Takeshi Takashima

We have been discussing a Jovian mission in 2020's under the cooperation between Europe and Japan. It is the same framework as Mercury mission BepiColombo. The proposed mission involves two Jovian orbiters and one satellite orbiter.

We are developing a Compton telescope based on high-resolution Si and CdTe detectors for astrophysical observations in sub-MeV/MeV gamma-ray region. Recently, we constructed a prototype Compton telescope which consists of six layers of double-sided Si strip detectors (DSSDs) and CdTe pixel detectors to demonstrate the basic performance of this new technology. By irradiating the detector with gamma rays from radio isotope sources, we have succeeded in Compton reconstruction of images and spectra. The obtained angular resolution is 3.9°(FWHM) at 511 keV, and the energy resolution is 14 keV (FWHM) at the same energy. In addition to the conventional Compton reconstruction, i.e., drawing cones in the sky, we also demonstrated a full reconstruction by tracking Compton recoil electrons using the signals detected in successive Si layers. By irradiating 137Cs source, we successfully obtained an image and a spectrum of 662 keV line emission with this method. As a next step, development of larger DSSDs with a size of 4 cm × 4 cm is under way to improve the effective area of the Compton telescope. We are also developing a new low-noise analog ASIC to handle the increasing number of channels. Initial results from these two new technologies are presented in this paper as well. © 2006 Elsevier B.V. All rights reserved.

We have tested APDs (Type spl 3989 and Z7966, Hamamatsu Photonics K.K.) using an electron beam. The Z7966, which has a depletion layer of 10 μ m, is firstly focused on and tested in our former paper [K. Ogasawara, K. Asamura, T. Mukai, Y. Saito, Nucl. Instr. and Meth. A 545(3) (2005) 744] for the energy range of 5-20 keV. The result shows that the pulse height distribution of the APD signal exhibits a significant peak for electrons with energies above 8 keV, and positions of their peaks show a good linearity. The condition of the peak production at energies below 8 keV was attributed to the thickness of the dead layer on the surface of APDs. Now we have tuned up our electron acceleration system up to 40 keV, and tested Z7966 by electrons of higher energies. The result shows that the output pulse height distributions of this Z7966 were distorted over 30 keV. In order to examine the distortion of pulse height distributions, we have made a Monte Carlo numerical simulation of particle transport inside the APD. The result shows that the highest energy limit is expected to be determined by the thickness of the depletion layer inside the APD. Therefore, we have tried an APD type spl 3989, which has a thicker depletion layer (30 μ m) and a thinner dead layer. As is expected, the spl 3989 responded to 2-40 keV electrons with fine peaks in the output pulse height distributions. The energy resolution was lower than 1 keV for 2-20 keV electrons, and 5 keV for 40 keV electrons. The linearity of the response was also good. According to the Monte Carlo simulation, electrons up to about 60 keV are expected to be well detectable. © 2006 Elsevier B.V. All rights reserved.

In order to operate an "improved" Fourier synthesis imager at higher photon energies than are accessible with solidstate detectors, we developed a new one-dimensional gamma-ray position sensor consisting of inorganic scintillators whose light is read out by a silicon strip detector (SSD). The SSD is read out through stripped p-electrodes (DC-coupled, 32 channels at 400 mu m pitch). The n-side of the SSD does not have a metallic surface, and has a relatively good optical transparancy. A stack of optically isolated thin CsI(TI) scintillator plates (10 mm x 10 mm in area and 0.3 mm thick) is attached, edge-on, to the n-side of the SSD. When a gamma-ray photon hits the stacked scintillator, the generated scintillation light is detected by the SSD, with the signal pulse-height largest in the p-electrode which is just beneath the CsI plate hit by the gamma-ray. The 32-channel outputs from the SSD are acquired simultaneously for each event with a low-noise analog ASIC. The gamma-ray position can be calculated as a weighted mean of pulse heights. Using this system, we have obtained a position resolution significantly finer than 1 mm for gamma-rays with energies greater than similar to 500 keV up to similar to 1300 keV.

