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ISSI Scientific Report manuscript No.(will be inserted by the editor)

David M. Smith

Hard X-ray and -ray Detectors

Received: date

ABSTRACT

The detection of photons above 10 keV through MeV and GeV energies ischallenging due to the penetrating nature of the radiation,which can require largedetector volumes, resulting in correspondingly high background. In this energyrange, most detectors in space are either scintillators or solid-state detectors. Thechoice of detector technology depends on the energy range ofinterest, expectedlevels of signal and background, required energy and spatial resolution, particleenvironment on orbit, and other factors. This section covers the materials and con-figurations commonly used from 10 keV to>1 GeV.

1 Introduction

Most high energy detectors in space are based on scintillators or solid-state de-tectors. Scintillators are the older technology and are generally used where largedetector volumes and lower cost are paramount. Solid-statediode detectors, e.g.germanium, silicon, and cadmium telluride (CdTe) or cadmium zinc telluride(Cd1xZnxTe or CZT), generally have better energy resolution than scintillatorsand a lower energy threshold, and can be more easily pixellated for fine spatialresolution if that is required. They are more expensive thanscintillators and moredifficult to produce, package and read out in large volumes.

For all these materials, photons are detected when their energy is transferredto electrons via photoelectric absorption, Compton scattering, or pair production(which also produces positrons, of course). The high energyparticles come toa stop in the detector volume, producing ionization that is detected by varyingmethods.

D. M. SmithPhysics Department and Santa Cruz Institute for Particle PhysicsUniversity of California, Santa Cruz, USA

http://arxiv.org/abs/1010.4069v1

2 David M. Smith

I will review the most commonly used solid-state and scintillator materialsand detector configurations. Considerable work has also been done worldwideon gas and liquid detectors and on newer or less common semiconductors andscintillators. For a much more detailed discussion of a wider range of detectors,as well as an excellent treatment of general considerationsin photon counting andelectronics, see the excellent textbook by Knoll [33].

2 Configurations and energy regimes

2.1 Thin and monolithic detectors

The optimum configuration and material for a detector dependmost strongly onthe energy range of the photons to be observed. Figure 1 showsthe cross-sectionsfor photoelectric absorption, Compton scattering, and pair production for photonsby elements commonly used in detectors: silicon and germanium (in solid-statedetectors) and iodine and bismuth (in common scintillators).

Simple efficiency calculations based on cross-sections canassist with instru-ment design, particularly when photoelectric interactions are dominant, but MonteCarlo simulation is the most powerful and flexible tool. It can be used to model theresponse to source and background radiation and to incidentparticles other thanphotons as well. The packages most commonly used are GEANT3 and GEANT4(GEometry ANd Tracking), originally developed at the European Organization forNuclear Research (CERN) for accelerator applications [17;1].

Since the cross-section for photoelectric absorption is large at low energies,low-energy detectors can be quite thin. Photoelectric absorption is also a strongfunction of atomic number, so that, for example, a silicon detector 1 mm thick willabsorb>50% of X-rays up to 23 keV, while a 1 mm CdTe detector will do thesameup to 110 keV. At these low energies, high spatial resolutionis often desired whenthere is an imaging system using focusing optics or a coded mask (see Chapter12). This can be accomplished with multiple small detectors, by pixellating theelectrodes of solid-state detectors, or by using multiple or position-sensitive pho-totubes to read out a scintillator (Anger camera configuration, after inventor HalOscar Anger).

At energies of more than about 300 keV, photoelectric cross sections are smalleven at high atomic number, and detectors must be made large enough that photonscan Compton scatter in the detector and still be photoelectrically absorbed after-wards. Even though the Compton cross section is nearly independent of atomicnumber, a high atomic number is still critical for stopping the dowscattered pho-ton before it escapes the detector carrying off some of its energy. A low atomicnumber can be desireable for the scattering plane of a Compton telescope or fora detector or shield designed to stop charged particles or X-rays and pass-raysthrough.

In many cases the optimum solution for maximizing sensitivity will be to haveseparate detectors for low energies (thin) and high energies (thick). Since mostcosmic sources have falling energy spectra, high-energy detectors will generallyneed larger area than low-energy detectors in order to reachcomparable sensitiv-ity.

Hard X-ray and-ray Detectors 3

Fig. 1 Cross-sections for photoelectric absorption (falling), pair production (rising above1 MeV), and Compton scattering (flat). Data from Berger et al.[8]. Dotted line: silicon. Dash/dotline: germanium. Dashed line: iodine. Solid line: bismuth.The cross sections of cadmium andtellurium (i.e. CdTe and CZT detectors) are similar to iodine. The approximate energy regimesfor basic detector configurations are shown (thin detectors, large monolithic detectors, Comptontelescopes, and pair-tracking telescopes). The pair-tracking regime extends beyond the plot tohundreds of GeV.

