Energy dispersive detectors for synchrotron radiation

Nuclear Instruments and Methods 201 (1982) 85-91 North-Holland Publishing Company

85

ENERGY DISPERSIVE DETECTORS FOR SYNCHROTRON RADIATION J.S. W O R G A N Science and Engineering Research Council, Daresbury Laboratory, Daresbury Warrington WA4 4AD, U.K.

A brief review is given of energy dispersive detectors and comparisons made for devices commonly used with synchrotron radiation. The principles of operation of semiconductor detectors and signal processing methods are described, from which simple estimates of performance expectations are derived. Synchrotron radiation applications, limitations and further prospects are discussed.

1. Introduction The energy dispersive method is frequently seen as an attractive alternative to traditional scanning X-ray spectroscopy largely because of the simplicity of the technique and the way in which spectral data can be collected in a simultaneous fashion. However, in spite of these advantages, relatively few applications survive due to technical limitations of one sort or another; in particular problems can occur with conventional X-ray sources because of uncertainties in spectral distribution and low intensity in the continuum relative to the characteristic line. It would therefore appear that synchrotron radiation from a storage ring is the ideal source for the method since the spectral distribution is broad, calculable and free from discontinuities. It is also available in large fluxes even at considerable distances from the source. Whilst these features can be exploited for some applications it is not possible to fully realise the potential of the method principally because of count rate limitations in detector systems. Even so at the Daresbury Synchrotron Radiation Source energy dispersive methods will be applied to topography, interferometry, powder diffraction, microprobe analysis and Compton scattering experiments.

scanning crystal methods for high resolution requirements and is limited to those applications where a modest resolution, usually not worse than a few per cent, can be tolerated. This can be difficult to achieve for low energy X-rays. The most useful EDD types are the scintillation counter, the gas detector and the semiconductor detector. For all these detectors the energy resolution is given by AEfwhm :

2.35

where F = Fano factor. E is the incident energy and ¢ is the energy required to create an electronhole pair. Approximate values for F and c are given in table 1. As a consequence it can be seen that the scintillation counter has a resolution no better than 25% at 20 keV and gets worse with decreasing energy. This detector does have a good signal/noise ratio due to a large "internal" device gain; it is compatible with high speed electronic signal processing and consequently is able to work at high rates. However, the poor resolution performance makes it difficult to take advantage of the rate capability in synchrotron radiation applications.

Table I

2. Detector requirements Perhaps the most important parameters of an energy dispersive detector (EDD) are energy resolving performance, useful spectral range and throughput rate. The E D D cannot compete with

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III. SOLID STATE DEVICES

86

J.S. Worgan / Energy dispersive detectors

The gas filled ion chamber is a simple and stable device, easily constructed with volume matched to the application [1]. When used as an energy analyser the charge contributions from electrons and positive ions must be measured for each event and the slow moving positive ion (saturation velocity typically 1000 c m / s ) will limit the count rate. The gas proportional counter, besides having internal gain does not suffer from the above limitation since the multiplication process takes place very close to the central wire, so that the charge due to electron motion is virtually independent of the location of the ionsing event. However even though it is not necessary to wait for the positive ion before taking an energy measurement of each event the space charge build-up at high rates will seriously degrade the energy resolving performance. An energy resolution of 11.6% at 5.9 keV for an Ar + Ne mixture is reported by Sipila [2] a result which is critically dependent on the purity and stability of gas mixtures used. The count rate is not quoted. Even better results are claimed for the gas scintilation proportional counter [3-5](8.5% at 5.9 keV) although at the expense of complexity in the apparatus and again the device, which has a very high sensitivity, is only suitable for measurements at relatively low count rates. The values of F and c quoted for the semiconductor detectors clearly show the superior energy resolving performance to be expected from this detector (1.6% at 5.9 keV), although in practice this figure is not achieved due to the small signal level relative to amplifier noise. Furthermore, acceptable performance is only possible by operation at liquid nitrogen temperatures for low energy X-ray measurements. In spite of these disadvantages virtually all EDD systems in use at synchrotron radiation facilities use the semiconductor detector and the remainder of this paper is devoted to the characteristics and applications of these devices.

