Search

In the MEG II experiment, the liquid xenon gamma-ray detector uses Multi-Pixel Photon Counters (MPPC) which are sensitive to vacuum ultraviolet (VUV) light under a high-intensity muon beam environment. In the commissioning phase of the detector with the beam, a significant degradation in the photon detection efficiency (PDE) for VUV light was found, while the degradation in the PDE for visible light was much less significant. This implies that the radiation damage is localized to the surface of the MPPC where incoming VUV photons create electron-hole pairs. It was also found that the PDE can recover to the original level by thermal annealing., 17 pages, 11 figures

Nuclear instruments & methods in physics research / A 887, 150 - 168 (2018). doi:10.1016/j.nima.2018.01.016, A highly granular electromagnetic calorimeter with scintillator strip readout is being developed for future linear collider experiments. A prototype of 21.5 $X_0$depth and $180 \times 180$ $mm^2$ transverse dimensions was constructed, consisting of 2160 individually read out $10\times 45\times 3$ $mm^3$ scintillator strips. This prototype was tested using electrons of 2–32 $Ge V$ at the Fermilab Test Beam Facility in 2009. Deviations from linear energy response were less than 1.1%, and the intrinsic energy resolution was determined to be $(12.5\pm 0.1(stat.)\pm 0.4(syst.))\%$ $/$ $\sqrt{E[Ge V]}\oplus \left( 1.2\pm 0.1(stat.)_{-0.7}^{+0.6}(syst.)\right)\%$, where the uncertainties correspond to statistical and systematic sources, respectively., Published by North-Holland Publ. Co., Amsterdam

MEG II experiment is an upgrade of MEG experiment to search for charged lepton flavor violating decay of muon, $\mu \rightarrow e\gamma$. Target sensitivity of MEG II is $6 \times 10^{-14}$ at 90% C.L., which is one order of magnitude better than MEG. Liquid xenon detector was used to detect $53 \,\mathrm{MeV}$ signal $\gamma$-ray in MEG, and this detector has been upgraded for MEG II. Replacement of 216 PMTs on the entrance face with 4092 newly developed VUV-sensitive MPPCs has been carried out to have better granularity and uniformity of the scintillation readout. In 2017, commissioning of this detector has been started. Sufficient performance has been confirmed for most of the MPPCs. Pilot run of this detector with muon beam was performed, and $\gamma$-ray near signal energy has been successfully detected. By the even-odd analysis of the obtained data, good timing resolution ($44 \,\mathrm{ps}$ for $50 \,\mathrm{MeV}$ $\gamma$-ray) has been achieved.

The MEG experiment at PSI searches for the decay μ→eγ at a level of ≈10^−13 on the branching ratio BR(μ→eγ/μ→tot), well beyond the present experimental limit (BR≤1.2×10^−11) and is sensitive to the predictions of SUSY-GUT theories. To reach this goal the experiment uses one of the most intense continuous surface muon beams available (≈10^8μ/s) and relies on advanced technology (LXe calorimetry, a gradient-field superconducting spectrometer as well as flexible and powerful trigger and acquisition systems). In order to maintain the highest possible energy, time and spatial resolutions for such detector, frequent calibration and monitoring, using a Cockcroft–Walton proton accelerator, are required. The proton beam is brought to the centre of MEG by a special bellows insertion system and travels in a direction opposite to the one of the normal μ-beam. Protons interact with a lithium tetraborate (Li2B4O7) nuclear target and produce one γ (17.6 MeV) from the reaction 3-7 Li(p, γ)8-4 or two coincident γs (11.67 and 4.4 MeV) from the reaction 11-5 B(p, γ)12-6 C*. The 17.6 MeV γ is used for calibrating and monitoring the LXe calorimeter (σEγ/Eγ=3.85±0.15% at 17.6 MeV) while the coincident 11.67 and 4.4 MeV γs are used to measure the relative timing of the calorimeter and the spectrometer timing counters (σ_delta_t=0.450±0.015ns).

We present a new result based on an analysis of the data collected by the MEG detector at the Paul Scherrer Institut in 2009 and 2010, in search of the lepton-flavor-violating decay μ→eγ. The likelihood analysis of the combined data sample, which corresponds to a total of 1.8×10^14 muon decays, gives a 90% C.L. upper limit of 2.4×10^-12 on the branching ratio of the μ→eγ decay, constituting the most stringent limit on the existence of this decay to date.

Tokyo group has performed first underground dark matter search experiment in 2001 through 2002 at Kamioka Observatory(2700m.w.e). The detector is eight LiF bolometers with total mass 168g aiming for the direct detection of WIMPs via spin-dependent interaction. With a total exposure of 4.1 kg days, we derived the limits in the a_p-a_n (WIMP-nucleon couplings) plane and excluded a large part of the parameter space allowed by the UKDMC experiment., 15 pages, 5 figures

A search for the decay μ+→e+γ, performed at PSI and based on data from the initial three months of operation of the MEG experiment, yields an upper limit on the branching ratio of BR(μ+→e+γ)2.8×10^−11 (90% C.L.). This corresponds to the measurement of positrons and photons from ~10^14 stopped μ+ decays by means of a superconducting positron spectrometer and a 900 litre liquid xenon photon detector.