Even large monolithic detectors can serve as elements for a coarse imag-ing system when placed in a large array. The INTErnational Gamma-Ray Astro-physics Laboratory (INTEGRAL) provides two good examples.The Spectrometeron INTEGRAL (SPI) [68] has coaxial germanium detectors witha characteristicsize of 7 cm serving as pixels below a large coded mask (see Chapter 12), and theImager on Board the INTEGRAL Spacecraft (IBIS) includes thick fingers of CsIserving as pixels beneath a finer mask than SPIs [66]. The large germanium detec-tors on the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)[61] sit below rotation modulation collimators (see Chapter 12) that do not requireposition sensitivity.

2.2 Compton and pair tracking telescopes

At MeV and GeV energies, the physics of the photon interactions in matter canbe exploited to reject background and determine the direction of the incomingphoton.

In the range of a few hundred keV to tens of MeV, large-volume detectors withposition sensitivity in three dimensions can record the entire sequence of Comp-

4 David M. Smith

a

b

Fig. 2 Design concepts for a modern Compton telescope (a) and pair telescope (b). The energiesand angles between interactions in the Compton telescope trace the incident photon to an annuluson the sky. Here the telescope is shown with low-Z scatteringdetectors and high-Z absorbingdetectors [11; 72; 63], although single-medium instruments are also being designed and built[13; 35; 4]. See Chapter 11 for more details. The pair telescope represents the GLAST design[6], with passive tungsten layers to convert the into ane+/e- pair, silicon detectors to track thepair, and a heavy calorimeter to absorb the remaining energy. Relativistic beaming of the pairand high spatial resolution allows reconstruction of the incident photons direction.

ton interactions in the detector volume. This allows the direction of the incomingphoton to be reconstructed and background to be rejected effectively (see Chapter11).

At energies of several tens of MeV and higher, where pair production is thedominant photon interaction with matter, a pair-conversion tracking system can beused. For example, the Large Area Telescope (LAT) [6] on the Fermi Gamma-raySpace Telescope consists of alternating thin layers of passive tungsten and activesilicon strip detectors (Figure 2b). Pair production takesplace in the tungsten,since the high atomic number gives a high cross-section (Figure 1). The elec-tron/positron pair has high enough energy to penetrate multiple W/Si layers; thepenetration depth gives the initial-ray energy, while the position-sensitive de-tectors allow the track to be extrapolated backwards to givethe arrival directionof the -ray. The highest-energy pairs that penetrate the tracker are stopped in acalorimeter. At GeV energies, this extrapolation can be very precise. The trackingdetectors are not required to measure deposited energy.

3 Detector materials

3.1 Scintillation detectors

Scintillators can be produced in large, monolithic volumes, and in a varietyof shapes. They can have low to high atomic numbers (and therefore stoppingpower), ranging from plastic scintillators at the low end tobismuth germanate

Hard X-ray and-ray Detectors 5

(Bi4Ge3O12 or BGO) at the high end. Plastic scintillators can be doped with high-Z atoms, like lead, to improve their stopping power.

In inorganic scintillators, ionization produces free electrons that can movearound the crystal until falling back into the valence band.In activated crystals,such as NaI(Tl) and CsI(Na), the trace activator element provides a fast route tothe valence band via intermediate states. Since the amount of activator is small,the crystal remains transparent to the scintillation photon emitted when the activa-tors excited state decays. In unactivated crystals such asBGO, one ion of the purecrystal (Bi3+) provides the scintillation photons, with a large enough shift betweenits emission and absorption frequencies that the crystal isstill transparent. In or-ganic scintillators (plastic and liquid), large moleculesare excited by the passingenergetic particles; the scintillation is produced when they relax to their groundstate. In all cases, scintillation light can be collected and multiplied by a sensorsuch as a photomultiplier tube (PMT), photodiode, or microchannel plate. Scin-tillation light can be reflected many times before being collected, so the PMT(s)need not have a direct line of sight to every part of the detector. In fact, uniformityof light collection and therefore energy resolution is sometimes improved bytreating the surfaces of the best-viewed parts of the detector so that they reflectscintillation light poorly.

