Manchanda, R K

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29th International Cosmic Ray Conference Pune (2005) 5, 247250High Energy X-ray Imaging Telescope : HEXITR.K. ManchandaDept. of Astronomy Astrophysics, Tata Institute of Fundamental ResearchHomi Bhabha Road, Mumbai 400 005, IndiaPresenter: R.K. Manchanda (ravi@tifr.res.in), ind-manchanda-R-abs2-og27-posterWe have developed a new balloon borne payload for hard X-ray observations in the energy region 20 to 800keV. The X-ray detector is 1200 sq cm area phoswich crystal, viewed by 13 phototubes with 3” dia, in an‘Anger’ mode. The first engineering flight of the payload was made on March 28, 2005. This paper describesthe details of the instrument and the detector behaviour at the ceiling altitude during the experiment.1. IntroductionThe development of imaging instruments using focusing by grazing incidence below 8 keV, have revolutionizedour understanding the X-ray universe at low energies. However, high resolution spatial and spectral mappingin the hard X-ray and low energy gamma ray region has been lacking mainly due to fundamental limitation ofthe sensitivity of the detector systems and imaging capability of the instruments. Imaging capability with goodangular resolution is essential for localizing the hard X-ray sources. An imager also allows the simultaneousmeasurement of background and the source, which is crucial at low flux levels. The best imaging techniquecurrently in use in the hard X-ray and gamma ray region is the use of coded aperture mask along with a positionsensitive detector.At energies above 30 keV, the only available detector system with high detection efficiency is the scintillationcounters. The detection efficiency even in xenon filled proportional counters is extremely small for energiesabove 30 keV and second, the observed energy loss spectra does not directly map into incident photon spectrumdue to large K-escape probability (85% in xenon). Monte-Carlo method used to de-convolve the incomingspectrum, critically depends on the knowledge of systematic effects. In contrast, scintillation detectors dohave high detection efficiency. but no spatial sensitivity. A light division technique can be however, used todetermine the point of interaction [1]. Satellite and balloon-borne imaging payloads employing single crystalscintillators are currently in use [2] [3] [4]. Imaging payloads using Large area pixilated CdTe and CdZnTearray detectors are currently in the development stages [5].The inherent background produced in a scintillator due to cosmic ray interaction and the Compton scatteringof the high energy photons dominates the aperture flux and therefore, the flux sensitivity does not directlyscale with the geometric area. The most favoured arrangement for reducing the detector background is the useof phoswich techniques using NaI(Tl) and CsI(Na), which provides a shielding factor of about 4-6 in cosmicenvironment.2. InstrumentHEXIT is a new balloon-borne hard X-ray imaging telescope, which combines a large area phoswich angercamera with a coded aperture URA mask. A passive slat collimator made from lead (.5 mm) laminated with tin(.25 mm) and copper (.15mm) on both sides is placed above the detector to limit the field of view to    	.In addition, to the main detector, a wide angle monitor payload, designed for real-time observations of the X-ray transients is also mounted on the payload248 R.K. ManchandaFigure 1. The Phoswich anger Camera (1A), Wide Angle monitor assembly (1B), Schematic for the PAC on-board elec-tronics (1C) and the 1D in the gure shows complete gondola assembly prior to launch.2.1 Phoswich Anger CameraThe HEXIT uses a new type of detector which we have been developing for the past few years [6] [7], in orderto build a high sensitivity imaging telescope in the hard X-ray range of 20-500 keV. The detector is a large areaphoswich assembly having 40 cm dia and made of NaI(Tl) and CsI(Na) scintillator crystals. Since the optimumthickness of the two scintillators in a phoswich detector depends on the targeted energy range, HEXIT assemblyconsists of 12 mm NaI(Tl) as the prime detector coupled to a 40 mm thick CsI(Na) crystal. The combinationis vacuum sealed with a 0.127 mm aluminum entrance window and a 10 mm thick Pyrex back plate. Thirteen76 mm phototubes are directly coupled to the optical window in a quadrant symmetric fashion. The large areadetector and the arrangement of the photomultiplier tubes is shown in the figure 1A. Since the signal separationfrom the prime and rear detector primarily depends on the pulse shape discrimination and the expected eventrate in the shield detector is much larger both due to large area and the higher detection efficiency, an ultra-fastRTD system is essential for a very low dead time and which can handle up to 10  counts/sec. The observedenergy resolution for an un-collimated source is  for the 60 keV line emission from Am  . Even thoughthe phototubes are relatively widely spaced in our geometry, the summed light output is uniform within overthe entire area of the detector. The coordinates determination in the case PAC is computed from the variationof the signal between different tubes. A linearized radial response was measured during the laboratory tests. Toobtain the location of the photon interaction over the surface, all signals from 13 phototubes are individuallypulse height analyzed and digitized data is transmitted to ground along with pulse height information and time-tag for each photon. To create a look-up table for quick computation of the event co-ordinates, the detectorwas calibrated using 25 and 60 keV lines from Am-241, at 5000 points. The energy resolution and the peakposition was measured for all the 13 tubes for each location of the source on the surface. The estimated positionresolution is be about 7mm even at 30 keV.High Energy X-ray Imaging Telescope..... 