portant advantages: (a) the energy required to create an ion pair is ten times less; and (b) for common semiconductor materials the negative and positive charge carrier velocities differ by no more than a factor of 4 or 5, contrasting very favourably with the factor of one thousand for the gaseous ion chamber. Since the detector dimensions can be quite small the detector has speed as well as resolution advantages. Typical energy resolution characteristics are shown in fig. 1 [6] where the two straight lines show the idealised performance of silicon and germanium with line broadening due to the statistical effects of the charge collection process only. The two curves indicate practical values for a silicon detector coupled to a measuring system with equivalent electronic noise values of 100 and 200 eV respectively. The effect of amplifier noise clearly dominates performance in the lower energy range. Here the resolution is given by:

+ ( .35 and for an equivalent noise value of e n = 100 eV (which represents current best performance) A E = 140 eV (2.3%) at 5.9 keY. Careful selection of electronics and shaping time constants is necessary to achieve such resolution. For conventional R C network shaping a time constant not less than l0 /~s is necessary which of course limits the counting capacity to a few thousand events per second if resolution and peak stability are to be preserved. A resolution of 105 eV achieved by Goulding

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The semiconductor detector can be regarded as a solid state ionisation chamber in which the charge collection mechanism is similar to that in the gaseous ion chamber. The detector has two im-

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J.S. Worgan / Energy dispersive detectors

[7] in 1970 with carefully selected components still represents the best reported performance to date. The input stage FET (2N 4416) remains the best low noise input system available. Since the mean square equivalent noise voltage is proportional to time constant in the high frequency case the effect of reduced time constant on system resolution can be estimated. This indicates that time constant reductions to 1/ts and 0.1 /~s will increase the energy resolution (at 6 keV) to 330 eV and 1 keV respectively. Thus a penalty in resolution performance is imposed when time constants are reduced to obtain faster counting rates, a common situation encountered with synchrotron radiation sources. One aspect of the problem, namely the small signal level from the detector can be derived from fig. 2 which represents a typical charge amplifier input stage. The input charge Q = eE/c = 5.3 × 10 -17 C / k e V of incident photon energy. C F is typically I pf so V0 --53 /~V/keV and the resolution figure of 140 eV represents an input variation of only 7.4 #V. A question frequently posed by synchrotron radiation users, namely, to what extent can the throughput rate be increased when energy resolution is not a prime importance, cannot be answered simply. The reason is that other related effects besides peak broadening become significant at high rates. Amongst these effects are base line shifts, peak positions shifts, emergence of false peaks and asymmetric distortion in spectral profiles, all or some of which may be unacceptable for a particular application. These effects are discussed in the literature where it can be seen that there is no simple generalised solution [8,9]. It is more useful here to indicate simply some approximate limiting

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values which can be used in the assessment of experimental applications.

4. Count rate limitations

The fundamental speed limitation (neglecting electronic curcuitry effects) will result from the finite charge collection time of the detector element itself. Typical charge carrier mobilities for holes and electrons in common detector materials are given in table2; values for gas detectors are given for comparison. It is seen that the common detector materials silicon and germanium have a clear advantage, germanium particularly, in speed of charge collection, although it should be noted that the cooling normally required for acceptable minimum noise performance with silicon and germanium reduces the carrier mobility by an order of magnitude relative to room temperature values and that materials which may have less cooling requirement - CdTe, HgI 2 - have large disparity between electron and hole mobilities. The limiting saturation velocity for a standard E D D is approximately 10 n s / m m ; thus for a typical detector depth of 5 mm the charge collection time is from 25 to 50 ns. It follows that if a limit of 5% distorted events due to detector pile-up is specified a worst case input flux restriction of 106 p h o t o n s / s is necessary. The electronics of a typical system cannot process events at this rate; in fact it is standard practice to include inspection circuitry to reject events which would interfere during the acceptance and measuring sequence which normally takes several microseconds. It is important to realise however that whilst the inspection process safeguards the integrity of electronic analysis and limits the observed output rate to modest levels it does not eliminate the effects of distortion within the detector itself. This situation, which is frequently encountered in synchrotron radiation applications can only be controlled by attenuating the X-ray flux incident on the detector which considerably reduces the potential of the method.

