Pulse Shape Spectra Research Articles

Development of GGAG alpha camera system for targeted alpha radionuclide therapy research

Cerium doped Gd3(GaAl)5O12 (GGAG) scintillator has a character in which the decay time for alpha particles differs from that of gamma photons. This is a good point for an imaging detector used for alpha autoradiography of a mouse administered Astatine-211 (At-211) in radionuclide therapy research. Thus, we developed an alpha camera system using a GGAG imaging detector combined with a charge-coupled device (CCD) camera. Our GGAG imaging detector is made of a 0.5 mm-thick GGAG plate optically coupled to a position sensitive photomultiplier tube (PSPMT) and set in a stand with the CCD camera positioned on the upper side of the imaging detector to take optical photos. We first measured the performance of our alpha camera system using an Am-241 alpha source and natural alpha emitting radionuclides with radon daughters. We then conducted imaging of the non-sliced organs of a mouse. After the imaging detector surface was covered with 2.5 µm-thick Mylar film, the organs of the At-211-administered mouse was set on the imaging detector after split or opened and the alpha emitter distributions were measured. We used pulse shape analysis to separate the alpha particles from the beta particles or gamma photons. We successfully measured the alpha particle distributions of the wet non-sliced organs of the At-211-administered mouse. Using the pulse shape spectrum, we could separate the alpha particle images and beta particle or gamma photon images. We conclude that the developed alpha camera system is promising for the imaging of alpha particles and separating of the images using pulse shape discrimination.

Optical fiber-based ZnS(Ag) detector for selectively detecting alpha particles

In alpha radionuclide therapy, an optical fiber-based alpha particle detector is a new tool that could possibly be employed for the direct detection of alpha particles in subjects. Thus, in the present study, we developed an optical fiber-based alpha particle detector. The alpha particle detector was made of a 1mm diameter, 10 cm long plastic double clad optical fiber drilled a 0.7 mm diameter, 2 mm depth open space at the one end of the fiber. Silver-doped zinc sulfide (ZnS (Ag)) was painted inside this open space to form a ZnS(Ag) small scintillation chamber. To conduct performance comparisons, we also developed a fiber detector using the same fiber in which a Ce-doped Lu1.8Y0·2SiO5 (LYSO(Ce)) scintillator with dimensions of 0.32 mm × 0.5 mm × 5 mm was inserted. Both fiber detectors were wrapped in aluminized Mylar and optically coupled to a position sensitive photomultiplier tube, before calculating the two-dimensional distributions, energy, and pulse shape spectra. For 5.5-MeV alpha particles, the ZnS(Ag) fiber detector produced ~ 5 times larger pulse heights and the count rate was ~2 times higher compared with those using the LYSO(Ce) fiber detector. For the maximum energy 2.28-MeV beta particles and 0.66-MeV gamma photons, the ZnS(Ag) fiber detector produced no counts, but it yielded small counts from natural alpha particles. Our results confirmed that the ZnS(Ag) fiber detector developed in this study could selectively detect alpha particles and it was insensitive to beta particles and gamma photons.

Feasibility evaluation of a direct detection method of alpha particles in water using YGAG plate with pulse shape analysis

Ce doped (YGd)3(GaAl)5O12:Ce (YGAG) is a ceramic scintillator with high light output, and its decay times are different between alpha particles and gamma photons or beta particles. This characteristic may be applied to the direct measurement of alpha particles in water where the gamma photons or beta particles become background counts. Thus we proposed a radiation detector using a YGAG plate dipped in water that contained alpha radionuclides to detect alpha particles in water. The scintillation photons from the YGAG plate were detected by a position sensitive photomultiplier tube (PSPMT) set in water that contained alpha radionuclides. A YGAG plate (20 × 20 × 0.5 mm) was set on the bottom of a glass cup filled with water containing alpha radionuclides that emit alpha particles. The alpha particles near the YGAG plate were detected by the scintillator, and the scintillation light was detected by the PSPMT . For radon-containing water sampled from a hot spring, pulse shape spectra showed two peaks: one for the alpha particles and another for the beta particles and the gamma photons. By pulse shape discrimination for the peak of the alpha particles, their count rate decreased with a decay of Rn-222 (3.8 days). For tap water, the pulse shape distribution showed only one peak and the count rate was almost constant, indicating these counts were mainly from the background beta particles or gamma photons. We could show a detection principle for alpha radionuclides in water and this detection principle for alpha radionuclides in water might be a new method for estimating the alpha radionuclide concentration in water.