In space applications, extremely high energy deposits, up to many GeV, canoccur in a detector due to the passage of a cosmic-ray iron nucleus (or other heavyelement) or the spallation of a nucleus in the detector by anycosmic ray. NaI [22]and CsI [29] scintillators display phosphoresence in whicha fraction of the lightis emitted over a much longer time (hundreds of milliseconds) than the primaryfluorescence. Thus, when a particularly large energy deposit occurs, the crystalcan glow rather brightly for up to a second; the effect on the data depends on thedesign of the electronics.

Pulse shape analysis, whether by analog electronics or via flash digitizationof the PMT signal, has multiple uses for scintillators. If two different scintillatorshave very different light decay times, they can be read out bya single phototubeif they are sandwiched together, with the energy deposited in each still separablein the PMT signal. This can be used as a form of active shielding to veto chargedparticles or photons that interact in both scintillators. This configuration is calleda phoswich (phosphor sandwich). Pulse shape analysis canalso be used to dis-tinguish neutron and cosmic-ray ion interactions from the interactions of photonsor electrons in CsI [10] and plastic and liquid scintillators [46].

Table 1 summarizes some properties of the scintillators most commonly usedin space, as well as two particularly promising lanthanum halide scintillators thathave recently become available. These new materials have good stopping powerand excellent energy resolution (34% FWHM at 662 keV versusabout 7% forthe industry standard NaI). They have a moderate internal radioactivity giving acount rate of 1.8 cm3s1, mostly from138La [49]. This is of the same order asthe background from other sources that an unshielded detector would receive inlow-Earth orbit, but could dominate the background if the detector is well shielded(see section 4.1 below).

6 David M. Smith

Table 1 Properties of Some Scintillators

Material Light Yield Decay Time Peak Density Max. Notes(photons/keV) (ns) (nm) (g/cm3) Atomic #

NaI(Tl) 38 230 415 3.67 53 1CsI(Na) 39 460, 4180 420 4.51 55 2CsI(Tl) 65 680, 3340 540 4.51 55 2BGO 8.2 300 480 7.13 83 3GSO 9.0 56, 400 440 6.71 64 4BC408 10.6 2.1 425 1.03 6 5LaBr3(Ce) 63 16 380 5.29 57 6LaCl3(Ce) 49 28 350 3.85 57 7

1 Good energy resolution. Hygroscopic; phosphorescent; Susceptible to thermal shock.2 Slightly hygroscopic; phosphorescent. Denser, less brittle than NaI. Pulse-shape discrimina-tion of particle types is possible.3 Excellent stopping power; inferior energy resolution; easily machined; non-hygroscopic.4 Gd2SiO5; non-hygroscopic. Used in the Hard X-ray Detector (HXD) on Suzaku [64].5 Acommonly used plastic.6 New material; some internal background from radioactivity. Best energy resolution, good stop-ping power.7 Similar to LaBr3, resolution and density not quite as high. Large crystals were developed ear-lier.Data are taken from Tables 8.1 and 8.3 of Knoll [33] and from Bicron [9].

3.2 Semiconductor detectors

In semiconductor detectors, the electrons and holes excited into the conductionband by the passage of energetic particles are swept toward opposite electrodeson the detector surface by an applied electric field. The image charges inducedon one or both electrodes, as they change with the movement ofelectrons andholes in the crystal, provide a small current pulse. This pulse is generally read bya charge-sensitive (integrating) preamplifier, followed by a shaping amplifier.

Semiconductor detectors are capable of much better energy resolution thanscintillators, since the collection of electrons in the conduction band is much morecomplete than the conversion to scintillation light and light collection in scintil-lators. The noise performance of the electronics must be excellent, however, ifthe natural resolution of the detector is to be approached i.e., the limit due tocounting statistics of the electron/hole pairs liberated (including the Fano factor[33, page 366]). In the preamplifier, noise currents are converted to voltage noiseproportional to the detector capacitance [62, page 33]; thus detectors with largevolume can be expected to show poorer resolution. Resolution can be preservedat large volumes if the electrode configuration has intrinsically low capacitance.Examples of such configurations are silicon drift detectors[54] and germaniumLO-AXTM [50] and drift [42] detectors, which have one large and one small,more pointlike electrode. Pixellating the anode and reading out each pixel as aseparate detector also results in low capacitance and excellent noise performance.Some of the most common semiconductor detector and electrode configurationsare sketched in Figure 3.