Diamonds have attractive properties for use as radiation detectors. Recent advances in diamond fabrication techniques have made it possible to produce almost perfect single crystal materials that have electrical properties suitable for radiation detectors. We therefore developed detectors made of high-purity synthetic IIa diamonds grown by a high-pressure, high-temperature method and investigated their basic properties. These detectors have Schottky and Ohmic electrodes, and the leakage current is less than 20 pA with reverse bias voltages up to 2.4 kV, demonstrating that the detector withstands high bias voltage. The results of testing the electrical properties show that the electrodes work well. The best detector achieved an energy resolution of 15.4 keV (FWHM) for 5.486 MeV alpha particles from (241) Am.

Alpha-ray detector (ARD) will be on-board SELENE, a Japanese lunar orbiter to be launched around 2006. Primary target is the alpha particles emitted by Rn-222 and Po-210. Rn-222 is produced by the decay of U-238 and emanates from the lunar surface. It is trapped by the lunar gravity and decays with the half-life of 3.8 days emitting 5.490 MeV alpha particle. In the decay sequence of Rn-222, Po-210 emits alpha particle with the energy of 5.305 MeV. Time scale of the activity is dominated by the 21-year half-life of Pb-210. Thus, alpha particle intensity from Po-210 is an indicator of the change of radon emanation rate and change of crust condition due to seismic activity or impact events for the time scale of similar to 50 years, while that of Rn-222 reflects the current emanation rate. Results from Apollo 15, 16, and recent Lunar Prospector mission indicate that the average amount of radon on the lunar surface is much smaller than expected, and the radon-alpha distribution suggests that radon comes out through gas emanation from fissures of the lunar surface. We developed a large-area detector of 326 cm(2) for the ARD, which is 15-20 times larger than the detectors of Apollo and Lunar Prospector. Reduction of the background was achieved with the anti-coincidence by rejecting cosmic-ray tracks. It will enable: (1) precise global mapping of the radioactive material on the lunar surface; (2) identification of gas emanation location; (3) study of the radon gas emanation mechanism on the lunar surface and the origin of the lunar atmosphere; (4) obtaining information on the crustal movement during the last similar to 50 years. (C) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

We have developed a new electrostatic analyzer which enables medium energy (200 keVq) plasma particle measurements with full solid angle coverage. The design of the test model realizes the uppermost measurement energy of ∼200 keVq with applied high voltages of ±5 kV. Laboratory experiments with the test model analyzer show that its performance agrees with numerical simulations. The test model design is well suited for combination with a mass analysis unit, while our new design can also be applied to medium energy electron measurements. Medium energy ion/electron sensors with this new design will surely be appreciated for upcoming space missions that will observe hot/energetic plasma structures in the regions such as the inner magnetosphere or reconnection region. © 2006 American Institute of Physics.

This paper reports on properties of energetic electrons observed by the Auroral Particle Detector (APD) on board the sounding rocket S-310-35, which was launched from Andøya Rocket Range, Norway, at 0033:00 UT on 13 December 2004 during the DELTA campaign. The APD was designed to measure energy spectra of energetic electrons in the range of 3.5 to 65 keV every 10 ms using avalanche photodiodes. The measurement was done at altitudes of 90-140 km (apogee height of the rocket flight), which corresponded to the collisional interaction region of precipitating electrons with the atmospheric constituents. The overall profile of energetic electron precipitations was consistent with auroral images taken from the ground. The downward fluxes almost always exceeded those of upward electrons, and the ratio of downward to upward fluxes increased with energy and also with altitude. This is reasonably understood in terms of the effect of collisions between the energetic electrons and the atmospheric constituents. An interesting feature in energy spectra of precipitating electrons is the existence of non-thermal electrons at higher energies, regardless of inside or outside of auroral arcs. In order to predict the incident downward spectra at the top of the atmosphere, we have applied an analytic method of Luhmann (1976) to evaluate the collisional effect on the electron spectra. As a result, most of the observed energy spectra of precipitating electrons are well expressed by kappa distributions with the thermal energy of a few hundreds of eV and kappa of 5-8, while the spectrum inside a strong arc is better fitted by the sum of a Maxwellian distribution on the lower energy side and a power law at higher energies. To the authors' knowledge, this is the first direct and reliable measurement of energy spectra of electrons in the 10-keV energy range in the auroral ionosphere. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS) The Seismological Society of Japan The Volcanological Society of Japan The Geodetic Society of Japan The Japanese Society for Planetary Sciences TERRAPUB.