A high resolution radiation imaging detector using a ceramic YGAG scintillator plate

Ce doped (YGd)3(GaAl)5O12:Ce (YGAG) is a ceramic scintillator originally developed for X-ray computed tomography (CT). It has high light output and is also a promising candidate for high resolution event-by-event radiation imaging detectors. Thus we developed a radiation imaging detector using a YGAG plate combined with a position sensitive photomultiplier tube (PSPMT) and evaluated the performance. The spatial resolution for Am-241 alpha particles (5.5 MeV) was ∼0.4 mm FWHM and the energy resolution was 12 % FWHM. The spatial resolution for Sr-Y-90 beta particles, Am-241 gammas (60 keV) and Cs-137 X-ray (∼35 keV) was 0.8 mm FWHM, 1.6 mm FWHM and 2.0 mm FWHM, respectively. The decay time difference of YGAG between 5.5 MeV alpha particles and 662 keV Cs-137 gammas was 37 ns and the pulse shape spectra for 5.5 MeV alpha particles and 662 keV Cs-137 gammas were separated without overlap with the figure of merit (FOM) of 1.14. It was possible to separate the alpha particle and gamma photon images using pulse shape discrimination. Using pulse shape discrimination, it was also possible to separate the alpha particles and gamma photons in the electrostatically collected radon daughters on the imaging detector. We conclude that the developed YGAG imaging detector is useful for high resolution radiation detector with pulse shape discrimination capability to discriminate the different types of radiations.

Response to Neutrons and γ-rays of Two Liquid Scintillators

UltimaGoldTM AB and OptiphaseTrisafe are two liquid scintillators made by Perkin Elmer and EG & G Company respectively. Both are commercially promoted as scintillation detectors for α and β particles. In this work, the responses to γ-rays and neutrons of UltimaGoldTM AB and OptiphaseTriSafe liquid scintillators, without and with reflector, have been measured aiming to use these scintillators as γ-rays and neutron detectors. Responses to γ-rays and neutrons were measured as pulse shape spectra in a multichannel analyzer. Scintillators were exposed to gamma rays produced by 137Cs, 54Mn, 22Na and 60Co sources. The response to neutrons was obtained with a 241AmBe neutron source that was measured to 25 and 50 cm from the scintillators. The pulse height spectra due to gamma rays are shifted to larger channels as the photon energy increases and these responses are different from the response due to neutrons. Thus, UltimaGoldTM AB and OptiphaseTrisafe can be used to detect γ-rays and neutrons.

Pulse shape discriminations of different types of radiation on GGAG imaging detector

Gd3(GaAl)5O12:Ce (GGAG) is a ceramic scintillator originally developed for X-ray CT, and it was also an excellent material for the development of an event-by-event-based radiation imaging detector when it was combined with a position sensitive photomultiplier (PSPMT). With the developed GGAG imaging detector, we found that the decay times for alpha particles and gamma photons were different. Also, we found that the decay times for alpha particles and beta particles were different. These characteristics are advantageous for developing an imaging detector for the simultaneous imaging of different types of radiation using pulse shape discrimination. Thus, we tested the separation of the images of the alpha particles and gamma photons using pulse shape discrimination. Also, we evaluated the separation of the alpha and beta particle images. In the pulse shape spectra, we could separate the peaks of Am-241 alpha particles and Cs-137 gamma photons with a peak-to-valley ratio (P/V) of 3.5. We obtained clearly separated images for Am-241 alpha particles and Cs-137 gamma photons using pulse shape discrimination. We could also separate the peaks of Am-241 alpha particles and Sr–Y-90 beta particles with a P/V of 1.5 in the pulse shape spectrum. We obtained separated images for Am-241 alpha particles and Sr–Y-90 beta particles using pulse shape discrimination. In addition, we could separate electrostatically collected natural alpha particles, Po-218 and Po-214, from the environmental beta particles and gamma photons using pulse shape discrimination. We conclude that the GGAG imaging detector is promising for simultaneous imaging and separating the images of different types of radiation using pulse shape discrimination.