2492.2 Wide Angle MonitorWide Angle monitor has been designed as a light weight add-on instrument on a conventional balloon-bornepayload with narrow field of view, to provide a wide angle coverage for transient events during the flight.However, the monitor can be used as a stand-alone all sky monitor both for the detection of transient events inhard X-rays and for long term monitoring of known X-ray sources.The detector assembly consists of 8 CsI(Tl) detectors with 45  area arranged on a geodesic frame and isshown in Fig. 1B. Such a geometry provides a large coverage of the sky. The detectors are viewed by miniature2” Hamamatsu photomultipliers. The field of view of each detector is restricted to 	FWHM by usinga honeycomb lead collimator. The collimators are so designed, as to give an overlapping view of the sameobject in at-least two detectors, both in the elevation and azimuthal axis. The simulations show that ratio of thecounting rates for bright events recorded by different detectors can give source location to few degrees.2.3 GondolaThe basic design of the balloon gondola is similar to proposed by [8]. Both detectors (WAM and HEXIT)are mounted on a frame, which is stabilized in azimuth using a servo, controlled Reaction wheel, using amagnetometer. The phoswich Anger Camera is mounted on bearings and can be rotated in zenith using DCmotor and a 12 bit optical shaft encoder in servo loop. The true North axis of the frame is calibrated on groundusing the pole star. The payload gondola is fully automatic and is controlled by an on-board microprocessorbased star tracker for payload orientation and source tracking. A GPS system is built into the unit to provideinstantaneous value of the balloon coordinates (lat, long) for the computation of the look angles.The front-end electronics for the two instruments is quite different. In the case of WAM the raw counting ratesare transmitted for each of the 8 detectors after pulse height analysis into 16 channels along with the detectoridentification code. The energy range for each of the 8 detectors was set at 20 to 200 keV. For the HEXITpayload the electronics is much more complex. The block diagram for the HEXIT on-board electronics isshown in Figure 1C. The main requirement are a large dynamic range since the energy band of operation forPAC was set at 20 to 1000 keV. The data from all the 13 tubes are digitized separately and transmitted to groundto compute the position of the incoming photons on the detector surface. In addition, a pulse rise time analysisis done to differentiate between the two types of pulses arising from two different detectors. A digitized sumsignal with an absolute time tag accuracy of 5 microseconds is also telemetered to the ground. An Am241radioactive source is mounted on the FOV collimator of the PAC to calibrated and check the detector gainduring the flight. The radioactive source is operated by ground command. The data is transmitted on a 100 kbitPCM link. The real-time analysis of the housekeeping and quick-look analysis of the PAC data are performedon ground. Similarly, the data from the WAM detectors is analyzed in real time to generate the alerts for anytransient phenomenon. All parameters of the instrument can also be commanded from ground if required,using manual-mode override. A complete assembly of the balloon payload in launch readiness is shown in Fig1D.3. Balloon flight and the payload performanceThe HEXIT payload was launched on March 25th, 2005 at 0305 IST using a 590,000 cubic-meter balloon. The650 kg. payload carrying an additional ballast of 200 kg. reached the ceiling altitude of 3.8 mbars. A total of6 hrs data was obtained on the float altitude before termination of the flight using a ground command. Beingthe first flight for this new payload, a large majority of the observations were devoted to the study of various250 R.K. ManchandaFigure 2. (a) Detector counts at ceiling altitude for the PAC and (b) detector 3 in the WAM assemblyengineering, control, stabilization, electronic and the event-selection parameters along with two source targets.Preliminary spectral data from the Phoswich Anger Camera and one of the WAM detector is shown in Figure 2.The figure represents the raw counting rates averaged over 30 minutes time bin and no corrections for detectionefficiency, atmospheric absorption or the Compton scattered contribution to the data have been applied to thedata. The data shown in the left panel represents the total counting rate observed in the phoswich anger camera.The observed counting rate at the ceiling altitude is ﬀﬂﬁﬂﬁﬂﬁ in the energy band 20 to 1000 keV. A strong linefeature seen at around 200 keV corresponds to the Iodine spallation line observed in the case of NaI detectorsdue to atmospheric neutrons [9]. In the right panel in figure 2, we have plated the typical spectrum of one ofthe detector in WAM assembly. The data corresponds to 20 to 200 keV. Once again no correction have beenapplied tot he data. The data show that the observed background spectrum is consistent with the observed forthe atmospheric gamma rays as seen in our earlier experiments. The analysis of the housekeeping data showsthat target acquisition and the orientation stability was with in 0.1 degree. All commands worked as expected.The detailed analysis of the flight data is in progress.4. AcknowledgementsIt is a pleasure to thank P.P. Madhwani, T.K. Manojkumar, J.P. Kayande, B.G. Bagade and Mrs. N. Kamble forfabrication of the gondola, electronics subsystems and the Ground support software.References[1] 1 H. O. Anger, Rev Sci. Instr., 29, 27 (1958).[2] J. Grindley et al., Astrophys. Sp Sci., 272, 733.(1992).[3] P. Laurent et al., Astron Astrophy., 278, 444. (1993).[4] Palmer et al., Astrophys. J, 412, 203. (1993).[5] T. Narita, P. Bloser, J. Grindlay, R. Sudharsanan, C. Reiche, C. Stenstrom, 1998, Proc SPIE 3446, 218.[6] R. K. Manchanda and R.K. Sood, Ind. J. Radio & Sp Phys, 20, 268(1991).[7] R. K. Sood R.K., Manchanda and R. Staubert, Adv. Space Res., 17 (9), 87 (1995).[8] R. K. Manchanda, Imaging in High Energy Astronomy, ed. Bassani & Di Cocco, Kluwar, p171, (1995).[9] G. Share , R. J. Murphy, B. R. Dennis et al., Solar Physics, 210, 357 (2002).

High Energy X-ray Imaging Telescope: HEXIT

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