Adv. Space Rec. Vol.1, pp.ll5—118 o273—1177/81/0401—0115$O5.OO/O©COSPAR, 1981. Printed in Great Britain.

AN IMAGING TELESCOPE FORSOFT GAMMA RAY ASTRONOMY

J.N. Carter,P. Charalambous,A.J. Dean,D. Ramsden,M. Badiali,1 P. Ubertini, G. Boella,2F. Perotti, G. Villa, G. di Cocco,3G. SpadaandA. Spizzichino

UniversityofSouthampton,Southampton,UKIstituto diAstrofisicaSpaziale/C.N.R.,Frascati, Italy

2lstitutodiFisica Cosmica/C.N.R., Milano, Italy3lstituto TE.S.R.E./C.N.R.,Bologna,Italy

ABSTBACT

A telescope capable of producing images of the gamma ray sky in the energy range0.2—20 MeV with an angular resolution of a few tenths of a degree is presented.This capability is achieved by means of a large array of Sodium Iodide positionsensitive elements together with a coded imaging mask. The expected performance,derived from calculations and preliminary laboratory tests, is described.

INTRODUCTION

During the past decade a series of satellite and balloon observations have detectedy—ray emission over the celestial sphere. Apart from objects in which a clear timesignature has permitted recognition, it has not been generally possible to identifyother 1—ray sources with other known astronomical objects, due to the limitedangular resolution of the telescopes employed. Although these y—ray measurementsare interesting within their own right, the depth of insight into the astrophysicalprocesses involved has been clearly limited by the narrow spectral range of thestudies.

It would seemapparent that in order to make a significant step forward in lowenergy 1—ray astronomy a telescope with high sensitivity and good (few arc minute)angular resolution is required. Present day 1—ray telescopes operate in the regimeof low signal—to—noise ratio. The background level is related to the intensity ofthe parent particle fluxes which in turn fluctuate in proportion to the localmagnetic field vector and other environmental parameters. Consequently, thebackground counting rate of the system varies continuously during the period ofobservation. Large systematic errors may be obtained in the estimated 1—raysource flux becauseof the background noise to be subtracted from the sourcemeasurementis time multiplexed. It is therefore desirable in the interest ofobtaining high sensitivity and reliable measurementsthat the background measurementis spatially multiplexed with the source measurements. In this way a continuousreal time background subtraction can be made. We therefore require that not only

does the instrument have fine angular resolution but also that it has an imagingcapability. Since 1—ray photons cannot be focused and telescopes utilizing doubleCompton scatters cannot resolve the direction of the incoming photons to betterthan a few degrees tl,2J the only other option available at the present day wouldappear to be some form of coded aperture imaging.

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116 J.N. Carter et al.

THE APPLICATION OF CODEDAPERTURE MASKSTO LOW ENERGY1-RAY ASTRONOMY

The operation of a position sensitive detection plane is fundamental to any imagingtelescope. Research in the field of nuclear medicine has been responsible for thedevelopment of position sensitive 1—ray detectors. The 1—camera [31 for example,is an extremely effective tool for tomography. However in this case the positionalsensitivity is obtained by locating a large number of photomultipliers so as tocover the underside of the detection crystal, making any active shield system bothunweildy and ineffective. The 1—camera may find a use in the field of 1—ray burtsastronomy for which a poorly shielded system is not such a limitation /2k 2.The other technical development which may be borrowed from the field of nuclearmedicine is the hybrid scanner [5,6,7]. The imaging telescope described in thispaper is based around the use of an arrangement of hybrid units. The codedaperture techniques in X—ray astronomy, whose value has been well demonstrated

L8,9,1OJ, are developments of ideas presented by Mertz [llJ and Dicke [l2J.The technique depends on the use of a coded aperture located some distance above aposition sensitive detector. The shadow of this mask is recorded on the detectorand the data contain information about the distribution of sources in the sky. Itis not immediately recgnisable as an image of the sky since the many small holescause the data to consist of many overlapping images. This data must be processedin order to provide useful information. Mathematically the imaging process may beexpressed as a cross—correlation between the sky, S(i,j), and an aperturetrasmission function A(i,j), resulting in the picture P(k,l). If the positionalresolution of the detector is the sane as that of the unit dimension in the mask,the distribution of photons arriving at the detector plane is given by:

P(k,l) = zi Z,.~ S(i,j) A(i+k,j+l)

DESCRIPTION OF THE IMAGING TELESCOPE

The instrument has an array of inorganic scintillation counters and a codedaperture mask in order to obtain a high angular resolution image of the sky in theenergy range 200 keV to 20 MeV. High sensitivity is achieved by the use of a largearea actively shielded detector. A schematic view of the instrument is shown inFigure 1. The 1—ray detector is designed around an array of Sodium Iodide positiomsensitive elements each having dimensions 5x5x50 cm. The Sodium Iodidescintillators are viewed by two photomultiplier tubes and are optically isolatedfrom adjacent elements and the veto counter by thin aluminium foils. A SodiumIodide anticoincidence shielding system is placed immediately below the barelements. The coded imaging mask is constructed from tungsten units capable of90% attenuation at 1 MeV photon eneriges and is placed 1~m away from detectionplane. Plastic scintillation detectors are placed close to the mask and detectorplane and are operated in electronic anticoincidence with the 1—ray data channel.

