A germanium detector array ruggedised for launch in a coded mask space telescope

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Nuclear Instruments and Methods in Physics Research A 385 ( 1997) 475-479

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NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

ELSEVIER

A germanium detector array ruggedised for launch in a coded mask space telescope J.R.H. Herring *, G.K. Skinner School r!f Ph~sic.s und Space Rrmmh,

University ojBirmingham.

Edgboston, Birmingham

B15 277: UK

Received 16 October 1996

Abstract The use of germanium detectors for gamma-ray astronomy from space has so far been restricted to single detectors providing no spatial information. We describe here the development of an array detector with up to sixteen independent germanium elements, which has been designed to withstand the environment associated with a rocket launch. It is shown that germanium elements in the assembly exhibit no significant degradation in performance when subjected to the appropriate vibration levels.

1. Introduction Nuclear line astronomy requires observations from satellites or high altitude balloons and detectors with extremely good energy resolution both to minimise the background within the line width and to permit detailed astrophysical investigations. Only germanium detectors offer the resolution necessary to observe, for example, the 0.01% Doppler shifts of the line energy associated with the 30 km/s velocities typical of motion within the galaxy, the subtle changes to line shape of comparable magnitude associated with various different conditions in the region of emission, or small “red-shifts” of lines from cosmological distances or originating close to a black hole. High altitude balloons offer exposures of typically oneday [ l-31, though an exposure of nine days has been achieved with a germanium detector system on a long duration balloon flight launched from Antarctica [ 41. Significantly longer observations are only possible from spacecraft, but because of the difficulties associated with the need to cool the detectors to < 100 K together with their delicate nature, very few germanium-based instruments have been used to make astrophysical observations from space. The HEAO-3 spectrometer with four germanium detectors was launched in 1979 on the HEAO-3 mission [ 51, a germanium-based gamma ray burst detector flew on the ISEE- mission in 1978 [6] and the Transient Gamma Ray Source instrument is now operational on the WIND spacecraft [ 71. None of these, nor any balloon-borne instruments, excepting our recent one men-

* Correspondingauthor, Tel. t44 e-mail [email protected]. 0168-9002/97/$17.00 PIISOl68-9002(96)01

121 414 6458/3,fax

+44 121 414 3722,

tioned below, have offered more than the most rudimentary imaging possible, for example, with simple collimators. The coded mask technique [ 8-141 in principle allows the formation of images at gamma-ray energies, but requires appropriate position sensitive detectors to register the pattern of radiation and shadow in a plane some distance from the mask. This paper describes the development of a cryostat capable of withstanding high vibration levels, whilst accommodating an array of detectors with minimal material between them, thus allowing for the free passage of compton scattered photons. We have previously reported [ 151 work leading to the evaluation of an array of 3 x 3 germanium detectors, each 15 x 15 x 50 mm, which was flown on a balloon in a small coded mask telescope, obtaining the first images of the Crab Nebula and Cygnus X-l with a high spectral resolution detector [ 161. The present work involves a 4 x 4 array of 20 x 20 x 50 mm elements, built to be vibration tested to levels typical of those experienced on spacecraft launched by large rockets.

2. Cryostat design The internal design of the cryostat was based upon the following requirements: (i) Each germanium crystal should be susceptible to easy and safe removal and replacement. (ii) The crystals must be firmly held without undue pressure, by forces sufficient to prevent any slippage under vibration.

CopyrIght @ 1997 Elsevier Science B.V. All rights reserved 159-X

j

GEWANIUM

BORE FOR HV GflOUND, SIGNAL

HIGii VOLTAGE C3NNECTIONS I-‘/E) .

SIGNAL LEAD

-

\ \ BfiAID

COPPEa PISTON

,’

Fig. 2. individual detector layout. showing mechanical support, and eiectrical and thermal contact arrangements. Fig. 1,The detector asembly. showing how the square wpport structure with provision for sixteen germatium detector elements is supported within P cylindrical “cage” by sxtecn tensioned wires. Radiation screens outside the cage, and between the detector structure and the cage have been removed.