Large energy deposits from cosmic rays in semiconductor detectors do notcarry the risk of long-duration detector response that theydo in scintillators. Itis very important, however, to test the response of theelectronics to huge energy

Hard X-ray and-ray Detectors 7

A

C

DB

E

Fig. 3 Common electrode configurations for solid state detectors.Each is shown with a 1 cmmarker to indicate the scale of a typical detector. A) Coaxial germanium detector (cross sectionof a cylindrically symmetrical shape). Electrodes are on the inner bore and the outer surface;the insulating surface is at the back (dashed line). B) Silicon strip detector (typically 300mthick, 1 mm strips). C) Monolithic CZT detector with coplanar (interleaved) electrode to nullthe signal from the holes and improve energy resolution [43]. D) Simple silicon p-i-n diodewith plane electrodes at top and bottom. E) CZT pixel detector. The pixel and strip detectorsare shown with guard rings around the edge, used to capture any leakage current on the sidesurfaces and keep it away from the electronics chains reading out the volume of the detector.Pixel and strip detectors have been made from all the common semiconductor materials (Ge, Si,CZT/CdTe).

deposits. Many designs can become paralyzed for a significant amount of time bya multi-GeV energy deposit, or else produce false counts by triggering on ringing.Interactions this energetic do not occur in the laboratory,where the only cosmicray particles are muons, so they ought to be simulated eitherat an accelerator orwith a pulser before any electronics design is declared ready for space flight.

An extensive treatment of semiconductor detectors and their electronics isgiven by Spieler [62].

3.2.1 Germanium

Germanium detectors are preeminent for spectroscopy in therange from hundredsof keV to a few MeV. Germanium crystals can be grown in large volumes at ex-tremely high purity, with single detectors up to 4 kg [57]. High purity guaranteesthat both electrons and holes can move untrapped through thewhole crystal vol-ume, and that the detector volume can be depleted of charge carriers due to im-purities by a manageable applied field of5001000 V/cm. The small bandgapgives good counting statistics for the liberated electron/hole pairs, but requireslow operating temperatures below 130 K, and preferably much lower to pre-vent thermal excitation into the conduction band and a largeleakage current [33,pg. 414].

Energy resolution of0.3% FWHM at 662 keV can be achieved with a gooddetector and optimized electronics; this compares to a corresponding value of7% for NaI(Tl) and34% for the new lanthanum halide scintillators. Onlyif this high resolution is scientifically important should germanium be considered.

8 David M. Smith

But high resolution should be considered important any timenarrow lines are be-ing observed, even if the exact profile of the line to be observed isnot needed,and even if it isnot necessary to separate nearby lines. Since-ray observationsin space are often dominated by background, a good energy resolution reducesthe amount of background against which the signal of a narrow-ray line is to bedetected, greatly increasing sensitivity.

Operation of germanium detectors in space is challenging due to the need tokeep them cold. Passive cooling is possible if a large area and solid angle of radi-ator can always be presented to deep space (avoiding the Sun,and, in low-Earthorbit, the Earth as well). This technique was used for the Transient Gamma-RaySpectrometer (TGRS) on the Wind spacecraft [59] and the Gamma-Ray Spec-trometer on Mars Odyssey [15]. Cryogens can also be used, butthey have a largemass and limited life; the germanium spectrometer on HEAO3ran out of cryo-gen after a pioneering 154-day mission [44]. Some recent instruments have reliedon Stirling-cycle mechanical coolers [68; 61; 24]. The design of the radiator forwaste heat is still critical in that case, but the requirements are not as severe as forpassive cooling. RHESSI, for example, often has the Earth nearly filling the fieldof view of its radiator for part of its orbit. Cryocoolers canbe very expensive toqualify for space flight and this should not be underestimated in mission planning.

All the germanium detectors mentioned above were in the closed-end coax-ial configuration (Figure 3a), with an outer contact on the sides and across thefront face of the detector, an inner contact lining a bore that goes most of the waythrough the crystal, and an intrinsic (insulating) surfaceon the back face. Thisconfiguration is a compromise between large volume, low capacitance (comparedto two flat electrodes on either side of a comparable crystal)and the lowest pos-sible distance between the electrodes (to keep the depletion voltage manageable).Thick germanium strip detectors are also being developed for Compton telescopesand other applications that require position sensitivity,and have already flown onthe Nuclear Compton Telescope balloon payload [13]. The x and y positions aremeasured by localizing the charge collection to individualstrips on each electrode(the strips on one side run perpendicular to those on the other), while the z po-sition is measured by the relative arrival times of the electrons and holes at theirrespective electrodes [3].

