Y. Yatsu

Recent progress of avalanche photodiodes in high-resolution X-rays and γ-rays detection

We have studied the performance of large area avalanche photodiodes (APDs) recently developed by Hamamatsu Photonics K.K, in high-resolution X-rays and Gamma-rays detections. We show that reach-through APD can be an excellent soft X-ray detector operating at room temperature or moderately cooled environment. We obtain the best energy resolution ever achieved with APDs, 6.4 % for 5.9 keV X-rays, and obtain the energy threshold as low as 0.5 keV measured at -20deg. Thanks to its fast timing response, signal carriers in the APD device are collected within a short time interval of 1.9 nsec (FWHM). This type of APDs can therefore be used as a low-energy, high-counting particle monitor onboard the forthcoming Pico-satellite Cute1.7. As a scintillation photon detector, reverse-type APDs have a good advantage of reducing the dark noise significantly. The best FWHM energy resolutions of 9.4+-0.3 % and 4.9+-0.2 % were obtained for 59.5 keV and 662 keV Gamma-rays, respectively, as measured with a CsI(Tl) crystal. Combination of APDs with various other scintillators (BGO, GSO, and YAP) also showed better results than that obtained with a photomultiplier tube (PMT). These results suggest that APD could be a promising device for replacing traditional PMT usage in some applications. In particular 2-dim APD array, which we present in this paper, will be a promising device for a wide-band X-ray and Gamma-ray imaging detector in future space research and nuclear medicine.

Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar

The Fermi γ-ray Space Telescope has revolutionized our knowledge of the γ-ray pulsar population, leading to the discovery of almost 100 γ-ray pulsars and dozens of γ-ray millisecond pulsars (MSPs). Although the outer-gap model predicts different sites of emission for the radio and γ-ray pulsars, until now all of the known γ-ray MSPs have been visible in the radio. Here we report the discovery of a radio-quiet γ-ray-emitting MSP candidate by using Fermi, Chandra, Swift, and optical observations. The X-ray and γ-ray properties of the source are consistent with known γ-ray pulsars. We also found a 4.63 hr orbital period in optical and X-ray data. We suggest that the source is a black widow-like MSP with a ~0.1 M ☉ late-type companion star. Based on the profile of the optical and X-ray light curves, the companion star is believed to be heated by the pulsar while the X-ray emissions originate from pulsar magnetosphere and/or from intrabinary shock. No radio detection of the source has been reported yet, and although no γ-ray/radio pulsation has been found we estimate that the spin period of the MSP is ~3-5 ms based on the inferred γ-ray luminosity.

An active gain-control system for Avalanche photo-diodes under moderate temperature variations

Avalanche photodiodes (APDs) are promising light sensor for various fields of experimental physics. It has been argued, however, that variation of APD gain with temperature could be a serious problem preventing APDs from replacing traditional photomultiplier tubes (PMTs) in some applications. Here we develop an active gain-control system to keep the APD gain stable under moderate temperature variations. As a performance demonstration of the proposed system, we have tested the response of a scintillation photon detector consisting of a 5x5 mm^2 reverse-type APD optically coupled with a CsI(Tl) crystal. We show that the APD gain was successfully controlled under a temperature variation of DT = 20deg, within a time-cycle of 6000 sec. The best FWHM energy resolution of 6.1+-0.2 % was obtained for 662 keV gamma-rays, and the energy threshold was as low as 6.5 keV, by integrating data from +20deg - 0deg cycles. The corresponding values for -20deg - 0deg cycles were 6.9+-0.2 % and 5.2 keV, respectively. These results are comparable, or only slightly worse than that obtained at a fixed temperature. Our results suggest new potential uses for APDs in various space researches and nuclear physics. As examples, we briefly introduce the NeXT and Cute-1.7 satellite missions that will carry the APDs as scientific instruments for the first time.

Expected radiation damage of reverse-type APDs for the Astro-H mission

Scheduled for launch in 2014, Astro-H is the sixth Japanese X-ray astronomy satellite mission. More than 60 silicon avalanche photodiodes (Si-APDs; hereafter APDs) will be used to read out BGO scintillators, which are implemented to generate a veto signal to reduce background contamination for the hard X-ray imager (HXI) and a soft gamma-ray detector (SGD). To date, however, APDs have rarely been used in space experiments. Moreover, strict environmental tests are necessary to guarantee APD performance for missions expected to extend beyond five years. The radiation hardness of APDs, as for most semiconductors, is particularly crucial, since radiation in the space environment is severe. In this paper, we present the results of radiation tests conducted on reverse-type APDs (provided by Hamamatsu Photonics) irradiated by gamma rays (60Co) and 150 MeV protons. We show that, even under the same 100 Gy dose, high energy protons can cause displacement (bulk) damage in the depletion region and possibly change the activation energy, whereas gamma-ray irradiation is less prone to cause damage, because ionization damage dominates only the surface region. We also present quantitative guidance on how to estimate APD noise deterioration over a range of temperatures and radiation doses. As a practical example, we discuss the expected degradation of the BGO energy threshold for the generation of veto signals, following several years of Astro-H operation in Low Earth Orbit (LEO), and directly compare it to experimental results obtained using a small BGO crystal.