(iii)

The thermal condu~tanceof paths between the crystals and their enclosure must be minimised, whilst that of their links to the cold finger must be maximised. (iv) The fundamental resonance frequency of the detector array must lie well above 100 Hz. Supporting the mass ( I .6 kg) of sixteen germanium detectors of the above dimensions while meeting these requirements presents a significant challenge. Schemes in which the necessary ~o~nbination of mechanical stiffness and low thermal conductivity was obtained by thin diaphragms or by low conductivity composite struts were avoided, because of problems with clamping and access in the one case and with possible contamination in the other. The objectives were met by the arrangement shown in Fig. I. The sixteen germanium detectors are mounted in a 4 x 4 array with 24 mm centre to centrc spacing in a module based on a rigid aluminiuIn support structure. A cylindrical “cage” is formed by two metal rings separated by spacers and the germanium detector module is suspended within this by sixteen stainless steel wires. Two wires, each 100 mm long and 1.3 mm diameter, stretch from each of the four lower corners of the module to the upper ring, and two from each upper corner to the lower ring. The ends of the wires were threaded and the pre-tension adjusted by nuts to -15 kg. A set of “Belleville” spring washers was used at one end of each wire to reduce the spring constants and

make the pre-tension less critically dependent on changes to differential thermat expansion. This arrangement has the advantage of introducing damping to reduce mechanical resonances. The individual detector elements (Fig. 2) are square section n-type coaxial, the signals being taken from a spring contact in the bore, which is close to ground potential. Each detector is held on chamfered plastic spacers. The lower spacers, which are lined with gold foil to provide electrical contact with the common negative high voltage. ride on a copper piston for thermal coupling. The assembly is held fnmly by a compression spring located below the piston, providing a compressive force of 3. I kg for a nom~nai sized crystal, capable of preventing movement under a peak acceleration of 30 g. Signal connections from the central anodes were taken by fine stainless steel wires to hermetically sealed feedthroughs and externally mounted preamplifiers with roomtemperature input FETs (Canberra Model 2002). To reduce the thermal leakage by radiation, two polished cylindrical aluminium radiation screens are fitted inside and outside the cage. The outer one is conductively coupled to the cage, while the inner is insulated from it. These assume intermediate temperatures defined by the balance of radiative and conductive heat flows. Significant conduction is associated only with the cage, which has outflow to the detector module along the support wires, and inflow from the enclosure along its thin stainless steel legs and through insulated adjustable buffers which provide transverse support. A screen and low emissivity surface was provided in front due, for example,

of the detector by a 12 pm aluminised Mylar inner window mounted on the module, which also helped in maintaining cleanliness. The overall detector assembly forms a sealed unit, which includes a copper cold finger which can be immersed in liquid nitrogen to provide cooling. The cold finger and the overall housing follow conventional laboratory design and are not necessarily representative of an arrangement which would be used in space. Molecular sieve in the region around the bottom of the cold finger is used to remove residual gasses in the sealed assembly in the usual way and a thin (300 pm Al) front window allows testing with photons down to -IO keV. The cold finger is linked thermally to the detector assembly by copper braids (which have swaged ends bolted home with indium gaskets) and so is mechanically decoupled from it and does not affect the vibrational testing of the detector module. However the need to conduct the vibration tests on the entire assembly. including the cold finger, necessitated the provision of an eight-spoked stainless steel “bicycle wheel” to support its top end.

.,

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1. ,’

Fig. 3. Vibration response obtained during

1 g sinesurvey.

material characteristics and perfect joints were assumed. the agreement is acceptable.

4. Vibration performance 3. Thermal performance Dummy detector elements made of stainless steel with an aluminium core, to simulate the weight and dimensions of real detectors and also to approximate their thermal capacity and conductance, were used for all the initial testing. This included an experimental run during which temperatures were monitored by platinum resistance thermometers attached to one of the dummy crystals and to other points in the system. Heat load was measured by monitoring the liquid nitrogen dewar weight for long periods with and without the detector assembly present. A simple computer model was developed in parallel with the hardware to provide guidance and insight; this gave a breakdown of the heat load as shown in Table 1. Dismantling and reassembly caused variations, but the best measured value of detector temperature was 97.5 K, and of heat load, 3.0 W. These figures can be compared with a prediction from the computer model of 99.7 K and 2.9 W. Given the uncertainties in surface emissivities, which were all taken in the model to he 0.125. and the fact that tabulated

The sixteen-wire suspension design of the detector module. as described under Section 3 above, was subjected to a finite element structural analysis. The frequencies of the lowest modes were also obtained by simple analytical modelling, and for the same assumptions the two predictions were in close agreement. The presence of the Belleville washers at the ends of the support wires complicates the analysis. If the a~angement is considered friction-free, the effective elastic constant of the wires is reduced by a factor of three compared with a situation in which the wires are considered clamped. The frequencies of the the fundamental horizontal and vertical resonances were predicted to be 192 and 199 Hz in the former extreme case or 332 and 344 Hz in the fatter. The measured values were expected to lie between these two Table 2 Specification of full level vibration test done both parallel and perpendicular to cold finger axis. It was preceded and followed by a I g Gne survey: S-2000 Hz. 2 octaves/min. see hg. 3