In addition to cooling, the other particular challenge for germanium detectorsin space is radiation damage. Defects in the crystal latticecaused by nuclear in-teractions of protons and neutrons create sites that can trap holes as they driftthrough the crystal. Electrons in the conduction band are not comparably affected.Since germanium detectors are designed to use both the electron and hole signals,hole trapping reduces energy resolution. Because protons lose energy rapidly byionization, they must have high energy to penetrate the layers of passive materialaround the detector (e.g. the cryostat) and penetrate beyond the outer surface ofthe crystal. Depending on the detector or cryostat configuration, the lower limit onrelevant proton energy is on the order of 100 MeV. Neutrons, on the other hand,can penetrate the full volume of the crystal regardless of their energy, and are rel-evant even at a few MeV [40]. Protons below 100 MeV can convertto neutronsvia spallation in spacecraft materials and therefore stillcause damage.

Strategies to reduce the effect of radiation damage includechoice of orbit,operating procedures, detector geometry, shielding, and annealing.

Hard X-ray and-ray Detectors 9

Fig. 4 Effect of radiation damage on-ray spectroscopic performance of a coaxial germaniumdetector. The 511 keV background line from positron annihilation is shown in data from theRHESSI satellite taken from 2002 to 2007. The symmetrical, narrow peak is from the start ofthe mission. The next two lines (dot-dash and dashed) show the effect of moderate to severedamage on the line resolution due to hole trapping. The last two lines (dotted and the nearly flatsolid line at the bottom) show the loss of effective area at very severe levels of damage due tovolumes in the crystal that are no longer depleted (active).At this point, the RHESSI detectorswere annealed. In general, an anneal would be performed at a much earlier stage of damage.

In low-Earth orbit, most radiation damage will come from radiation belt pro-tons seen during passage through the South Atlantic Anomaly(SAA). This can beavoided if the orbit is equatorial (inclination less than about 10o). This is the mostbenign orbit available, since the magnetosphere also protects the instrument fromsolar energetic particles and a large fraction of Galactic cosmic rays. In high-Earthorbit or interplanetary space, damage from Galactic cosmicrays usually dominates[34], unless a large solar energetic particle event occurs,in which case a largeamount of damage can be inflicted in a short time [52; 49]. An orbit that spendsmuch of its time in the heart of the radiation belts has by far the highest dose ofall, and would certainly prohibit the use of germanium. The Space EnvironmentInformation System (SPENVIS) webpage [27] is an extremely valuable resourcefor estimating the irradiation by radiation-belt and solarprotons in various orbits.

A good choice of detector geometry can limit the severity of the effect of radi-ation damage by limiting the amount of germanium that the holes must traverse.In the coaxial configuration, most of the volume is near the outside of the detector.Thus, by applying negative high voltage (HV) to the outer contact, the holes aremade to take the shorter path for the majority of interactions [51]. The result is aline shape with a sharp peak and a long tail (due to the few interactions near thebore). This is shown in Figure 4. This polarity provides gooduniformity of field

10 David M. Smith

within the crystal for slightly n-type material. The opposite polarity (used withp-type material) will show broadening of the line much earlier and more severelyas the holes migrate all the way to the central bore.

Shielding the detectors can be very effective at blocking solar and radiation-belt (SAA) protons, but not cosmic rays, which are much more energetic and pen-etrating. Very often, shielding is also desired to reduce background (see section4.1 below).

Keeping the detectors very cold reduces the amount of trapping for a givenradiation dose as long as the detector is never warmed up [16]. Raising the HVon a damaged detector, when possible, can reduce the effect of damage somewhat[34]. If large currents are passed through a damaged crystal, many of the holetraps will fill, and the effect of radiation damage will be much less severe for afew minutes, until these traps empty again. This occurs whena spacecraft passesthrough the SAA. The opposite effect occurs for damaged n-type germanium de-tectors when the HV is turned off: when it is turned back on, there are temporarilymore unfilled traps than in equilibrium, and the resolution will be degraded untilan equilibrium between detrapping and hole production is reached on the sametimescale of minutes [32].

Even if all these factors are taken into account in design, virtually any germa-nium detector that isnot in a low-Earth, equatorial orbit will have to be annealed.When the crystal is heated to well above operating temperatures, many of the dam-age sites become de-activated (not repaired, since the anneal temperatures are fartoo low to actually move atoms around in the lattice). The mechanism is not wellunderstood. The literature includes many small-scale experiments that dont givea good, overall formula for estimating the efficacy of the anneal process given de-tector type, damage history, and anneal temperature and duration. Temperaturesof 50o to 100oC and durations of days to weeks are typical of operations in space[41; 15]; when in doubt, the longer and warmer, the better [16]. Some anneal-ing does take place at room temperature [53] but takes longerto be effective, andcannot eliminate trapping completely.

