S. Miyazaki

Hard X-Ray Emission from the Galactic Ridge

Hard X-ray and γ-ray emissions from the Galactic ridge were studied with the large area proportional counter (LAC) on the Ginga satellite and a balloon-borne detector Welcome-1. In the scanning observations with the LAC, diffuse hard X-rays were detected along the Galactic plane between l = -20° and l = 40°. The measured spectrum shows that a hard component exists in the Galactic ridge emission above 10 keV, in addition to the hot plasma component. The estimated luminosity of the Galactic ridge emission is around 2 × 1038 ergs s-1 in the 3-16 keV band. Welcome-1 observed the γ-ray ridge emission at around l ~ 345° between 50 and 600 keV. These two results and a recent COMPTEL study suggest that the spectrum of the diffuse Galactic ridge emission extends over the keV-MeV range. From the observed spectral slope, bremsstrahlung by electrons is shown to be the dominant emission mechanism. This implies that low-energy electrons must be supplied continuously to sustain emission in the hard X-ray band. We propose a scenario in which the thermal electrons in the hot plasma responsible for the strong Fe K X-ray emission are shock-accelerated continuously in supernova remnants (SNRs), producing the observed hard X-ray and γ-ray emissions from the Galactic ridge.

Well-type phoswich counter for low-flux X-ray/ gamma -ray detection

Novel phoswich counters have been developed that are capable of detecting low flux hard X-rays gamma -rays from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator, hard X-rays/ gamma -rays that have deposited energy only in the detection part can be selected. The first model counter was built by using a new scintillator, GSO, in the detection part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area approximately 740 cm/sup 2/) was flown on board a balloon, setting a limit to the /sup 57/Co line flux from SN 1987A at around 10/sup -4//cm/sup 2/-s. The sensitivity for continuum flux was around a few*10/sup -6//cm/sup 2/-s-keV between 100 and 200 keV. >

Use of a large area photodiode in CsI(Tl) scintillation counters

Abstract A newly developed large area photodiode has been studied as the photon sensor for medium-size CsI(Tl) scintillation counters. The photodiode has a sensitive area of 18 × 18 mm 2 and a thickness of 500 μm. When combined with a CsI(Tl) crystal of 2.5 × 2.5 × 5.0 cm 3 , it gives an energy resolution of around 5.1% (FWHM) for 1.33 MeV gamma-rays. Characteristics of the low noise preamplifier designed for the CsI(Tl)-photodiode counter will also be given.

Newly developed low background hard X-ray/gamma-ray telescope with the well-type phoswich counters

A low background hard X-ray/gamma-ray telescope (Welcome-1) for balloon-borne experiments in the energy range from 40 keV to 800-1000 keV is discussed. The detector is based on newly developed well-type phoswich counters. In the first well-type phoswich counter, GSO(Ce) (Gd/sub 2/SiO/sub 5/ doped with Ce) is used as the detection part and CsI(Tl) as the shielding part. Welcome-1 consists of 64 GSO/CsI well-type phoswich counters assembled in the compound-eye configuration, and the effective area is 740 cm/sup 2/ at 122 keV and 222 cm/sup 2/ at 511 keV line. The well-type phoswich counter and the compound eye configuration reduce background significantly both externally and internally, and allow a high signal-to-noise ratio in balloon-borne experiments. The background levels at an attitude of 4.5 g/cm/sup 2/ are 1*10/sup -4//cm/sup 2//s/keV at 122 keV. Crab nebula is observed with the signal to background ratio better than unity between 100 and 200 keV. The design of the telescope and the flight performance are presented. >

Well-type phoswich counters for low-flux x-ray/gamma-ray detection

Novel phoswich counters have been developed that are capable of detecting low flux hard X-rays gamma -rays from localized sources. The counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). The well-shaped shielding part acts as an active collimator as well as an active shield. The whole assembly is viewed by a phototube from the exterior bottom surface of the shielding part. By using an appropriate pulse-shape discriminator, hard X-rays/ gamma -rays that have deposited energy only in the detection part can be selected. The first model counter was built by using a new scintillator, GSO, in the detection part and CsI(Tl) in the shielding part. A detector system consisting of 64 such phoswich counters (total area approximately 740 cm/sup 2/) was flown on board a balloon, setting a limit to the /sup 57/Co line flux from SN 1987A at around 10/sup -4//cm/sup 2/-s. The sensitivity for continuum flux was around a few*10/sup -6//cm/sup 2/-s-keV between 100 and 200 keV.<<ETX>>