The Earth's inner magnetosphere (inside 10 Re) is a region where particle energy increases to the relativistic energy range. This region is very important as a laboratory where high-energy particle acceleration can be directly measured in a dipolar field configuration, as well as for human activities in space including space weather prediction. Despite abundant in situ satellite measurements, this region has been "missing" because of several difficulties arising from the measurements, such as high-energy particle contamination of low-energy particle measurement, protection against the possible incidence of radiation belt particles on the satellite, and the difficulties of measuring three-dimensional particles over a broad energy range, from a few electron volts to more than 10 MeV. In this paper, we address important scientific topics and propose a possible configuration of small satellites termed Energization and Radiation in Geospace (ERG), which would provide new insights into the dynamics of the inner magnetosphere and strongly contribute to the International Living With a Star project. © 2005 COSPAR.

As the detection part of an improved Fourier Synthesis imager, we developed a new one-dimensional γ-ray position sensor consisting of inorganic scintillators whose lights are read out by a silicon strip detector (SSD). The employed SSD has no metal anode, with its Al-cathode divided into 32 channels of 400 μm pitch. Electrodes of all channels are Albonded to a low noise analog ASIC, so that signals can be read out from all channels simultaneously. We fabricated a "stacked CsI scintillator", consisting of 20 CsI(Tl) scintillator plates each having a dimension of 10 mm × 10 mm × 0.3 mm, and 0.07 mm-thick light reflective sheets inserted between adjacent CsI plates. Coupling the scintillator and the SSD, γ-ray with energies ≳ 500 keV were successfully detected, with a position resolution of ∼0.37 mm. © 2005 IEEE.

We are developing low noise analog ASICs for hard X-ray and gamma-ray astronomy. They are based on a sub-micron CMOS process. Our design includes a finely-adjustable high-resistance circuit, which, for example, enables us to realize a pole-zero-cancellation in the analog chain. A two-dimensional ASIC with a fast read-out scheme and a one-dimensional ASIC with an improved analog design are developed and verified. With the latter ASIC connected with a CdTe detector, we have obtained a radioactive source spectrum with an energy resolution of 4.6 keV (FWHM) at 59.5 keV. © 2005 IEEE.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six layered double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 122 keV to 662 keV. The energy resolution is 9.1 keV and 14 keV at 356 keV and 511 keV, respectively. © 2005 IEEE.

The massive flare of 27 December 2004 from the soft γ-ray repeater SGR 1806-20, a possible magnetar, saturated almost all γ-ray detectors, meaning that the profile of the pulse was poorly characterized. An accurate profile is essential to determine physically what was happening at the source. Here we report the unsaturated γ-ray profile for the first 600 ms of the flare, with a time resolution of 5.48 ms. The peak of the profile (of the order of 107 photons cm-2s-1) was reached ∼50 ms after the onset of the flare, and was then followed by a gradual decrease with superposed oscillatory modulations possibly representing repeated energy injections with ∼60-ms intervals. The implied total energy is comparable to the stored magnetic energy in a magnetar (∼1047 erg) based on the dipole magnetic field intensity (∼1015 G), suggesting either that the energy release mechanism was extremely efficient or that the interior magnetic field is much stronger than the external dipole field.

Radiation detector made of synthetic diamond has many advantages as high resolution and pulse mode radiation detector. Synthetic diamond detector has been investigated its characteristics with electric tests and radiation sources in laboratories. In order to investigate more detail of its character, high energy heavy ion beams from HIMAC were irradiated to the synthetic diamond detector. Because these beams can penetrate the diamond detector and give it much deposit energy, characteristic at deeper part of synthetic diamond detector can be investigated. Using heavy ion beams like Iron 500 MeV/u, Silicon 800 MeV and so on, saturation curve for bias voltage, polarization effect, which is a phenomenon of decreasing of output signals and energy resolution of a diamond detector for deposit energy were investigated. © 2005 IEEE.

Radiation detector made of synthetic diamond has many advantages as high resolution and pulse mode radiation detector. Synthetic diamond detector has been investigated its characteristics with electric tests and radiation sources in laboratories. In order to investigate more detail of its character, high energy heavy ion beams from HIMAC were irradiated to the synthetic diamond detector. Because these beams can penetrate the diamond detector and give it much deposit energy, characteristic at deeper part of synthetic diamond detector can be investigated. Using heavy ion beams like Iron 500 MeV/u, Silicon 800 MeV and so on, saturation curve for bias voltage, polarization effect, which is a phenomenon of decreasing of output signals and energy resolution of a diamond detector for deposit energy were investigated.