Development of high resolution phoswich depth-of-interaction block detectors utilizing Mg co-doped new scintillators

To correct for parallax error in positron emission tomography (PET), phoswich depth-of-interaction (DOI) detector using multiple scintillators with different decay times is a practical approach. However not many scintillator combinations suitable for phoswich DOI detector have been reported. Ce doped Gd3Ga3Al2O12 (GFAG) is a newly developed promising scintillator for PET detector, which has high density, high light output, appropriate light emission wavelength for silicon-photomultiplier (Si-PM) and faster decay time than that of Ce doped Gd3Al2Ga3O12 (GAGG). In this study, we developed a Si-PM based phoswich DOI block detector of GFAG with GAGG crystal arrays and evaluated its performance. We assembled a GFAG block and a GAGG block and they were optically coupled in depth direction to form a phoswich detector block. The phoswich block was optically coupled to a Si-PM array with a 1 mm thick light guide. The sizes of the GFAG and GAGG pixels were 0.9 mm x 0.9 mm x 7.5 mm and they were arranged into 24 x 24 matrix with 0.1 mm thick BaSO4 as reflector. We conducted the performance evaluation for two types of configurations; GFAG block arranged in upper layer (GFAG/GAGG) and GAGG arranged in upper layer (GAGG/GFAG). The measured two dimensional position histograms of these block detectors showed good separation and pulse shape spectra produced two distinct peaks for both configurations although some difference in energy spectra were observed. These results indicate phoswich block detectors composed of GFAG and GAGG are promising for high resolution DOI PET systems.

Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11475026 and 11175023).

Enhanced electron–positron pair production by frequency chirping in one- and two-color laser pulse fields is investigated by solving the quantum Vlasov equation. A small frequency chirp shifts the momentum spectrum along the momentum axis. The positive and negative frequency chirp parameters play the same role in increasing the pair number density. The sign change of the frequency chirp parameter at the moment leads the pulse shape and momentum spectrum to be symmetric, and the number density to be increased. The number density of produced pairs in the two-color pulse field is much higher than that in the one-color pulse field and the larger frequency chirp pulse field dominates more strongly. In the two-color pulse fields, the relation between the frequency ratio of two colors and the number density is not sensitive to the parameters of small frequency chirp added in either a low frequency strong field or a high frequency weak field but sensitive to the parameters of large frequency chirp added in a high frequency weak field.

Timing performance measurements of Si-PM-based LGSO phoswich detectors

Since the timing resolution was significantly improved using silicon photomultipliers (Si-PMs) combined with fast scintillators, we expect that phoswich detectors will be used in future TOF-PET systems. However, no practical phoswich detector has been proposed for TOF-PET detectors. We conducted timing performance measurements of phoswich detectors comprised of two types of Ce-doped LGSO scintillators with different decay times coupled to Si-PMs and digitized the output signals using a high bandwidth digital oscilloscope. We prepared three types of LGSOs (LGSO-fast, LGSO-standard, and LGSO-slow) with different Ce concentrations. After measuring the decay time, the energy performance, and the timing performance of each LGSO, we conducted pulse shape analysis and timing resolution measurements for two versions of phoswich LGSOs: LGSO-standard/LGSO-fast and LGSO-slow/LGSO-fast combinations. The pulse shape spectra for a 10-mm-long crystal LGSO-slow/LGSO-fast combination showed good separation of the front and back crystals with a peak-to-valley ratio of 2.0. The timing resolutions for the 20-mm-long crystal LGSO-slow/LGSO-fast combination were ~300ps FWHM. The timing resolutions for the phoswich LGSOs were slightly inferior than that measured with the individual LGSO fast, but the acquired timing resolution for the phoswich configuration, ~300ps with a LGSO-slow/LGSO-fast combination, is adequate for TOF-PET systems. We conclude that LGSO phoswich detectors are promising for TOF-DOI-PET systems.

