New detectors for astronomy

Nuclear

Instruments

and Methods

in Physics

Research

A 387 (1997)

19-23

__

NUCLEAR INSTRUMENTS a METNom IN PNVSICS

R%%T”

f!I!!FI ELSEVIER

New detectors for astronomy Law-ant Vigroux CEAIDSMIDAPNIA,

Service d’Astrophysique.

1. Introduction The last twenty years have been a revolution in the field of detectors for astronomy. They have progressed from hybrid systems, to monolithic solid state detectors. Readout electronics have been incorporated to the detection layer, producing compact and reliable systems. The photometric and spectroscopic performances have been improved. These new generation detectors are now available for the whole range of electromagnetic radiation, from gamma rays to radio domain. In this short review, I shall describe the progress made in the field of direct detection. I shall leave apart the domain of heterodyne detection which is limited to the ratio domain, for wavelengths longer than I mm.

2. Detectors for the visible domain In the astronomical community, the visible extends from the atmospheric cutoff at 300nm to the near infrared. The conventional red cutoff is set near 1 pm which is the Si cutoff. This domain has experienced a first revolution with the photographic plates at the beginning of the century. The photographic plates remained the main detector in astronomical observations for almost 50 years. They were challenged only by the electronographic camera. In these detectors. the photons are transformed in electrons on a photocathode. Then the electrons are focused on a photographic emulsion by an electrostatic or a magnetic lens. The first electronographic camera was designed in 1938 by Professor Lallemand at the Paris Observatory. Compared to photographic plates, an electronographic camera offers a much better linearity, and an improved sensitivity. However, they were much more difficult to use, and these operations required trained personnel. It was the main reason for which these detectors never became widely used. When the first TV cameras had started to be developed. the idea to couple these cameras with image intensifiers was immediate. A large variety of such systems was designed between 1970 and 1980 with almost all possible image intensifiers. The main advantage was to provide an electronic signal which can be digitized, and directly stored on a computer, avoiding the cumbersome 0168-9002/97/$17.00 Copyright PII SO168-9002(96)01200-4

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use of a microdensitometer which was needed to scan the photographic emulsions. The drawback was the nonlinearity of these detectors and the field distortion. The major achievement in these detectors was a two-dimensional photon counting camera which was designed by Professor Bocksenberg at the University College of London in the mid seventies. A space qualified version of these cameras is used in the Faint Object Camera which was developed by the European Space Agency for the Hubble Space Telescope. All these detectors were superseded by the Charge Coupled Devices (CCD). The first CCD camera was set up in operation in 1978, with small CCDs, 100 X 100 pixels. Their main advantages were the high quantum efficiency, the low noise and the very good linearity, which make them almost perfect detectors. Since then, there was a continuous improvement in the CCD performances. Thinning and back side illumination has allowed their use in the near UV The on chip amplifier design has been improved, and noise as low as a few electrons by pixels are common features of modem CCDs. The size has been increased. The new CCD formats are very impressive. 2000 X 4000 pixels seems to be the new standard, but new devices offering 2000 X 6000 (EEV) or even 4000 X 7000 pixels (Phillips) have been produced with reasonable yields. In addition, the packaging has been upgraded to allow to build large mosaics with small dead zones between adjacent CCDs. So far, the largest CCD camera has 8000 X 8000 pixels. It was built by J. Lupino from the University of Hawaii. Larger CCD cameras, with up to 16000 X 16000 pixels are under construction, such has the MEGACAM project proposed by the CEA/DAPNIA for the prime focus of the Canada France Hawaii telescope. Modern CCDs are now the detector in use on all ground based telescopes in the visible for all observations, either imaging or spectroscopic measurements. Their characteristics are now well understood. However, the procurement of large format CCDs remains a problem. Several industrial companies claim to have large devices, thin and back side illuminated, on their catalogs, but none have been able to demonstrate convincingly that they are able to provide them on a commercial basis. A large effort is now made to develop such capabilities, either in the US, SITE, ORBIT, or in Europe, EEV, Philips. The need for these realizations

0 1997 Elsevier Science B.V. All rights reserved 1. INTRODUCTORY

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I Nucl. Insrr. and Meth. in Phys. Res. A 387 (1997) 19-23

is boosted by the large number of CCDs to be set up in the instruments of the new 8 m telescope generation. In 2000, there will be more 8 m telescopes in operation than the number of 4 m class telescopes in 1980.

