivalent rates from a pulse height distribution obtained by a 76.2 mm phi spherical NaI(Tl) scintillation spectrometer was described. Weak leakage radiation from nuclear facilities were also measured and dose equivalent conversion factor and effective energy of leakage radiation were evaluated from 1 cm dose equivalent rate and exposure rate.

７．５ｃｍφ球形ＮａＩ（Ｔｌ）シンチレーション検出器による地上レベルにおける５ＭｅＶ以下の宇宙線波高分布スペクトル

Ground level cosmic ray pulse height spectrum of a 7.5cm diameter spherical NaI (T1) scintillation detector was evaluated through stripping operation based on a pulse height spectrum measured on the sea and high precision response functions of the detector for U-series, Th-series and 40K. The exposure rate calculated from the determined cosmic ray pulse height spectrum was 0.21μR/h eq., which agreed well with that obtained from another method. The shape of the pulse height spectrum showed similarity to that measured at the altitude of 16, 000 ft, especially in the energy region of 0 to 3MeV. The principle of the adopted method is rather plain, however, the reliability of the spectrum is satisfactory. As the pulse height spectrum is a common information to any analytical method for environmental gamma ray using NaI (T1) scintillation detector, it is expected to be used for simple and precise separation of cosmic ray component involved in the enviromental pulse height spectrum.

Measurement of Cosmic Ray Induced Ionisation Intensity

A 20.186 litre sealed type standard ionisation chamber of 760 mmHg pressure and a 3" diam. spherical NaI(Tl) scintillation spectrometer were used to measure terrestrial ? radiation and cosmic ray induced ionisation intensity at sea-level, ground-level and high altitudes in Taiwan. The net ionising component measured by the ion chamber was directly converted into total exposure rate in air. The terrestrial exposure rate was calculated from measurements of the photon spectrum with the NaI(Tl) spectrometer. The cosmic ray induced ionising intensity was obtained by subtracting the exposure rate of the NaI(Tl) method from that of the ion chamber method. Data show that the cosmic ray induced ionising intensity at sea-level and ground-level (<100 m above sea-level) are 2.96 and 3.09 µR.h-1 respectively which are in good agreement with the data given in the UNSCEAR report (1982). The altitude dependence of cosmic ray induced ionising intensity was measured at Mt Alishan (2450 m above sea-level). The exposure rate from cosmic radiation was doubled at 2100 m altitude as compared with that at sea-level.

Ground Level Cosmic Ray Energy Spectrum in the Energy Range 0 to 4 MeV - Observed with a Spherical NaI(Tl) Spectrometer

Ground Level Cosmic Ray Energy Spectrum in the Energy Range 0 to 4 MeV - Observed with a Spherical NaI(Tl) Spectrometer

Estimation of exposure rates due to gamma-rays from standard sources used for calibration of exposure-rate-meters.

Exposure rates due to gamma-rays from cobalt-60 and cesium-137 sources, which were prepared for establishment of the institutionally standardized method of calibration for exposure-rate-meters, were measured by using a 3″φ spherical NaI (Tl) scintillation spectrometer to discuss on the increased exposure rates caused by the backscattered gamma-rays from holders of the standard sources, and on the non-uniform angular distributions of the exposure rates given by the standard sources.The exposure rates were obtained at points of interest as a function of atomic number of the source base or the holder material, and they had a maximum near the atomic number of the iron or the copper material. Another series of exposure rates were obtained as a function of angle θ between the base normal and the line from the source to the detector. The detectable amount of variations of exposure rates could be seen at an angle of around 60 degree in case of cobalt-60 source, and at an angle of around 30 degree in case of cesium-137 source.

Estimation of Photon Energy Spectra Based on 34×34 Response Matrix

The 34×34 response matrix whose energy divisions were chosen as 0.02 MeV betwee 0 and 0.1 MeV and chosen as 0.1 MeV between 0.1 and 3 MeV was constructed, and pulse-height distributions obtained by 3φ spherical NaI (T1) scintillation spectrometer were unfolded by a response matrix method into true photon energy spectra. The derived spectra were discussed precisely in the energy region from 0 to 3 MeV. Exposure rates were calculated from the spectra, and these values were compared with those measured with an ionization chamber to make sure the total number of incident photons on the detector.

Intrinsic efficiencies of spherical NaI(Tl) detectors

The intrinsic efficiencies of spherical NaI(Tl) detectors are considered with respect to the efficiencies of cylindrical NaI(Tl) detectors. A simple method of calculating the intrinsic efficiencies of spherical detectors is presented. It is shown that the problems introduced by the dependence of detection efficiency of the angular distribution of the incident gamma-ray flux for cylindrical detectors can either be eliminated or greatly reduced by the use of spherical detectors.

Absorption of Gamma Radiation in NaI Well Crystals

The results of Monte Carlo calculations on the detection and total absorption peak efficiencies of spherical NaI crystals are presented. A method of applying these data to cylindrical well crystals of arbitrary size and well geometry is described and the results are compared with experimental data. The Monte Carlo computations cover the gamma-ray energy range from 200 kev to 2 Mev and crystal radii from 1.5 to 12.0 cm. Histograms are presented of the number of Compton scattering events which a 0.662-Mev gamma ray undergoes in crystals of different sizes.