C. Ida

First underground results with NEWAGE-0.3a direction-sensitive dark matter detector

Abstract A direction-sensitive dark matter search experiment at Kamioka underground laboratory with the NEWAGE-0.3a detector was performed. The NEWAGE-0.3a detector is a gaseous micro-time-projection chamber filled with CF4 gas at 152 Torr. The fiducial volume and target mass are 20 × 25 × 31 cm 3 and 0.0115 kg, respectively. With an exposure of 0.524 kg days, improved spin-dependent weakly interacting massive particle (WIMP)-proton cross section limits by a direction-sensitive method were achieved including a new record of 5400 pb for 150 GeV / c 2 WIMPs. We studied the remaining background and found that ambient γ-rays contributed about one-fifth of the remaining background and radioactive contaminants inside the gas chamber contributed the rest.

Developments of a large area VUV sensitive gas PMT with GEM/μPIC

A new large area UV photon detector with micro pattern gaseous detectors is developed and evaluated. A semitransparent CsI photocathode deposited on a MgF2 window was combined with 10cm ? 10cm GEM and ?PIC. Using Ar+C2H6 (10%) gas, we achieved the gas gain of more than 105 which is enough to detect single photoelectron. We, then, irradiated vacuum UV photons (VUV, around 172nm) from the newly developed LaF3(Nd) scintillator to the detector and the single photoelectrons were successfully detected. We also demonstrated the imaging capability of the detector with ?PIC readout systems.

Performance of 8

We have developed a LaBr3:Ce scintillator array consisting of 8 × 8 pixels with a size of 5.8 mm×5.8 mm×15.0 mm, which serves as an absorber of scattered gamma rays with energies from 0.1 to 1 MeV in a Compton camera. The pixels were cut from two pieces of LaBr3:Ce crystal with a diameter of 38 mm and a length of 38 mm with full width at half-maximum (FWHM) energy resolutions of 4.1 ± 0.1% and 3.0 ± 0.1% at 356 and 662 keV, respectively, measured with a single anode photomultiplier tube (PMT). The crystal had the following volumetric uniformities: light outputs with a difference of 0.5% (standard deviation: SD) and energy resolutions with that of 2% (SD) at 356 keV. In contrast, for each pixel in the array, the average and SD FWHM energy resolutions over 64 pixels, measured with a single-anode PMT and a collimator, were 5.8 ± 0.9% at 356 keV. The array was then coupled to a 64-channel multi-anode PMT (Hamamatsu H8500), the anode pitch of which was the same as the LaBr3:Ce pixel pitch of 6.1 mm. When the 64 anodes were read out from four channels in a resistor chain by the charge division method, the FWHM energy resolution of all 64 pixels was 7.0 ± 0.5% at 662 keV, whereas that of the inner 6×6 pixels was 5.8 ± 0.4% at 662 keV. In addition, we measured a Gd2SiO5:Ce (GSO:Ce) scintillator array consisting of 8×8 pixels with a size of 5.9 mm×5.9 mm×13.0 mm to compare its performance with that of the LaBr3:Ce array. The FWHM energy resolution of all 64 GSO:Ce pixels was 10.8 ± 0.3% at 662 keV. With these energy resolutions, FWHM angular resolutions of the Compton camera using the LaBr3:Ce and GSO:Ce arrays are expected to be 4.6° and 5.3°, respectively, at 662 keV.

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We have developed a gamma camera based on an array of LaBr3(Ce) scintillator pixels coupled to a multi-anode photomultiplier tube (MAPMT). It consisted of an 8times8 array of LaBr3(Ce) pixels with a size of 5.8times5.8times15 mm3 and a 64-channel MAPMT (Hamamatsu flat-panel H8500) with an effective area of 49times49 mm2. The pixels of the LaBr3(Ce) array were made from two LaBr3(Ce) monolithic crystals with a diameter of 38 mm and a height of 38 mm, which had energy resolutions of 4.33plusmn0.02% at 356 keV and 3.25plusmn0.01% at 662 keV. They were assembled into the array with a reflector between pixels, and sealed hermetically by our own technique. The pitch of the LaBr3(Ce) pixels, 6.1 mm, was determined to be the same as that of the anodes. The thickness of pixels was 15 mm to have moderate detection efficiency for sub-MeV/MeV gamma rays. We evaluated the performance as follows. At first, in order to remove the effect of the gain variance among anodes of the MAPMT, the array was coupled to a single-anode PMT, and collimated gamma rays from isotopes were irradiated to one pixel in the array. The energy resolutions (FWHM) were 5.4 (average) plusmn1.0 (RMS) % at 356 keV and 4.5plusmn1.0 % at 662 keV. Next, in order to obtain a gamma-ray image of 64 pixels by readout of only four channels, we used a resistor-chain readout system in the charge division method. In flood field irradiation of gamma rays, each pixel was clearly resolved. The energy resolutions (FWHM) of 64 pixels were 8.6plusmn1.0% at 356 keV and 5.8plusmn0.9 % at 662 keV. The averages were represented by (5.8plusmn0.7) (E/662 keV)-052 plusmn 0.02 % at energies from 122 keV to 835 keV.