3. Infrared

detectors

The infrared detectors developments are much more recent than the CCD developments. The first arrays for astronomical use appeared in 1985. There was a rapid improvement of these devices since then, mainly pushed by the military programs. All these detectors are hybrid. A first layer is used to detect the photons. In the last generation, this layer is an extrinsic photoconductor. The photoconductor is hybridized on a readout electronics by small indium bumps. A specific photoconductor is selected according to the wavelength range. Table 1 summarizes the main photoconductors presently in use for astronomical observations. While for military applications or remote sensing, the HgCdTe can be tuned to have its peak sensitivity at longer wavelength, e.g. 7 km, it is used only in the near infrared for astronomical applications. This is due to the existence of the atmospheric window at 10 km which is beyond the capabilities of the HgCdTe. In space observatories, the HgCdTe does not provide a broad wavelength coverage as the gallium-doped silicon. However, the new generation detectors in HgCdTe, in near infrared, presents outstanding performances which are not very far from CCDs. They have large formats, 1000 X 1000 pixels, low readout noise, <20 electrons/pixel and low dark current. Larger formats, 2000 X 2000 are under development by Rockwell, but. if successful, this development will be the maximum size of these detectors. There is an intrinsic limit due to the size of the pixel which must be larger than the indium bump diameter, >I0 pm, and the overall size of the array which must remain compatible with the differential thermal

dilatation between the HgCdTe and the silicon readout circuit. At longer wavelengths, extrinsic silicon remains the rule. Several doping materials can be used. The most commonly used is the gallium-doped silicon which can be used from 5 to 18 pm. The first detector in use was the 32 X 32 pixels detector which was designed by the CEAl LETI/Laboratoire d’Infrarouge for ISOCAM. This detector was also the first to have a 100% filling factor since the pixels are not delimited by opaque metallic grids on the front surface, but by an electrode located on the back surface, between the indium bumps. The bias voltage is applied on the front surface to a transparent conducting layer created by implantation. New generation devices have now larger formats, up to 256 X 256 pixels. They have also larger integration capacitance to cope with the high thermal background generated by the Earth atmosphere in ground-based observatories. At wavelengths longer than 30 km, the siliqon must be replaced by germanium. This make the detectors even more difficult to realize since the metallurgy of the germanium is not as developed as the silicon technology. Several impurities can be used, Be near 50 pm and Ga for 100 pm. It is even possible to stress the photoconductor to mechanically decrease the gap of the impurities. Stressed Ge:Ga have been used in the two far infrared instruments on board of ISO. All these detectors are not available on arrays. They are single pixels which can be assembled to form an array, but with 2 poor filling factor. A new technology has been recently developed to increase the wavelength coverage of these detectors, the Block Impurity Band (BIB) technology. The idea is to create an impurity band which does not extend on the whole thickness of the photoconductor. When a negative bias is applied, a depletion layer is created inside the detector, and the energy levels are bent towards low energy, thus increasing the detection at long wavelength. BIB detectors are available only in the US. They exist on silicon, Si:As and on germanium, Ge:Ga. This is the only