Earliest Detection of the Optical Afterglow of GRB 030329 and its Variability

We report the earliest detection of an extremely bright optical afterglow of the gamma‐ray burst GRB 030329 using a 30 cm telescope at the Tokyo Institute of Technology (Tokyo, Japan). Our observation started 67 minutes after the burst and continued for two succeeding nights. Combining our data with those reported in GCN Circulars, we find that the early afterglow light curve of the first half day is described by a broken power‐law (∝ t−α) function with indices α1 = 0.88 ± 0.01 (0.047 days < t < tb1), α2 = 1.18 ± 0.01 (tb1 < t < tb2), and α3 = 1.81 ± 0.04 (tb2 < t < 1.2 days), where tb1 ∼ 0.26 days and tb2 ∼ 0.54 days, respectively. The change of the power‐law index at the first break at t ∼ 0.26 days is consistent with that expected from a “cooling break”.

Development of X-ray/gamma-ray imaging spectrometers using reach-through APD arrays

We present spectroscopic capability of a position sensitive detector using a large area reach-through avalanche photodiode (APD) array, mainly for astronomical applications. It is quite important to obtain wide band spectra of high energy astrophysical phenomena simultaneously in order to probe emission processes or structures. Especially observations of transient objects, such as gamma-ray bursts of active galactic nuclei, require detectors with wide energy band coverage for the sake of an efficient spectroscopy within limited time windows. An APD is a compact semiconductor photon sensor with an internal gain which is often up to ~ 100. A reach-through type APD has a thicker depletion layer thus higher efficiency for direct X-ray detection compared to a reverse type APD. We have developed 1-dimensional reach-through APD arrays which consist of 8 and 16 segments with a pixel size of 2.2 × 16 and 1.1 × 16 mm2. We demonstrated quite uniform gain and energy resolution for 5.9 keV X-ray over the pixels of these arrays. Subsequently we constructed X-ray/gamma-ray detector using the APD array optically coupled to a conventional CsI(Tl) scintillator which demonstrated energy coverage typically from 1 keV to 1 MeV.

The small satellite "tsubame" for polarimetry of GRBs

“Tsubame” is a university‐built small satellite mission that measures polarization of hard X‐ray photons (30–100 keV) from gamma‐ray bursts (GRB) using azimuthal angle anisotropy of Compton scattering. The satellite is intended for a piggy‐back launch on the H‐IIA rocket. It uses Control moment gyros to quickly orient itself to GRBs within 15 seconds from triggers. We are now developing the engineering model of the two detector systems on Tsubame, the polarimeter and the bust monitor. In this paper, we present an overview of the Tsubame mission, and detail of development of engineering model of our polarimeter and burst monitor.

The Observation Campaign of SS 433 in April 2006

A radio-IR-optical-X-ray observation campaign of SS 433 has been performed in April 2006, when the jet axis is almost perpendicular to the line of sight. Five flares have been detected during the campaign by radio monitoring observation with RATAN-600. The X-ray astronomical satellite Suzaku observed the source in and out of eclipse. In the X-ray data out of eclipse, the flux shows a significant variation with a time scale of hours. The source seems to be in the active state during the campaign. The observation logs and preliminary results are presented.

In-orbit performance of avalanche photodiode as radiation detector onboard a pico-satellite Cute-1.7+APD II

Cute-1.7+APD II is the third pico-satellite developed by students at the Tokyo Institute of Technology. One of the primary goals of the mission is to validate the use of avalanche photodiodes (APDs) as a radiation detector for the first time in a space experiment. The satellite was successfully launched by an ISRO PSLV-C9 rocket in Apr 2008 and has since been in operation for more than 20 months. Cute-1.7+APD II carries two reversetype APDs to monitor the distribution of low energy particles down to 9.2 keV trapped in a Low Earth Orbit (LEO), including South Atlantic Anomaly (SAA) as well as aurora bands. We present the design parameters and various preflight tests of the APDs prior to launch, particularly, the high counting response and active gain control system for the Cute-1.7+APD II mission. Examples of electron/proton distribution, obtained in continuous 12-hour observations, will be presented to demonstrate the initial flight performance of the APDs in orbit.

Development of the X-ray polarimetry small satellite "TSUBAME"

TSUBAME is a university‐built small satellite mission to measure polarization of hard X‐ray photons (30–100 keV) from gamma‐ray bursts using azimuthal angle anisotropy of Compton‐scattered photons. Polarimetry in the hard X‐ray and soft gamma‐ray band should play a crucial role in the understanding of high energy emission mechanisms and the distribution of magnetic fields and radiation fields. TSUBAME has two instruments: the Wide‐field Bust Monitor (WBM) and the Hard X‐ray Compton Polarimeter (HXCP). The WBM detects a burst and determines on board the direction of the burst occurrence with an accuracy of 10 degrees. The spacecraft is then slewed to the GRB in 15 seconds from the trigger using CMG, a high speed attitude control device. HXCP will measure the polarized X‐ray photons from the GRB while the spacecraft is slowly spinning around the bore sight. In this paper, we present an overview of the TSUBAME mission, its expected performance of X‐ray polarization measurement based on Monte Carlo simulation...