8 Pixel LaBr

We have developed a new imaging device consisting of a VUV scintillator coupled to a gas photomultiplier (gas PMT) with a CsI photocathode, micro pixel chamber (μ-PIC) and gas electron multipliers (GEMs). Generally, the VUV scintillator has a short decay time of less than ~ 10 nsec. Thus the new detector could be used under the high rate counting ( ~ 10 MHz) in the hard X-ray to soft-gamma ray region. Our goal in this paper is to obtain a soft gamma-ray image at 0.1 MeV with our gas PMT combined with the VUV scintillator as a high rate counter, and we have optimized electric fields in the gas PMT and developed a new VUV scintillator with a higher light output. In order to obtain a higher collection efficiency of photoelectrons and suppress the ion feedback in the gas PMT, we first optimized the electric field. Then we decided the electric field in the drift, transfer, and induction region to be 0.25, 1.0 and 3.0 kV/cm, respectively. The total gas gain of the gas PMT was approximately 2 × 105, and the gas PMT was estimated to have a quantum efficiency (QE) of 0.7% at 178 nm. Additionally, as a consequence of new VUV scintillators search, Nd:LuLiF4 and Nd:LuF3 with a volume of 10 mm × 10 mm × 5 mm were found to have higher light outputs than Nd:LaF3, which is a conventional VUV scintillator, by a factor of 2.1, and 2.6, respectively, and the Nd:LuF3 irradiated with 5.5−MeV alpha−rays had a light output of approximately 400 photons. Finally, we succeeded in obtaining the crystal images upon 5.5 MeV alpha and 0.122 MeV gamma rays excitation from 241Am− and 57Co sources, respectively, using the gas PMT with the Nd:LuF3.

_{3}

We are developing an Electron-Tracking Compton imaging Camera (ETCC) based on a gaseous Time Projection Chamber (TPC) and a scintillation camera. The ETCC detects the energy and the direction of the incident gamma ray using the information of the recoil electron and the scattered gamma ray. We have developed the ETCC with a detection volume of 23 × 28 × 30 cm3 which consists of the 23 × 28 × 30 cm3 gaseous TPC and 30 × 30 cm2 GSO(Ce) scintillation camera. And we obtained the gamma-ray image and investigated the performances of the ETCC. The Angular Resolution Measure (ARM) and the Scatter Plane Deviation (SPD) are 6.1 degree and 64.5 degree (HWHM) at 662keV, respectively, and the energy resolution is 18.0%(FWHM) at 662keV.

:Ce and Gd

We have developed a low-power wide-dynamic-range readout system for a 64-channel multi-anode photomultiplier (PMT) of a scintillation gamma camera. Each anode is individually read with the system that contains discrete devices of amplifiers, comparators, sample-hold ADCs, and FPGAs. The size of the system which is designed for a two-dimensional array of Hamamatsu flat panel PMT H8500 is 5×5×14 cm3. The input dynamic range is variable by replacing the feedback capacitor of the preamplifier (e.g., 700 pC and 4000 pC for GSO(Ce) and LaBr3(Ce) crystals, respectively). The serialized ADC data are sent to a VME sequence module. The total power consumption is 1.6 W per 64 channels. With this system we have developed a gamma camera using an 8×8 array of GSO(Ce) pixels with a pixel size of 6×6×13 mm3 coupled to an H8500, and obtained flood-field irradiation images at energies from 30 keV to 1.3 MeV. The energy resolution was 10.8±0.4% (FWHM) at 662 keV. In addition, we used the readout system for an 8×8 array of LaBr3(Ce) pixels with a pixel size of 6×6×15 mm3 and obtained a flood-field irradiation image at 662 keV.