In the first instance 1—ray events in each detector element are defined by the

coincidence between the outputs of the two associated photomultiplier tubes in theabsence of pulses from adjacent detector units or the veto systems. Pulse heightanalysis of these two outputs provides information on the energy and position ofthe photon. In this way it is possible to have a large sensitive area which isactively shielded by inorganic scintillators which are themselves also used asdetection elements. This confisiiration enables us to improve the quality of thespectral information and to possibly reduce the background counting rates for

1—ray energies greater than 1 MeV by operating the instrument as an annihilationpair spectrometer. The position sensitive detection bars were originally used asan integral part of a radio—isotope mapping instrument [5,6,7]. The detectionelements are constructed from discrete slices of Sodium Iodide crystals which are

An Imaging Telescope for Soft Gamma Ray Astronomy 117

then optically coupled such that a small amount of light is lost at each interface,resulting in a logarithnic attenuation along the crystal. Figure 2 shows thepositional resolution for 1 MeV y—rays as a function of position of incidence alongthe detection bar derived from a Monte—Carlo simulation. When photopeak eventsare selected the positional resolution is typically 7 mm. Multiple interactionsand the escape of Compton photons degrade the positional resolution to about 10 mm.The calculated energy resolution at 1 MeV is typically 15% FWHM.

The theory of optimum coded masks indicates that the preferred arrays, in units oftheir fundamental element, are i x j such that i.j = 2~-i, where n is aninteger [13]. To obtain a unique sky, every point within the defined field ofview must project a unique image of the mask onto the detector which implies thatthe mask should be at most the double of the detector in linear size.A mask having element sizes of 50 x 20 mm, when locates at 14 m will provide abasic imaging quality of 17 x 143 arc minutes. Stronger point sources will belocated with a precision of a few arc minutes if the image profile is studiedstatistically.

~-

~ __

‘~~:° ~ V . 2S

• StOS VIEW ,,S’A,C! PROM C!~V!~ O~~Y~C?O~ (CM

Fig. 1 Schematic view of the telescope. Fig. 2 Positional resolution for 1 MeV1—rays as a function of incidence alongthe detection bar.

CONCLUSIONS

The arrangement described in this paper enables a large area (‘i. 1 m2) actively

shielded, high detection efficiency (~20%) low energy 1—ray telescope to be builtwhich operates in the energy range 0.2 to 20 MeV. Since the background noise andsource measurements are made at the same time, many of the present day problemsassociated with the estimation of source fluxes are avoided. This telescopehaving arc minute imaging capability should achieve an intrinsic sourcesensitivity of about one order of magnitude higher than present day instruments.

REFERENCES

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I.E.E.E. Tr.Nuc.Sc., 26,p. 506—512, (1979)3. H.0. Anger, ISA Transactions 5, 311, (1966)14. H. Horstman, E. Horstman Moretti, F. Fuligni, G. Di Cocco, W. Dusi, F.Frontera,

E.Morelli, and A. Spizzichino, COSPARSymposium, x-ray Astronony,3, 539,(l979)5. T.P. Davis, H. Martone, Journal of Nuclear Medicine, T, 1114, (1966)

118 J.N. Carter et al.

6. J.c.w. Crawley, N. Veall, Medical Radioisotope Scintigraphy, vol. l,p. 105—112(IAEA) Vienna (1973)

7. E.Vauramo, A.Virjo, British Journal of Radiology, 50, 868, (1977)8. J. Gunson, B. Ploychronopoulos, Mon.Nat.Roy.Ast.Soc., 177, 1485, (1976)9. R.J. Proctor, G.K. Skinner, A.P. Wilmore, Mon.Nat.Roy.Ast.Soc., 179, l85,(l978)10. E.E. Fenlinore, T.M. Cannon, Applied Optics, 17, 337, (1978)11. L. Mertz, Transformations in Optics, John Wiley & Sons Inc., (1965)

12. R.H. Dicke, Ap.J., 153, LlOl—6, (1968)13. F.J. MacWilliams, N.J.A. Sloane, I.E.E.E., 614, 1715, (1976)