As the detection part of an improved Fourier Synthesis imager, we developed a new one-dimensional gamma-ray position sensor consisting of inorganic scintillators whose lights are read out by a silicon strip detector (SSD). The employed SSD has no metal anode, with its Al-cathode divided into 32 channels of 400 pm pitch. Electrodes of all channels are Al-bonded to a low noise analog ASIC, so that signals can be read out from all channels simultaneously. We fabricated a "stacked CsI scintillator", consisting of 20 CsI(Tl) scintillator plates each having a dimension of 10 rum x 10 mm x 0.3 mm, and 0.07 mm-thick light reflective sheets inserted between adjacent CA plates. Coupling the scintillator and the SSD, gamma-ray with energies greater than or similar to 500 keV were successfully detected, with a position resolution of similar to 0.37 mm.

This paper summarizes the current status of the BepiColombo/MMO (Mercury Magnetospheric Orbiter) spacecraft design. The MMO is a spinning spacecraft of 223 kg mass whose spin axis is nearly perpendicular to the Mercury orbital plane. The current status of the overall MMO system and subsystems such as thermal control, communication, power etc are described. The critical technologies are also outlined. Furthermore, the outline of the international cooperation between JAXA and ESA are also presented.

A Compton camera is the most promising approach for gamma-ray detection in the energy region from several hundred kiloelectronvolts to megaelectronvolts, especially for application in high energy astrophysics. In order to obtain good angular resolution, semiconductor detectors such as silicon, germanium and cadmium telluride(CdTe) have several advantages over scintillation detectors, which have been used so far. Based on the recent advances of high resolution CdTe and silicon imaging detectors, we are working on a Si/CdTe Compton camera. We have developed 64-pixel CdTe detectors with a pixel size of 2 mm x 2 mm and double-sided Si strip detectors(DSSDs) with a position resolution of 800 mum. As a prototype Si/CdTe Compton camera, we use a DSSD as a scatterer and two CdTe pixel detectors as an absorber. In order to verify its performance, we irradiate the camera with 100% linearly polarized 170 keV gamma-rays and demonstrate the system works properly as a Compton camera. The resolution of the reconstructed scattering angle is 22degrees(full-width at half-maximum). Measurement of polarization is also reported. The polarimetric modulation factor is obtained to be 43 %, which is consistent with the prediction of Monte Carlo simulations.

Diamonds have attractive properties for use as radiation detectors. Recent advances in diamond fabrication techniques have made it possible to produce almost perfect single crystal materials that have electrical properties suitable for radiation detectors. We developed detectors made of high-purity synthetic IIa diamond grown by the high-pressure, high-temperature method and investigated their basic properties. The leakage current is less than 20 mu A with reverse bias voltages up to 2.4 kV, and the diode current is 0.1 mu A with 1 V forward bias voltage. The energy, resolution for the best detector is 15.4 keV (FWHM) for 5.486 MeV alpha particles from Am-241.

Diamonds have attractive properties for use as radiation detectors. Recent advances in diamond fabrication techniques have made it possible to produce almost perfect single crystal materials that have electrical properties suitable for radiation detectors. We developed detectors made of high-purity synthetic Ha diamond grown by the high-pressure, high-temperature method and investigated their basic properties. The leakage current is less than 20 pA with reverse bias voltages up to 2.4 kV, and the diode current is 0.1 μA with 1 V forward bias voltage. The energy resolution for the best detector is 15.4 keV (FWHM) for 5.486 MeV alpha particles from 241Am. © 2004 IEEE.

We are developing a medium energy particle detector in the high-count rate environment in order to solve the acceleration mechanism and the process of high-energy particle on the collision less plasma shock region in the space. The key observation is to measure the maximum energy of accelerated particles correctly. It is important to reduce the pile up events in order to measure the correct maximum energy. The new developed detector for high-count rate environment consists of the Double-sided Silicon Strip Detector (DSSD) and the readout LSI chips VA32TA made by IDEA. The performance of the new DSSD system for charged particles was tested using proton beams with energy of 6 MeV from the medium energy accelerator in HIMAC of the National Institute for Radiological Science. It is result that the new DSSD system works well and has high performance to detect not only x-rays but also charged particles. It is certain that this new DSSD system can measure correct energy of incident particles in high-count rate environment using the accelerator.