Table 1 Main infrared

detectors

in use for astronomical

observations

Photoconductor

Wavelength

Operating temperature

Size

Manufacturer

HgCdTe InSb Si:Ga

I-24 pm l-5 p,rn 5-18 pm

80 K 20 K 5K

lOOOX1OQO 256 X 256 198 X 126

Rockwell Santa Barbara LETIlLIR

Si:B Si:As

1O-28 pm 8-28 pm

3K 5K

single pixel 256 X 256

Rockwell

30-50 firn 50-100 pm 100-200 pm

2K 2K 2K

single pixel single pixel single pixel

50- 180 pm

2K

single pixel

(BIB) Ge:Be Ge:Ga Ge:Ga stresed Ge:Ga (BIB)

L. Vigrousl

Nucl. Instr. und Meth. in Phys. Res. A 387 (1997)

way to produce array detectors in the mid infrared. If the technology is well mature for silicon-based BIB detectors, it is still a major development effort for germanium-based detectors.

4. Far infrared and submillimeter

detectors

This domain is still a major area of developments. It is also a domain where the detection physics is the most uncertain. The wavelength is large compared to detector size, or the beam size. Diffraction play a major role in all the optical designs in this wavelength range. Experts in this domain speak a different language, according to their personal background, radio techniques, or direct photon detection. Radio techniques have been limited in going towards wavelengths shorter than a few millimeters due to the high frequency involved, and direct detection systems were lacking. In the last years, progresses have been done in both ways. Modem superconducting junctions are now able to work at wavelengths as short as 400 pm, while low thermal capacitance bolometers have become available, allowing their use for direct detection. In this review, we shall focus only on this last development. A submillimeter bolometer should have a very low thermal capacitance to be able to detect the very low energy of the photons. Progress was made in the design of small transmuted germanium crystal of size smaller than 0.5 mm’. When cooled to temperature equal or lower than 0.3 K, these bolometers provide a good detection for wavelengths in the range 400 km to a few millimeters. Since these detectors are smaller than the incident wavelength, they should be placed inside an integrating cavity, and coupling between the cavity and the optical system is provided by feedhoms. Based on this general design, there are two different horn designs. Either corrugated horns which offer a multimode coupling, or Weston horns which offer a monomode coupling. Single bolometers have been realized with the Ge crystal supported in the middle of the integrating cavity by high thermal impedance struds. However. the assembly of these bolometers is very difficult and requires a high workmanship expertise. Such systems are limited to single bolometers or small arrays. The largest array so far is the SCUBA instrument for the James Clerck Maxwell Telescope, which has 91 individual bolometers. This detector has been commissioned in June 1996. New progresses are expected in the coupling of silicon technology with the bolometer design. The first step in this direction has been made by a Caltech group. In their design, the Ge crystal is glued on a web realized in silicon nitride by chemical etching. This web provides a very stiff mechanical support, together with a high thermal impedance. The main problem remains the connection between the web and the Ge thermometer. Further developments have been proposed by the LETIILIR. In this new design,

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the web is metallized, and can be used for the absorption of the incident waves, tuning the web pattern to the wavelength to be detected. In this new design, the system includes two resonant grids above a A/4 reflector cavity. The thermometer might be a small Ge crystal, as in the conventional design, or better, doped silicon. Such a system can be realized using standard silicon technology which has been developed for micro-captors. Arrays of detectors would be very easy to produce with this technology. We have started a Research and Development program at the LETIILIR, to produce a prototype of such an array which can cope with the requirements of the future submillimeter space observatory of the European Space Agency, FIRST, which should be launched in 2007.