We succeeded in separating the STJ upper electrode events from other electrode events. This study is for X-ray spectroscopy of slow ions. With a SQUID amplifier, it is clearly seen that the upper electrode signal has a sharp peak with a narrow base, while the other has a relatively gentle peak with a wide base. The difference in signal width was used to separate the upper electrode events from other electrode events. Then the X-ray spectroscopy of slow highly charged ions was carried out at RIKEN. The results are shown here.

The superconducting tunnel junction (STJ) is one of the most promising devices to use as a fast detector for heavy ions. The energy deposited due to the passage of a heavy ion through an STJ forms a region called a hot spot, where the superconductivity of the region is broken. As a result, a reduction of the critical current (I/sub c/) in the STJ occurs. If the bias current exceeds this reduced I/sub c/, the output voltage from the STJ switches from 0 V to its gap voltage, which is recognizable as a signal due to the heavy ion passage. Nb-based STJs were fabricated for this heavy ion detector and an experiment was performed at the RIKEN Ring Cyclotron Facility to investigate the response to heavy ions. Instantaneous switching to the voltage state of the STJ in response to the decrease in I/sub c/ induced by a heavy ion beam was successfully observed by introducing /sup 40/Ar particles with a kinetic energy of 95 MeV/nucleon into the STJ.

Instantaneous switching to the voltage state of a superconducting tunnel junction (STJ) following a decrease in the critical current induced by a heavy ion beam was observed by introducing 40Ar particles with a kinetic energy of 95 MeV/nucleon into an Nb/Al–AlOx/Nb STJ. The particle detection by using this rapid switching of STJ could realize fast timing measurements at the level of ps.

We are developing high-resolution X-ray detectors based on superconducting tunnel junctions (STJs). A two-stage superconducting quantum-interference device (SQUID) amplifier with high gain, low noise and high sensitivity was employed as a readout electronic device. We have developed the STJ readout circuit which makes it possible to optimize the STJ bias condition without disturbing the operating point of the SQUID amplifier. High-resolution X-ray detectors using our system is discussed.

X-ray detectors based on Nb/Al/AlO x /Al/Nb superconducting tunnel junctions (STJ) have been fabricated. X-ray responses of the fabricated STJs were measured by illumination with X-rays from a 55 Fe electron capture source. The energy resolution of 41, 58 and 65 eV (FWHM) at 5.9 keV were obtained for 20×20, 100×100, 200×200 μm 2 , square-shaped devices, respectively. The X-ray measurements were performed at 0.35 K using a charge-sensitive preamplifier at room temperature. We also discussed the capability to suppress the substrate phonon events using Al 2 O 3 insulation layer. An advantage for the combined use of the Al 2 O 3 insulation layer and the sapphire substrate was demonstrated.

Superconducting tunnel junctions (STJ), with a buffer layer between the silicon substrate and junction, are being developed for use as high-resolution X-ray detectors. Aluminum-oxide (Al/sub 2/O/sub 3/) is employed as the buffer layer in order to suppress the phonon mediated background from the Si substrate. The extent of phonon insulation was studied by measuring the X-ray spectra of STJs as a function of buffer layer thickness. The phonon insulation ability of Al/sub 2/O/sub 3/ was compared to that of magnesium-oxide. The Al/sub 2/O/sub 3/ layer should be a good phonon insulator, with the ability to suppress phonons with a single buffer layer.

A compact system for an STJ-based high resolution X-ray detection system using a SQUID amplifier has been developed. A noise level of less than 4.8 mV at the SQUID output pulse height of 1.2 V for Nb-based STJ signals was achieved in the temperature range of 1.7-4.2 K with good reproducibility under an external magnetic field of about 0.01 Tesla, which was applied to suppress the Josephson current of the STJ. The results show the possibility of constructing a compact STJ-based X-ray detection system with an 'on-site' low noise SQUID preamplifier.

The performance of a 21-g lithium fluoride bolometer is presented. The background spectrum was measured in the surface laboratory. We derive an exclusion plot for the spin-dependent coupled Weakly Interacting Massive Particles (WIMPs) cross section.

Bolometers are being developed at the University of Tokyo aiming at a search for galactic dark matter. We have succeeded in constructing a small-sized bolometer with 2.8 gram of lithium fluoride crystal with an rms energy resolution of 3.8 ke V for 60-ke V γ rays. Both lithium and fluorine are expected to be suitable for the detection of possible particle dark matter with a spin-dependent interaction like supersymmetric neutralinos. Fabrication of Neutron Transmutation Doped (NTD) germanium is also in progress to make a number of thermistors with uniform sensitivities to be used for multi-module bolometers. When the development of a full-sized detector with a total mass of order one kilogram is completed, we plan to install it in the Kamioka underground laboratory.