To bring a breakthrough in the observation of the gamma-ray universe, we are working on the development of Semiconductor Multi-Compton Telescope(SMCT). In the SMCT, all the energy, positional and timing resolution must be high to ensure high sensitivity. The imaging device based on the high resolution CdTe diode and Si, such as CdTe pixel detectors and Double-sided Si Strip Detectors (DSSDs), are promising candidates for the components of the SMCT. Here we report the results of a CdTe pixel detector connected with a low noise analog ASIC. We obtained an energy resolution of 2.5 keV(FWHM) at 122 keV, with a positional resolution of 2 mm. Performance of the first prototype Compton Camera using a DSSD and two CdTe pixel detectors is also described. We irradiated 100% linearly polarised 170 keV gamma-ray line to the system and obtained the polarimetric modulation factor of 43%.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six stacked double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 80 keV to 662 keV. The energy resolution (FWHM) is 10 keV and 16 keV at 356 keV and 511 keV, respectively.

We are developing a whale ecology observation satellite system, comprised of a small satellite (WEOS), a ground operation center and probes attached to whales in the ocean. The NASDA (JAXA at present) has successfully launched the small satellite on 14th December, 2002 as one of the piggyback payloads of H-IIA-4 rocket from Tanegashima Space Center. The orbit of the WEOS is sun synchronous with the height of 800 km, and the attitude is stabilized by gravity gradient torque caused by a deployable mast of 3 m long. The satellite is manufactured by using industrial parts through careful environmental tests. Performance of the WEOS on the orbit is perfect, and its antennas for communications are kept facing to the earth. Operations for attaching the probes to whales are going on. The probes send ecological data to the WEOS, in which position data from a GPS receiver onboard the probes are included. The WEOS has a space GPS receiver on board, and the position data on the orbit are sent by telemetry to the ground station together with the Doppler shift data obtained by a PLL receiver of the satellite. We can thus locate the probe on the ocean by inversion analysis, which contributes as an auxiliary tool for studying migration of whales. Based on the experience of this mission, we are proposing a formation flight of 8 to 10 of such small satellites uniformly spaced on two polar orbital planes, which allows a cost effective realization of an advanced multi purpose satellite system. This paper describes the outline of the WEOS system, and the future prospect realized by the formation flight of similar satellites on the earth.

We are developing a Compton camera based on Si and CdTe semiconductor imaging devices with high energy resolution. In this paper, results from the most recent prototype are reported. The Compton camera consists of six stacked double-sided Si Strip detectors and CdTe pixel detectors, which are read out with low noise analog ASICs, VA32TAs. We obtained Compton reconstructed images and spectra of line gamma-rays from 80 keV to 662 keV. The energy resolution (FWHM) is 10 keV and 16 keV at 356 keV and 511 keV, respectively. © 2004 IEEE.

The development of Gamma-Ray Spectrometer (GRS) is reviewed. Japanese lunar polar orbiter SELENE carries Gamma-Ray Spectrometer (GRS) which uses a high purity Ge detector cooled down to 80-90 K by a Stirling mechanical cooler for the first time in the lunar mission. GRS consists of n-type Ge detector as a main detector and EGO and plastic scintillators as an active shielding detector. The 250 cc Ge-detector is encapsulated in the aluminum canister. Its excellent energy resolution (< 3 keV in fwhm for K-40 gamma-ray line) is attained for engineering model even after severe vibration tests were made. GRS will provide the global mapping for major elements of O, Mg, Al, Si, Ti, Fe, etc. and natural radioactive elements of K, Th and U with a high precision. GRS has such an excellent energy resolution that it would identify prompt gamma-ray line from hydrogen, if there exists water ice at the surface of the lunar polar regions.