Development of dual-layer GSO depth-of-interaction block detector using angled optical fiber

A PET system for small animals requires a small detector ring to obtain high-spatial resolution images. However, when we use a relatively large size of photodetector such as a position-sensitive photomultiplier tube (PSPMT), the detector ring is arranged in a hexagonal- or octagonal-shape, and the PET system has large gaps between the block detectors. The large gaps produce image distortion, and the reconstruction algorithm is difficult. To solve these problems, we proposed to arrange two scintillator blocks on one PSPMT using two angled optical fiber-based image guides. We could set two scintillator blocks angled at 22.5° on a PSPMT so that these scintillator blocks are arranged in a nearly circular (hexadecagonal) shape with eight developed block detectors. We used Gd2SiO5 (GSO) scintillators with Ce concentrations of 1.5mol% (decay time: 39ns) and 0.4mol% (decay time: 63ns). Sizes of these GSO cells were 1.6×2.4×7.0mm3 and 1.6×2.4×8.0mm3 for 1.5mol% Ce and 0.4mol% Ce, respectively. These two types of GSO were arranged in an 11×15 matrix and optically coupled in the depth direction to form a depth-of-interaction (DOI) detector. Two GSO blocks and two optical fiber-based image guides were optically coupled to a 2-in. PSPMT (Hamamatsu Photonics H8500: 8×8 anodes). We measured the performances of the block detector with Cs-137 gamma photons (662-keV). We could resolve almost all pixels clearly in a two-dimensional position histogram. The average peak-to-valley ratios (P/Vs) of the two-dimensional position histogram along profiles were 2.6 and 4.8 in horizontal and vertical directions, respectively. The energy resolution was 28.4% full-width at half-maximum (FWHM). The pulse shape spectra showed good separation with a P/V of 5.2. The developed block detector performed well and shows promise for the development of high-sensitivity and high-spatial resolution PET systems.

Development of GAGG depth-of-interaction (DOI) block detectors based on pulse shape analysis

A depth-of-interaction (DOI) detector is required for developing a high resolution and high sensitivity PET system. Ce-doped Gd3Al2Ga3O12 (GAGG fast: GAGG-F) is a promising scintillator for PET applications with high light output, no natural radioisotope and suitable light emission wavelength for semiconductor based photodetectors. However, no DOI detector based on pulse shape analysis with GAGG-F has been developed to date, due to the lack of appropriate scintillators of pairing. Recently a new variation of this scintillator with different Al/Ga ratios—Ce-doped Gd3Al2.6Ga2.4O12 (GAGG slow: GAGG-S), which has slower decay time was developed. The combination of GAGG-F and GAGG-S may allow us to realize high resolution DOI detectors based on pulse shape analysis. We developed and tested two GAGG phoswich DOI block detectors comprised of pixelated GAGG-F and GAGG-S scintillation crystals. One phoswich block detector comprised of 2×2×5mmpixel that were assembled into a 5×5 matrix. The DOI block was optically coupled to a silicon photomultiplier (Si-PM) array (Hamamatsu MPPC S11064-050P) with a 2-mm thick light guide. The other phoswich block detector comprised of 0.5×0.5×5mm (GAGG-F) and 0.5×0.5×6mm3 (GAGG-S) pixels that were assembled into a 20×20 matrix. The DOI block was also optically coupled to the same Si-PM array with a 2-mm thick light guide. In the block detector of 2-mm crystal pixels (5×5 matrix), the 2-dimensional histogram revealed excellent separation with an average energy resolution of 14.1% for 662-keV gamma photons. The pulse shape spectrum displayed good separation with a peak-to-valley ratio of 8.7. In the block detector that used 0.5-mm crystal pixels (20×20 matrix), the 2-dimensional histogram also showed good separation with energy resolution of 27.5% for the 662-keV gamma photons. The pulse shape spectrum displayed good separation with a peak-to-valley ratio of 6.5. These results indicate that phoswich DOI detectors with the two types of GAGGs are promising for developing a high resolution PET system.

Horizontal refraction and whispering gallery sound waves in area of curvilinear coastal wedge in shallow water

Horizontal refraction of sound signals in area of coastal wedge determines specific structure of the sound field in horizontal plane, including multipath areas and caustics (in the framework of approximation of horizontal rays and vertical modes). This structure depends on mode number and frequency. In the paper sound field of signal, radiated by the point source is studied in the shallow water area which can be considered as a wedge with curvilinear coastal line (lake, gulf, cape, etc.). It is shown that for some distances from the source to the beach whispering gallery waves (whispering gallery horizontal rays) exist, propagating along the coast, where energy is concentrated in narrow area in horizontal plane. Structure and characteristic of these rays depend on mode number, frequency and waveguide parameters (in particular, steepness of sea floor and radius of curvature). Intensity fluctuations as well as variation of pulse shape and frequency spectrum are studied; analytical estimations are presented as well as discussion of possible experimental observations. [Work was supported by RFBR and BSF.]