3.2.2 Silicon

Silicon can be grown in large volumes but not to as high a purity as germanium,and is therefore harder to deplete. Silicon detectors are therefore generally thin(typically 300m), and used for purposes where that is appropriate. The thickestsilicon detectors (up to1 cm) are made from slightly p-type material and havelithium ions drifted through the crystal bulk to compensatethe intrinsic impuri-ties, a technique formerly used for germanium before high-purity material becameavailable.

Small, simple planar p-i-n detectors are often used for X-ray detection up toa few 10s of keV, as in the top detector layer of the hard X-ray instrument onthe Suzaku spacecraft [64]. Small Si drift detectors (SDDs), in which the field isshaped to lead the electrons to a small collecting contact, show improved resolu-tion over p-i-n detectors due to their smaller capacitance [54] and can also provideposition sensitivity when the drift time is measured in the electronics.

Large, thin Si strip detectors single or double sided) can beused when positionresolution is important but energy resolution and low energy threshold are not,

Hard X-ray and-ray Detectors 11

such as in the silicon tracker on the Fermi Gamma-ray Space Telescope, wherethe requirement is to register the passage of high-energy electrons. Thick Si stripdetectors have been proposed for a Compton telescope operating in a mode wherea final photoelectric absorption is not necessary [35].

Si detectors benefit from cooling to reduce leakage current,but at a more mod-est level than germanium (temperatures of 20o to 0oC). This can be accomplishedby a careful passive cooling design or the use of simple thermoelectric (Peltier)coolers.

3.2.3 Cadmium Telluride and Cadmium Zinc Telluride

Cadmium telluride (CdTe) and cadmium zinc telluride (Cd1xZnxTe) offer twoadvantages relative to germanium: they can be operated at room temperature andthey have better photoelectric stopping power. It is difficult to grow large crystalsof high quality, and the largest detectors available are 14cm3 [18]. Efficient de-tection in the MeV range therefores require a three-dimensional array of detectors[47] to take the place of a single large germanium coaxial detector, with very care-ful control of passive material within the detector volume to prevent undetectedCompton scatters.

When energy response greater than 30 keV is needed but it is not necessaryto go above a few hundred keV, a single layer of CZT/CdTe detectors is often thebest choice. If pixels of a few mm or larger are desired, an array can be madeout of individual detectors, as was done for the Burst Alert Telescope (BAT) [7]on the Swift mission (CZT) and the front detector layer of theIBIS imager onINTEGRAL (CdTe) [37]. For smaller pixels, it can be advantageous to pixellatethe electrode on one side of a larger detector and read signals out of each pixel.Not only does this provide greater position resolution withsmaller gaps, the smallpixels have very low capacitance and excellent energy resolution. Pixellated CZTdetectors have been used on two hard X-ray focusing balloon payloads: the HighEnergy Focusing Telescope (HEFT) [14] and the INternational Focusing OpticsCollaboration forCrab Sensitivity (InFOCs) [65]. For InFOCs, signal traceswere routed away from the detector to the ASIC electronics, while for HEFT thepreamplifiers were put onto the ASIC with the same spacing as the detector pixelsand bump-bonded directly to the detector. The HEFT detectortechnology is beingadapted for the Nuclear Spectroscopic Telescope ARray (NuSTAR), an upcomingNASA Small Explorer satellite using focusing optics [25].

CZT and CdTe (and other compound semiconductors) differ from germaniumand silicon in that holes are much less mobile than electrons, and suffer trappingeven in crystals that are not radiation-damaged. Thus the best energy resolutionis obtained when only the electrons contribute to the energysignal. This is notpossible for a simple planar configuration (a rectangular detector with plane elec-trodes on opposite sides), but there are a number of ways to improve the situation.Coplanar grid [43; 2] and pseudo-Frisch grid [45] electrodeconfigurations cancancel most of the contribution of the holes to the signal forthick (1 or 2 cm)single detectors, resulting in an electron-only signal that recovers the excellentenergy resolution of a very thin detector. If a pixellated detector is desired for spa-tial resolution or low capacitance anyway, pixellating theanode also ensures thatthe electrons contribute most of the detected signal as theyget very near the anode

12 David M. Smith

(the small pixel effect). For a thick CZT or Cd/Te detector, it is also possible tomeasure the depth of the interaction within the crystal by measuring the risetimeof the current pulse and use this information to correct the energy measurementeither by analog or digital techniques downstream [55]. This technique was usedfor INTEGRAL/IBIS [37].

4 General Considerations

4.1 Background

Since most-ray detectors in space are not detecting focused radiation, count ratesare often dominated by background rather than signal. Bright transient events suchas cosmic-ray bursts, solar flares, and terrestrial-ray flashes are often source-dominated, but for most applications bothreducing and estimating backgroundlevels is important.