A simple pulse shape discrimination method for the phoswich counter

A simple pulse shape discriminator (PSD) has been developed for applications where a large number of channels are used within limited space and electric power. In the PSD, the output signal from a photomultiplier is differentiated and integrated to form a triple-polar pulse. The time interval between the first and second zero crossings depends on the decay time of the scintillation light. The circuit selects signals from the proper scintillator by using this interval. One can significantly reduce the effect of time slewing in the low energy region using this method. Sixty-four channels of the PSD have been successfully operated in a balloon-borne hard X-ray experiment.<<ETX>>

A limit on the Co-57 gamma-ray flux from SN 1987A

We studied the gamma-ray flux from SN 1987A with a balloon-borne detector in Brazil on November 29, 1990. By comparing on- and off-source data, we obtained limits to the total gamma-ray flux due to Co-57 decays and presumed pulsar activity. If the pulsar contribution is neglected, an upper limit is set to the total Co-57 contribution above 60 keV at 2.7 x 10 exp 4/sq cm/s where the 122 keV line contribution is 1.0 x 10 exp 4/sq cm/s. The corresponding limit on the abundance ratio of Co-57 and Co-56 is 3.4 times that of the sun. The present upper limit implies that the reported leveling off in the light curve of SN 1987A is primarily due to the pulsar activity. If the pulsar is Crab-like, the light curve requires an energy outflow greater than 0.95 x 10 exp 37 ergs/s (above 1 keV) now being supplied to SN 1987A by its presumed pulsar. 23 refs.

New hard X-ray/gamma-ray telescope - Welcome-1

We have developed a low background hard X-ray/gamma-ray telescope for balloon-borne experiments. The telescope called Welcome-1 (Well type Compound Eye) utilizes newly developed well-type phoswich counters. In the well-type phoswich counter, the background from external and internal sources are reduced significantly, Welcome-1 is designed for observation in the energy range from 60 keV to 800-1000 keV. The effective area of Welcome-1 is 740 sq cm at 122 keV and 222 sq cm at 511 keV line. We flew Welcome-1 in 1990 and 1991 to detect hard X-rays from SN1987A, PSR1509-58, Cen-A and others. The background levels at an altitude of 4g/sq cm are 1 x 10 exp -4/sq cm photons/s/keV at 122 keV and 3 x 10 exp -5 photons/sq cm/s/keV at 511 keV. The data obtained during the flight shows that the detector in fact has the 3 sigma sensitivity of about a few x 10 exp -6 photons/sq cm/s/keV and about 10 exp -4 photons/sq cm/s in a 10 exp 4 s observation for the continuum spectrum and line spectrum, respectively.

Hard x‐ray observation of Cen A and a break in the energy spectrum

We studied hard X‐ray/γ‐ray emission from Cen A (NGC 5128) in a balloon experiment with a low background detector (Welcome‐1) in Brazil. The energy spectrum of CenA is obtained from 40 keV to 600 keV. We combined the energy spectrum obtained by the Ginga satellite in a similar state. The combined spectrum indicates that there is a break at 185±22 keV. The spectrum is fitted to the broken power law model with the photon index of α1=−1.79 up to 188 keV, and α2=−3.7−1.7+0.9 above the break.

Detector to study low-flux hard X-ray/gamma-ray sources

Abstract We have developed a new kind of phoswich counters that will be capable of detecting low flux hard X-rays/gamma-rays from astronomical objects. The new phoswich counter consists of a small inorganic scintillator with a fast decay time (the detection part) glued to the interior bottom surface of a rectangular well-shaped block of another inorganic scintillator with a slow decay time (the shielding part). Here, the well-shaped shielding part acts as an active collimator as well as an active shield. We have built a detector system consisting of 64 such phoswich counters: newly developed scintillator (GSO) is used for the detection part and CsI( Tl ) is used for the shielding part. The total geometrical area of the 64 detection parts is about 740cm 2 and its 3σ sensitivity is expected to reach below 10 −5 cm −2 s −1 keV −1 up to 700keV. With several improvements such detectors will be able to detect hard X-rays/gamma-rays at a flux level around 10 −6 cm −2 s −1 keV −1 upto 2 MeV.