Three-layer GSO depth-of-interaction detector for high-energy gamma camera

Using Ce-doped Gd2SiO5 (GSO) of different Ce concentrations, three-layer DOI block detectors were developed to reduce the parallax error at the edges of a pinhole gamma camera for high-energy gamma photons. GSOs with Ce concentrations of 1.5mol% (decay time ~40ns), 0.5mol% crystal (~60ns), 0.4mol% (~80ns) were selected for the depth of interaction (DOI) detectors. These three types of GSOs were optically coupled in the depth direction, arranged in a 22×22 matrix and coupled to a flat panel photomultiplier tube (FP-PMT, Hamamatsu H8500). Sizes of these GSO cells were 1.9mm×1.9mm×4mm, 1.9mm×1.9mm×5mm, and 1.9mm×1.9mm×6mm for 1.5mol%, 0.5mol%, and 0.4mol%, respectively. With these combinations of GSOs, all spots corresponding to GSO cells were clearly resolved in the position histogram. Pulse shape spectra showed three peaks for these three decay times of GSOs. The block detector was contained in a 2-cm-thick tungsten shield, and a pinhole collimator with a 0.5-mm aperture was mounted. With pulse shape discrimination, we separated the point source images of the Cs-137 for each DOI layer. The point source image of the lower layer was detected at the most central part of the field-of-view, and the distribution was the smallest. The point source image of the higher layer was detected at the most peripheral part of the field-of-view, and the distribution was widest. With this information, the spatial resolution of the pinhole gamma camera can be improved. We conclude that DOI detection is effective for pinhole gamma cameras for high energy gamma photons.

Basic performance evaluation of a Si-PM array-based LGSO phoswich DOI block detector for a high-resolution small animal PET system

The silicon photomultiplier (Si-PM) is a promising photodetector for PET. However, it remains unclear whether Si-PM can be used for a depth-of-interaction (DOI) detector based on the decay time differences of the scintillator where pulse shape analysis is used. For clarification, we tested the Hamamatsu 4 × 4 Si-PM array (S11065-025P) combined with scintillators that used different decay times to develop DOI block detectors using the pulse shape analysis. First, Ce-doped Gd(2)SiO(5) (GSO) scintillators of 0.5 mol% Ce were arranged in a 4 × 4 matrix and were optically coupled to the center of each pixel of the Si-PM array for measurement of the energy resolution as well as its gain variations according to the temperature. Then two types of Ce-doped Lu(1.9)Gd(0.1)Si0(5) (LGSO) scintillators, 0.025 mol% Ce (decay time: ~31 ns) and 0.75 mol% Ce (decay time: ~46 ns), were optically coupled in the DOI direction, arranged in a 11 × 7 matrix, and optically coupled to a Si-PM array for testing of the possibility of a high-resolution DOI detector. The energy resolution of the Si-PM array-based GSO block detector was 18 ± 4.4 % FWHM for a Cs-137 gamma source (662 keV). Less than 1 mm crystals were clearly resolved in the position map of the LGSO DOI block detector. The peak-to-valley ratio (P/V) derived from the pulse shape spectra of the LGSO DOI block detector was 2.2. These results confirmed that Si-PM array-based DOI block detectors are promising for high-resolution small animal PET systems.

ZnS quantum dot based nanocomposite scintillators for thermal neutron detection

Water-soluble ZnS quantum dots (QDs) were synthesized using a co-precipitation method. Both undoped and Eu3+-doped ZnS QDs showed emission bands peaking at around 420nm. The highest photoluminescence (PL) quantum yield (QY) was measured to be 17% for undoped ZnS QDs capped with SiO2. Thermal neutron induced pulse height and pulse shape spectra were measured for composites composed of 6LiF nanoparticles, undoped ZnS QDs, and poly(vinyl alcohol) (PVA) matrix. The pulse height spectra from the nanocomposites were analyzed with a track-structure model. The pulse decay time of scintillation photons is 10–30ns. The fast decay rates and clear pulse height peaks induced by thermal neutrons indicate that ZnS QD based nanocomposites are promising scintillators for detecting high-flux thermal neutrons.