For an unshielded detector in low-Earth orbit, the dominantsources of contin-uum background are the cosmic diffuse radiation (below about 150 keV) and thealbedo glow of s from the Earths atmosphere due to interactions of cosmicrays (above 150 keV) [20; 23]. From a few hundred keV to a few MeV, radioac-tivity in detectors themselves and, to a lesser extent, in passive materials nearbycan be a significant source of both line and continuum background. Radioactivitycan be natural (e.g. from40K and the daughters of238U), or induced by cosmicrays, solar or radiation-belt protons, or neutrons createdin the spacecraft or inthe Earths atmosphere. Induced radioactivity can be prompt when short-lived nu-clear states have been excited, or have a half-life from seconds to years. Above afew MeV, the dominant component is likely to be from minimum-ionizing cosmicrays clipping the corners of the detector. Recently, there has been a great deal ofeffort put into refining tools for simulating the expected backgrounds [20; 69] (seeFigure 5).

Reducing background should first be approached by selecting the rightdetectorthickness (no thicker than necessary) and material (one that is not intrinsicallyradioactive and does not become badly so when exposed to cosmic rays on orbit).But if the instrument is not meant to observe the entire sky, there should usuallyalso be some shielding. Below 100 keV, passive shielding can be adequate. Itis often arranged in a graded configuration, with a high-Z material like lead ortungsten on the outside followed by one or two layers of lower-Z material, each ofwhich is meant to stop K-shell X-rays from the previous layerbefore they reachthe detector.

At Compton-dominated energies (see Figure 1), thick passive material wouldbe necessary to stop incomings. In space, however, such a shield can actuallycreate more background than it stops, due to reprocessing ofincident cosmic raysinto neutrons,s, and multiple charged particles. Thus graded-Z and other passiveshields shouldnt be more than a few millimeters thick. However,active shieldingwith several centimeters of inorganic scintillator can be very effective. In this case,cosmic rays are vetoed along with their daughter particles produced in the shield,and only a single interaction is necessary to veto a background photon, even ifit then interacts in the central detector. An active shield will also veto photonsfrom the target that interact in the central detector but scatter out of it (Compton

Hard X-ray and-ray Detectors 13

Fig. 5 State-of-the-arta priori modeling of -ray instrumental background: data fromWind/TGRS and a simulation of all important background components using MGGPOD. FromWeidenspointner et al. [69].

shield mode). Even active shields produce background via neutron production[48]. A study for INTEGRAL/SPI found that 5 cm of BGO was the optimumshield thickness for its orbit [21].

At the highest energies, a thin, active plastic shield can veto the prompt com-ponents due to cosmic rays: clipping of the detector by the cosmic rays them-selves and prompt nuclear de-excitations in passive materials near the detectors.But it should be established that these background components will be importantin the energy range of interest before choosing to use a plastic veto. The abilityto veto charged particles that arenot cosmic rays is desirable for an orbit outsidethe Earths magnetosphere (for solar particles) or a low-Earth orbit that goes tohigh magnetic latitudes (for precipitating outer-belt electrons and, for nearly polarorbits, solar particles as well).

Estimating background is less important for detectors in imaging configura-tions (e.g. coded mask, rotating grid, or Compton telescope) that have ways to re-ject background based on incident direction. In these cases, it is enough to predictthe background accurately enough to have confidence in the instruments sensitiv-ity. But for non-imaging detectors, it may be necessary to know the backgroundto 1% or better to study faint sources. This cannot be done viaa priori modeling,if only because cosmic ray fluxes fluctuate much more than this. Instead, back-ground is subtracted by finding a period of time when the source is not visible butthe background is expected to match that during the observation. For highly col-limated instruments, this is best accomplished by chopping between the sourceposition and an empty field nearby, as was done with the Oriented ScintillationSpectrometer Experiment (OSSE) on CGRO and the High-EnergyX-ray TimingExperiment (HEXTE) on RXTE [31; 56]. For uncollimated or wide-field instru-

14 David M. Smith

ments viewing a transient event like a cosmic-ray burst or a solar flare, timeintervals just before and after the event, or (in low-Earth orbit) 15 orbits (one day) away often provide an excellent background measurement. The case of anon-chopping instrument measuring a non-transient sourceis the most difficult. Avariety of techniques can be mixed, combining observationsand modeling, includ-ing the use of the Earth as an occulter and the generation of background databasesincorporating large amounts of data from throughout the mission. Such a databasecan be used to extract the dependence of background on parameters such as orbitalposition and cosmic ray flux [30; 60].