We are developing a liquid xenon calorimeter for the MEG experiment. This experiment, to be performed at the Paul Scherrer Institut (PSI) in Villigen (CH), was designed by an Italian-Japanese-Russian-Swiss collaboration to search for the /spl mu//sup +//spl rarr/e/sup +//spl gamma/ decay, forbidden in the standard model of electroweak and strong interactions. The photon four momentum is to be measured by an innovative C-shaped 800 litres liquid xenon calorimeter read by more than 800 photo-multiplier tubes. An absorption length for Xe scintillation light larger than 100 cm has been measured. The performance of such a detector has been measured using a large volume prototype (100 litres) in a test beam at PSI, using high energy photons from decays of neutral pions from a charge exchange reaction of negative pions on protons. A resolution of 5% FWHM was obtained.

A cryogenic thermal detector with a 2.8 g LiF crystal is constructed. A photoelectric peak of 60 keV γ-rays is clearly seen with LiF crystal with an RMS energy resolution of 3.8 keV. LiF is suitable for the detection of possible particle dark matter with a spin-dependent interaction like neutralinos since both Li and F are expected to have high interaction rates with such particles.

An experiment to search for hypothetical particle dark matter using cryogenic thermal detector, or bolometer is ongoing. The bolometer consists of eight pieces of 21 g LiF absorbers and sensitive NTD germanium thermistors attached to them and is installed in the Nokogiriyama underground cell which is a shallow depth site ($\sim 15$ m w.e.). We report on the results from the first running for about ten days using this arrayed bolometer system together with appropriate shieldings and muon veto counters. From the obtained energy spectra the exclusion limits for the cross section of the elastic neutralino-proton scattering are derived under commonly accepted astrophysical assumptions. The sensitivity for the light neutralino with a mass below 5 GeV is improved by this work., 8 pages, Revtex, 4 figures

Sensitive n-type and p-type germanium thermistors were fabricated by the melt doping technique and by the neutron transmutation doping (NTD) technique, respectively, aiming at a use for the cryogenic thermal detector, or bolometer of Tokyo dark matter search programme. We report on the measurements of the sensitivities of these thermistors. In particular, the p-type thermistors are sensitive enough to scale up our existing prototype LiF bolometer and realize a multiple array of the bolometers with the total absorber mass of about 1\,kg., Comment: 14 pages, revtex, with 4 postscript figures appended (uuencoded)

Niobium-based superconducting tunnel junctions (STJs) with aluminum trapping layers as X-ray detectors were fabricated. We applied a fabrication technique for reducing the leakage current and improving the energy resolution for X-rays at around 6 keV. The edges of two Al layers sandwiching a tunneling barrier were oxidized by plasma discharge. The performance of these new STJs was compared with other types of STJs to which thermal oxidization in an O2 atmosphere had been applied. The oxidization process in O2 plasma significantly improved the energy resolution for X-rays, which reached values (FWHM) of 41 eV, 58 eV, 65 eV, and 129 eV for STJs with the size of 20×20 µm2, 100×100 µm2, 200×200 µm2, and 500×500 µm2, respectively.

The spectra of substrate phonon events induced by illuminating with X-rays obtained with Nb-based superconducting tunnel junctions (STJs) fabricated on a sapphire substrate were examined. We constructed a geometrical model in which the collection efficiency of phonons is proportional to the solid angle subtended to the STJ from the position of the absorption. By using the Monte-Carlo simulation, we succeeded in fairly reproducing the features of the spectra obtained by our measurements. A comparison of the model spectra with the measured ones revealed that a mismatch of the acoustic impedance at the junction-substrate interface and the spatial spread of a phonon cloud play an important role in the determination of the spectra. By including these effects, we show that the measured spectra can be well reproduced by adjusting only one free parameter, the radius of the phonon cloud. We estimated this radius in a sapphire substrate to be ∼20 µm under 5.9 keV X-ray illumination.

The MEG (Mu to Electron Gamma) experiment has been running at the Paul Scherrer Institut (PSI), Switzerland since 2008 to search for the decay μ +→e+ γ by using one of the most intense continuous μ + beams in the world. This paper presents the MEG components: the positron spectrometer, including a thin target, a superconducting magnet, a set of drift chambers for measuring the muon decay vertex and the positron momentum, a timing counter for measuring the positron time, and a liquid xenon detector for measuring the photon energy, position and time. The trigger system, the read-out electronics and the data acquisition system are also presented in detail. The paper is completed with a description of the equipment and techniques developed for the calibration in time and energy and the simulation of the whole apparatus., The European Physical Journal C, 73 (4), ISSN:1434-6044, ISSN:1434-6052