Higher-order effects on self-similar parabolic pulse in the microstructured fibre amplifier

By considering higher-order effects, the properties of self-similar parabolic pulses propagating in the microstructured fibre amplifier with a normal group-velocity dispersion have been investigated. The numerical results indicate that the higher-order effects can badly distort self-similar parabolic pulse shape and optical spectrum, and at the same time the peak shift and oscillation appear, while the pulse still reveals highly linear chirp but grows into asymmetry. The influence of different higher-order effects on self-similar parabolic pulse propagation has been analysed. It shows that the self-steepening plays a more important role. We can manipulate the geometrical parameters of the microstructured fibre amplifier to gain a suitable dispersion and nonlinearity coefficient which will keep high-quality self-similar parabolic pulse propagation. These results are significant for the further study of self-similar parabolic pulse propagation.

Low-frequency pulse propagation over irregular terrain

The shape of a low-frequency pulse that propagates to the far side of a hill depends primarily on two physical mechanisms: a diffracted wave and a surface wave. Both of these mechanisms depend on frequency, so that the pulse shape and frequency spectrum of the received pulse differ significantly from those near the source. As is well known, diffraction acts like a low-pass filter. The surface wave has a more complicated frequency dependence that depends on the terrain, the frequency dependence of the ground impedance, and the near-ground sound speed profile. For a given range, these parameters determine a frequency band that contains most of the surface wave energy. The surface wave thus functions as a low-frequency bandpass filter. The combined effects of diffraction and surface wave propagation on pulse propagation over a hill are studied using a parabolic equation model and a piecewise linear mapping to flatten the irregular terrain. Numerical calculations are presented for selected terrain features, ground impedances, and sound speed profiles. [Research supported by the U.S. Army Armament, Research Development and Engineering Center (ARDEC).]

Determination of complex microcalorimeter parameters with impedance measurements

The proper understanding and modeling of a microcalorimeter's response requires accurate knowledge of a handful of parameters, such as C, G, α . While a few of these parameters are directly determined from the IV characteristics, some others, notoriously the heat capacity ( C) and α , appear in degenerate combinations in most measurable quantities. The consideration of a complex microcalorimeter leads to an added ambiguity in the determination of the parameters. In general, the dependence of the microcalorimeter's complex impedance on these various parameters varies with frequency. This dependence allows us to determine individual parameters by fitting the prediction of the microcalorimeter model to impedance data. In this paper we describe efforts at characterizing the Goddard X-ray microcalorimeters. With the parameters determined by this method, we compare the pulse shape and noise spectra predictions to data taken with the same devices.

Multifrequency dynamical model of pulsed CO2 lasers

A six-temperature multifrequency dynamical model of pulsed CO2 lasers is presented, taking account of the collision-dependent overlap of rotational lines, the effect of hot bands and sequence bands on the gain spectrum, and non-Lorentzian line overlap effect in a wide range of pressures from 20 torr to 20 atm. Theoretical calculations of pulse shape and laser spectrum have good agreement with experimental results. This model should be useful in the understanding of pulsed CO2 laser dynamics, in the evaluation of alternative pump schemes, and in the investigation of tunable characteristics. Thus it can provide theoretical support for the design of pulsed CO2 lasers.

Plutonium Determination in Compacted Bentonite by Using PERALS

The determination of plutonium activities at low levels was occurred during study on diffusion of plutonium onto compacted bentonite, a candidate buffer material of multi-barrier system suggested for high-level radioactive waste disposal in Japan. It was performed by using photo/electron-rejecting alpha liquid scintillation (PERALS) spectrometry, model 8100AB (ORDELA INC.), followed by plutonium extraction from slices of the bentonite by hydrochloric acid (HC1). Pulse shape spectrum was chosen because of a high counting efficiency and only one alpha emitter existing. For an accurate determination, the optimum pulse shaped discrimination (PSD) setting was obtained. Effects resulted from HC1 concentration used for plutonium extraction and sample-loading (SL) were also investigated by PERALS spectrometry from the view of its application. The results showed that peak region in pulse shape spectrum shifted with changes of the HC1 concentrations and sample-loading. Pulse shape spectrum of background blank (liquid scintillator, Ultima Gold AB) showed an obvious peak located on the left side of alpha region. This peak influenced seriously plutonium counting by means of peak overlap when total plutonium activity in sample was less than several Bq. The low limit of detective concentration was estimated to be 0.79 Bq/g-bentonite.