Table I Thermal leakage componrnts, Breakdown nccording to modellinp Path

Load

Rudiutirv Front window Sides

0.63

/w I

5-30 30-100

I I mm pk-pk 20 g

I 30

Condrcctivc~ Wires and Support C~~UCIUI‘C “Btcycle wheel” cupport TOGtl

0.38 0.56 1.57

20- IO0

I OO-so0 500-2000

+6 dB/Oct

0.2g?/Hz -6 dB/Oct

J.R.H. Herring,

478

Fig. (%o

4. Repeated measurements lines, before

of energy

G.K. SXinner/Nucl.

resolution

(full

width

Instr. and

nt half maximum.

Mrth.in

FWHM)

P/I!:F. Rrs. A 3X5 (1997) 475-479

for two germanium

for the 1173 and 1332 keV

prevtbn

0

postvibn

0

extremes, the lower values being expected if the friction is not high enough to prevent washer movement. During an initial vibration test before installation of any real detectors it was possible to place a monitor accelerometer on one of the sixteen dummy detectors. Fig. 3 shows the response of such an accelerometer during a 1 g sine survey with the input perpendicular to the axis of the instrument. The fundamental resonance is at approximately 200 Hz, consistent with the above analysis. The response with input parallel to the axis was very similar. Both before and after germanium detector elements were installed, vibration tests were performed with input both parallel and perpendicular to the cold finger, or cryostat axis, to the full specification given in Table 2. These are typical of the qualification levels for an instrument to be flown on a spacecraft.

5. Detector

prwbn

of FWHM

an 226Ra source. before

with

l

1000

500 Photon Fig. 5. Variation

detector elements

and after vibration.

enerqy

energy

(keV)

for both detectors.

and after vibration.

obtained

using

performance

Following initial tests, two of the dummy elements were replaced with fully operational germanium detectors, installed by Canberra Semiconductor NV, Belgium. The performance of the detectors in this set-up was characterised by resolution measurements, before and after vibration tests to the specification in Table 2. The vibration was conducted with the detectors at room temperature and the cryostat was repumped following the test. Fig. 4 shows measurements of the energy resolution for the two hoCo gamma lines at 1173 and 1332 keV, and Fig. 5 shows similar data over a range of energies, using the many lines from an 2’6Ra source. The energy resolution obtained (“Co pre-vibration average: 2.6 and 2.7 keV for detector

Gel and 2.4 and 2.5 keV for Ge2) is not quite as good as is possible with the best cylindrical detectors with cooled preamplifier input transistors. but in neither case is there any evidence for degradation of detector performance due to the vibration.

6. Discussion and conclusions One of the objectives was to achieve close packing of the individual blocks of germanium, to make efficient use of the available detection area, with little or no material between them, so that high energy electrons and Compton scattered photons could pass from one to the next without loss in the total detected energy. The packing density achieved with 20 mm square detectors on 24 mm centres means that 69.4% of the frontal area is active. Between the detectors there is no additional material over most of the 50 mm length. support structure being limited to a 4 mm lip at the front of the detector and an 8 mm one at the rear. There is however a thin layer of boron implanted p-type material over the n:hole outer surface of the Ge crystals which. being conductillg, will have no electric field and so be “dead”. This however is only 0.3 pm thick. For scattered photons. the surface layer will have essentially no effect and even photons crossing between detector elements in the 24% of the length over which there is intervening material will usually traverse the small amounts of low-Z material without interacting (interaction probability
and thermal properties. The design was such that rather larger arrays based on the same principles could be envisioned. In a flight design attention would have to be given to reducing radiative heat leakage, for example by using multilayer insulation, but it has been shown that the crucial conductive components can be kept to a low level. This development makes possible the utilisation of detectors containing an array of small germanium elements in space-borne coded mask telescopes to provide both imaging capability and high spectral resolution.

Acknowledgement Many thanks are due to IMichel Ceuppens and Patrick Vermeulen of Canberra Semiconductor NV, who provided a great deal of assistance with the design, and also fabricated and installed the detectors. Thanks are also due to Chris Farr of Webster and Horsefall Ltd. for supplying various samples of high performance stainless steel wire and helpful advice on fastening them. to Chris Cooper of the University of Bi~ingham Materials Science IRC for carrying out pull tests and failure analyses on wires held in various ways, and to John Yates of the Rutherford Appleton Laboratory for conducting the vibration tests.

References

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