5. X-rays detectors In astronomy, the X-rays domain is defined according to the telescope technology. It covers the energy range where a grazing incidence telescope can be used. It extends from 0.1 keV to 10 keV. The first solid state detectors for X-rays were single silicon lithium diodes. They provided excellent energy resolution, 150 eV, and required only a moderate cooling to liquid nitrogen temperature. Several instruments were designed in the years from 1975 to 198.5 with such diodes. These instruments were set up in rockets or in satellites. Later, the developments move along two directions, CCDs and bolometers. The CCDs are very good detectors for X-rays. In astronomical conditions, the X-ray fluxes are low enough to allow the detection of individual photons. The charge created by the capture of a photon is proportional to its energy. CCDs can be used to make spectroscopy at a moderate resolution, together with imagery. Typical energy resolution achieved by CCDs are in the range of 140 eV at 6 keV. This is the main advantage of the CCDs for X-ray astronomy. Specific CCDs have been designed to optimize their performances in the X-ray domain. They are designed on high resistivity substrate to increase the thickness of the depletion layer and therefore to increase the efficiency at high energy, above 3 keV They are also thinned and backside illuminated to increase the quantum efficiency at low energy, below I keV. The drawback of CCDs is the slow readout which prevent making the very short observations which would be required by the rapid variability of some astronomical sources. Another disadvantage is the sensitivity of CCD to charged particles. X-ray photons are mixed together with a large number of charged particles from the cosmic rays. The background generated by the cosmic rays is about 100 times larger than the X-ray flux of normal sources. Background rejection system must be part of the instmments. For long mission, or for satellites which cross the Van Allen belts, the radiation-integrated dose becomes large, larger than 100 krad. The CCDs can be severely

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I Nucl. Instr. and Meth. in Phys. Rex A 387 (1997)

degraded by such doses. The main effect is to degrade the charge transfer efficiency. The original performances can be recovered by annealing the CCDs at temperatures above 100°C. The first X-rays instrument developed around CCDs is on board of a Japanese satellite, ASCA. They will be the main detectors in the two main X-rays observatories that will be launched before the end of the century, AXAF by NASA in 1998, and XMM by ESA in 1999. XMM will have 3 cameras, two of them will have each 7 CCDs of 1000 X 1000 pixels. A new type of X-rays detector has been developed by the Max Planck Institute in Munich for XMM. Instead of CMOS type storing capacitance in regular CCDs, the detection cell are p-n junctions. An electrode system, similar to CCDs, allows the charge transfer toward the readout amplifier. The advantages of these devices are a much higher quantum efficiency at high energy thanks to the larger thickness of the detectors. The energy resolution is similar to CCDs. These detectors will be the heart of the third spectra-imaging camera of XMM. The next step toward higher spectral resolution is the bolometer. For semiconductors, there is a fundamental limitation near lOOeV, due to the Fano noise in the mechanism of a pair creation after the capture of a photon. This noise is absent in bolometers. Much better spectral resolution, of the order of 20-30eV can be achieved by bolometers cooled to 0.1 K. The drawback of this technology is due to the difficulty of having a cryogenic system working at 0.1 K and space qualified. Another limitation is due to the manufacturing of the bolometers, which prevents building large arrays. In the present state of the art, X-rays bolometers are limited to spectroscopic instruments. They are part of the AXAF payload, but were not incorporated in the XMM payload. A new detector technology is now emerging. It is based on superconducting Josephson junctions. These junctions are based on niobium electrodes separated by an insulating layer, generally an aluminum oxide. A magnetic field of 500 to 1000 Gauss is applied perpendicular to the junction direction. The quality of this detector arises from the low energy required to break a Cooper pair and to create quasi particles inside the superconductor material. The quasiparticles can be detected by the tunneling of electrons through the insulating layer. These detectors are still in a development phase. Several groups in the world are working on these superconductor technologies, in particular a European group at ESTEC in The Netherlands. They are very promising. Very good spectral resolution, 20-3OeV, have been achieved in the X-rays domain. Compared to bolometers, these detectors are easier to build, and require cooling at 1 K only, compared to 0.1 K for X-rays bolometers. The very low energy requires to break the Cooper pair, 1.5 meV for niobium, allows to use these detectors in single photon counting mode even in the UV, the visible and the near infrared. Test detectors have demonstrated a photon counting capability up to 2 mm.

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While these detectors are still in a prototype phase, they represent a very promising direction of developments for the detector of the future for a broad range of applications. They are of special interest for low energy X-rays and hard UV, where no good detectors are presently available.