_{2}

We have developed an Electron-Tracking Compton Camera (ETCC) based on a gaseous micro Time Projection Chamber (ETCC) based on a gaseous micro Time Projection Chamber (μ-TPC) which measures the direction and the energy of the recoil electron and a GSO(Ce) scintillation camera which surrounds the μ-TPC and measures the Compton scattered gamma ray. If not measuring a direction of a recoil electron, a direction of the incident gamma-ray could only be reconstructed as a circle. Measuring the direction of the recoil electron reduces the Compton cone to a point, and thus reconstructs the incident direction completely for a single photon and realizes the strong background rejection. Using the ETCC with a detection volume of about 10cm×10cm×15cm, we had the balloon-borne experiment supported by ISAS/JAXA in 2006 for the purpose of the observation of diffuse cosmic and atmospheric gamma rays. The ETCC obtained about 200 photons with FOV of 3 str in 3 hours in the energy range from 100 keV to 1 MeV, and the obtained flux was consistent with previous observations. On the basis of the results, we are developing the large size ETCC in order to improve the effective area for the next balloon experiment. The large size ETCC has the detection volume of 23cm ×28cm×30cm which consists of the 23cm×28cm×30cm μ-TPC and the 30cm×30cm×1.3cm scintillation camera. Then we obtained the gamma-ray image and investigated the first performances of the large size ETCC. The Angular Resolution Measure (ARM) and the Scatter Plane Deviation (SPD) are 12.1 degree and 117 degree (FWHM) at 662keV, respectively, and the energy resolution is 16.9%(FWHM) at 662keV.

SiO

We have developed the Electron tracking Compton Camera (ETCC) with reconstructing the 3-D tracks of the scattered electron in Compton process for both gamma-ray astronomy and medical imaging [1–3]. By measuring both the directions and energies of a recoil gamma ray and a scattered electron, the direction of the incident gamma ray is determined for an individual photon. Furthermore, a residual measured angle between the recoil electron and scattered gamma ray is powerful for the kinematical background-rejection. For the 3-D tracking of the electrons, the Micro Time Projection Chamber (μ-TPC) was developed, which consists of a new type of the micro pattern gas detector, or a Micro Pixel Gas Chamber (μ-PIC). The ETCC consists of this μ-TPC and the GSO crystal pixel arrays below the μ̃TPC for detecting the recoil gamma rays. The ETCC provided the gamma ray images of point sources between 120keV and ∼1 MeV with the angular resolution of 6 degree (FWHM) at 511keV of 18F ion, respectively. Also the angle of the scattered electron was measured with the resolution of ∼80 degree. Two mobile ETCCs with 10cm-cube TPC for small animal and 30cm-cube TPC for human body, are now being operated for Medical Imaging test. We have studied the imaging performances using both phantoms and small animals (rats and mice) for conventional radioisotopes of 131I and 18F-FDG. In particular, new ETCC with LaBr3 pixel scintillator provides good images similar to SPECT for 131I and human PET for 511keV, respectively, where a clear concentration to tumors in a mouse is observed The 30cm-cube ETCC can get an image for 1m-size length objects in one measurement. Thus, we have carried out several comparisons of our images with those of SPECT and PET. Multi-tracer image using I-131 and FDG for small animal and the image for higher energy gamma ray above 511keV for plants using 54Mn have been carried out successfully. Also several new biomarkers and new radio nuclides were examined to verify the merits of ETCC for medical imaging.

_{5}

We have developed a sub-MeV and MeV gamma-ray imaging Compton camera for use in gamma-ray astronomy; it consists of a gaseous time-projection chamber (TPC) to convert the Compton scattering events and a scintillator array to absorb photons. The TPC measures the energy and three-dimensional tracks of Compton-recoil electrons, while the pixel scintillator arrays measure the energy and positions of scattered gamma rays. Therefore, our camera can reconstruct the incident gamma rays, event by event, over a wide field of view of approximately 3 str. We are now developing a Compton camera for a balloon-borne experiment.