4.2 Livetime

Instrumental livetime is generally of concern only when background is not i.e.when very bright cosmic, solar or terrestrial transients are of primary interest. Inthese cases, all stages of the signal chain should be analyzed to make sure thatthe highest expected count rate can be recorded. The intrinsic response time ofthe detector material (scintillation light decay or electron and hole drift times ina solid state detector) may be a consideration in detector choice, but only if pulseshaping times and throughput in the rest of the electronics can be designed to keepup with the detectors capability. Large detectors can be pixellated or replacedwith many small ones to reduce deadtime, at the expense of an increase in thenumber of electronics chains needed. The deadtime caused byan active shieldveto should always be estimated, particularly when the instrument is very large orwill be studying bright transients. When scattering between detector elements ispart of the source detection (such as in a Compton telescope or polarimeter), thefrequency with which two independent background or source interactions will fallwithin the instruments coincidence time window by chance and be mistaken fora scatter should always be calculated.

4.3 Spectral Response

Lastly, it is important to understand the energy response ofany detector design dueto the physics of the high-energy photon interactions. Incomplete collection of theincident photon energy is important for both line and continuum spectroscopy, butis most obvious to the eye when a narrow line is being observed. At energies whereCompton scattering becomes important, a Compton continuumbelow the incidentenergy appears in the spectrum due to scattering either intoor out of the detector.At low energies (within a factor of2 the K-edge of the detector material), a K-shell X-ray escape peak appears since absorption occurs very close to the surface.At MeV energies, two escape peaks appear corresponding to the escape of one orboth 511 keV photons following pair production and annihilation of the positron.Figure 6 shows the photopeak line, Compton backscatter feature, and first 511 keVescape peak from a RHESSI observation of the 2.223 MeV line from a solar flare,from neutron capture by a proton producing deuterium.

These effects combined with the blurring effect of finite energy resolution combine to make up the response matrix, the function thatmaps the inputspectrum of incoming photons to the output spectrum of detector counts. Good

Hard X-ray and-ray Detectors 15

Fig. 6 RHESSI spectrum of the solar flare of 28 October, 2003. The emission in this energyrange is dominated by the response to the 2.223 MeV line, which includes the photopeak, theCompton continuum from photons that scatter nearly 180o out of the detector (cutting off around2000 keV), and the first 511 keV escape peak (1712 keV). There is an underlying, falling con-tinuum due to other flare components as well.

stopping power (high atomic number) and an active Compton shield can keep thediagonal components of this matrix dominant, making interpretation of the spec-trum easier. It is not possible to unambiguously invert a nondiagonal instrumentresponse matrix and calculate a unique photon spectrum given the observed countspectrum. The usual practice is to convolve models of the expected spectrum withthe response matrix and compare the results to the observed count spectrum.

Monte Carlo tools such as GEANT should be used to model and predict theresponse matrix for any new design.

5 Outlook

Germanium (or, at hard X-ray energies, the other solid statedetectors) provideshigh enough energy resolution for most astrophysical purposes> 10 keV. BGOprovides stopping power and large volume up to the mass limitthat a launch ve-hicle can reasonably haul. So the current challenges in detector development arecombining high resolution with high volume, and adding fine,2- or 3-dimensionalspatial resolution for Compton telescopes, coded masks, orimagers. Germaniumstrip detectors provide an appealing compromise of very high spatial and energyresolution with moderate detector volume and stopping power. The key devel-opment issues are the difficulty, power and expense of cooling and minimizingthe amount of passive material in and around the array, material needed for ther-

16 David M. Smith

mal control as well as mechanical and electrical connections. When very highspatial and spectral resolution are not required, the new lanthanum halide scin-tillators are becoming a popular option in proposals for space instruments, havespectral resolution better than other scintillators and very good stopping power.They are appealing if their intrinsic background can be tolerated. Recently, high-performance Anger camera prototypes have been developed using SDDs to readout a large scintillator [38; 36]; SDDs have also been coupled to a physically pixel-lated scintillator [71]. These technologies may provide aninteresting alternativeto pixellated semiconductors for moderate to high spatial resolution.

Development is also always in progress on other technologies that are promis-ing but still present technical challenges, such as liquid xenon [4; 5], semiconduc-tors such as TlBr [18; 28] and HgI2 [67], and scintillators such as SrI2 [70; 26]and BaI2 [19].

I would like to thank Mark McConnell for very helpful suggestions on themanuscript of this chapter.

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1 Introduction2 Configurations and energy regimes3 Detector materials4 General Considerations5 Outlook