A series of corotating energetic helium events with 3–5 MeV/n has been observed by the HEP telescopesonboard the GEOTAIL satellite at 1 AU from October 1993 to August 1994. We have examined the relationship between solar wind (SW) speeds observed at 1 AU and the intensity of He in the series of the corotating interaction region (CIR) events. The speed difference of the high-speed SW from slow-speed SW, which forms a CIR in the outer heliosphere, shows a reasonable correlation with the peak intensity of He at 3–5 MeV/n. The speed difference of those SWs is considered to be related to the shock strength which develops in a CIR at a distance from the Sun. Such a correlation between energetic ions associated with CIRs and SW speed has not been shown so far for the events during the period of 1978-1986. We have also investigated the dependence of the relation between the CIR ion events and the SW speeds during the period between 1978 and 1995 which corresponds to the high solar active phase of Solar Cycle 22. There is a good correlation between the SW speed and the peak intensity of CIR energetic ions in the quiet phase of solar activity, but not in the active phase.

The real-time measurement of radiation environment was made with an improved real-time radiation monitoring device (RRMD)-II onboard Space Shuttle STS-79 (S/MM#4: 4th Shuttle MIR Mission, at an inclination angle of 51.6° and an altitude of 250–400 km) for 199 h during 17–25 September, 1996. The observation of the detector covered the linear energy transfer (LET) range of 3.5–6000 keV/µm. The Shuttle orbital profile in this mission was equivalent to that of the currently planned Space Station, and provided an opportunity to investigate variations in count rate and dose equivalent rate depending on altitude, longitude, and latitude in detail. Particle count rate and dose equivalent rate were mapped geographically during the mission. Based on the map of count rate, an analysis was made by dividing whole region into three regions: South Atlantic Anomaly (SAA) region, high latitude region and other regions. The averaged absorbed dose rate during the mission was 39.3 µGy/day for a LET range of 3.5–6000 keV/µm. The corresponding average dose equivalent rates during the mission are estimated to be 293 µSv/day with quality factors from International Commission on Radiological Protection (ICRP)-Pub. 60 and 270 µSv/day with quality factors from ICRP-Pub. 26. The effective quality factors for ICRP-Pub. 60 and 26 are 7.45 and 6.88, respectively. From the present data for particles of LET > 3.5 keV/µm, we conclude that the average dose equivalent rate is dominated by the contribution of galactic cosmic ray (GCR) particles. The dose-detector depth dependence was also investigated.

A new telescope consisting of three two-dimensional position-sensitive silicon detectors which can measure the linear energy transfer(LET) distribution over the range from 0.2 to 400 keV/µ m has been developed as a real-time radiation monitor in manned spacecraft. First, the principle of LET measurement and its design method are described. Second, suitable electronic parameters for the LET measurement are experimentally determined. Finally, the telescope performance is investigated by using relativistic heavy ions. The first in-flight test of this type of telescope on the US Space Shuttle (STS-84) is scheduled for May, 1997.

A series of observations of Centaurus X-3 was made with Ginga on 1989 March 22-24, over a complete orbital cycle. The data set includes a pre-eclipse dip, the eclipse ingress and egress, the eclipse itself, and a phase of high, steady emission after egress. Pulse timing analysis of these data has provided a set of orbital parameters which confirms the change in the orbital period known previously and yields an improved rate of change of orbital period: P(orb)/P(orb) = -1.738 x 10(-6) yr-1. We also detected a double-peaked structure in the pulse profile at low energies which is rarely observed from Cen X-3. The spectrum after egress was found to be the cutoff power law typical of pulsars in general. At mid-eclipse, scattering by optically thin matter can account for the observed spectrum with the addition of a soft thermal component, while during the pre-eclipse dip, the spectrum has three components: a highly absorbed, pulsating one; a scattered non-pulsating one; and the same soft thermal component as seen at mid-eclipse. An iron emission line was detected at all binary phases. After egress, when the source was brightest, the energy of the iron emission line was predominantly at 6.4 keV due to fluorescence by cold circumstellar matter. However, during eclipse the iron emission was more complex: both 6.4 and 6.7 keV iron lines were observed from this binary system, with some evidence of a separate line at approximately 8.5 keV. It was found from the orbital phase dependence of the line intensities that the fluorescent 6.4 keV iron line is produced by cold matter located relatively close to the neutron star, whereas the 6.7 keV line is attributed to the hot highly ionized plasma spread out over the surface of the companion. The present observations give an estimate for the size of such a hot plasma zone formed by the X-ray irradiation: D6.7 > 8 x 10(11) cm.