6. Detectors for gamma rays At energy higher than 10 keV, the grazing incident telescope cannot be used, and other imaging techniques, such as a coded aperture should be used. This is the usual limit that astronomers set between X-rays and gamma rays, while in other domain, the limit is nearer a few 100 keV. This domain has experienced the same evolution as all the others. While the first detectors were derived from high energy physics, like sparks chambers, they have been progressively replaced by hybrid detectors, such as scintillators coupled to photomultipliers, and more recently by solid state detectors. The next major program for the gamma rays domain is the European Space Agency mission called INTEGRAL. INTEGRAL will be launched in 2001. It is based on two main instruments, an imager working between 20 keV and 5 MeV, and a spectrograph working between 100 keV and 1OMeV. The imager is designed around a two stages detector. The first layer is built with CdTe crystals. The CdTe provides a very good spectral resolution. Resolution of 7 keV at 122 keV has been achieved at room temperature on the prototype detectors for INTEGRAL in our lab. To get a good quantum efficiency at high energy, the thickness of the material should be equal or larger than 2 mm. In these conditions, there are charge losses due to recombination inside the substrate. However, there is a good correlation between the rising time of the pulse generated on the electrode by the capture of a photon, and the charge loss, the charge loss being the largest for slow rising pulses. Pulse rising times are of the order of a few microseconds and can be very easily measured. A correction scheme can be implemented, either by hardware, or software, depending on the environment to make a charge loss correction, and improve the efficiency of the detector and its spectral resolution. The imager of INTEGRAL will have 16384 individual crystals of 4 mm X 4 mm and 2 mm thick. The second stage is based on CsI. It will work at higher energy, from 200 keV to 5 MeV As far as the energy resolution is concerned, the CsI is not as good as the CdTe, and it cannot be used at low energy. However, it offers a much higher quantum efficiency at high energy. The INTEGRAL imager will have 4096 crystals of 9 mm X 9mm and 30 mm thick. The spectrograph on board of INTEGRAL is designed around germanium diodes. There will be 19 diodes of 27 mm X 27 mm and 70mm thick. They will be used between 100keV and 10 MeV. The spectral resolution is

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2 keV at 1 MeV These detectors will be cooled to 85 K to achieve the spectral resolution required by INTEGRAL. These detectors have already been used for Nuclear Physics instruments, but it will be the first space mission that will use these detectors. In the future, the solution is certainly a combined spectra-imager based on pixelized germanium. There are already developments along these lines by several groups. But this technology was not advanced enough to be incorporated in the INTEGRAL design. It was the main reason to separate the two functions, imaging and spectroscopy, and to have two different instruments.

7. Conclusions New technology detectors are, or will be shortly, available from y-rays to submillimeter. They represent a breakthrough in detection efficiencies compared to previous technology detectors. The main improvement areas are: l the size of the detectors, l their sensitivity, l the low noise, l the spectral characteristics,

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the operation simplicity. the reliability. However, these detectors have generated a new problem. The technologies on which these detectors are based, are beyond the normal laboratory capabilities. They can be developed only in dedicated plants with special equipment, room, ion units, etching clean implantation units, . . , which can be found only in the semiconductor industry. These units belong to large companies like Thomson in France, and the procurement possibilities depend on industrial strategy not under the control of scientists interested in fundamental research. Generally speaking, there is a mismatch between the industry priorities and the need for detectors suitable for our needs. Even if a common interest can be found, the large overhead induced by the environment make the development cost beyond the funding capabilities of research grants. The lack of small European companies interested in scientific detector developments make new detector developments difficult. The only possible solution for the future is to encourage the creation of joint UniversityIndustry laboratories specialized in detector developments and to develop R&T on detectors by special funds from the space agencies, or European scientific organizations. l

l

1. INTRODUCTORY PAPERS