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Turning from the limiting factors of the detector itself a simple estimate of the limits which will III. SOLID STATE DEVICES

J.S. Worgan // Energy dispersive detectors

88 Table 2 Some properties of detector materials

Energy per charge pair (eV) Band gap (eV) Electron mobility a) (300 K) Electron mobility a) (77 K) Hole mobility a) (300 K) Hole mobility a) (77 K)

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Germanium

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4.46 1.4 1000 2x 104 100 500

4.2 2.1 100 400 4 50

~ 30 5× 103 1.7

"~ Mobility in cm2/Vs.

be present in an idealised electronic processor can be derived. A typical arrangement is shown in fig. 3 which consists of detector and charge preamplifier mounted in a liquid nitrogen cryostat, a pulse processor containing precision integrator (I) or some form of time variant filter, a pulse pile up protection circuit (P) and a base line stabiliser (B) all controlled by logic circuits set up by discriminators (D) which inspect the incoming pulse information. The performance requirements of the pulse processor are particularly demanding when one considers the low level of signal from the detector, the resolution demands and linear energy conversion from about 3 keV to beyond 60 keV. The theory and design of analogue pulse amplifiers is treated extensively in the literature [6,10-12] including a novel approach to the analysis of various pulse shaping systems [13]. In a wide range system, recognition of a genuine

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single photon event requires at least twice the charge collection time even for an ideal amplifier. A finite period is required for measurement and the subsequent restoration of the amplifier base line such that even if the predominant time restriction (noise filtering) were removed each event would require at the very least a processing time of about 125 ns resulting in an idealised maximum count rate of 8 × l05 pulses/s at the pulse processor output. Commercially available systems operate at around 10 4 pps and systems aimed at 105 pps are under development. These figures refer to the output or throughput rate. Where manufacturers' specifications show performance characteristics plotted as a function of count rate extending to high values this is invariably to illustrate the system stability in the presence of high input rates. Output rates are considerably lower due to pile-up rejection. It is important that the above values are

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J.S. Worgan / Energy dispersive detectors

borne in mind when planning future experiments since without major technological advances significant improvements cannot be expecte d .

6. Applications to synchrotron radiation Applications range from the simple use of a detector with "monochromatic" radiation experiments where isolation of harmonic content is important, to wide range simultaneous "white beam" experiments. Typical arrangements are discussed in refs. 14-17. Since many applications use the "white beam" of synchrotron radiation it is useful to consider the intensities encountered. In the region from 0.5 ,A to 2.A total flux rates from a storage ring of 1016 p h o t o n s / s / m r a d are nOt unusual which, allowing for typical divergences ifl this spectral range, will give flux rates of 2.5 × 1013 photons/mm2/s and 1.5 × 1012 phot o n s / m m 2 / s at 20 and 80 m from the source origin respectively. Using a 10 × 10 micron collimator as a practical limiting device reduces the above to 2.5 × 10 9 and 1.5 × 108 photons/s so clearly the detector cannot be used in the direct beam without an absorber attenuating by at least three orders of magnitude. Corrections for the absorber function detract somewhat from the usefulness of the detector for direct spectral measurements.

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Conversely however the detector can perform very well with weak scattering samples where simple experimental geometry can easily give spatial resolution resulting in an energy spread which is negligible compared with the basic detector resolution. For small angle (mrad) scattering experiments attenuations of 106 or greater can provide adequate signals. Since the spatial resolution component varies inversely with the scattering angle the detector aperture can be increased for large angle studies with very weak scattering systems. This very high sensitivity and the source characteristics can be used to advantage in X-ray fluorescence analysis [ 18].

7. Device selection Where the technique is applicable the factors which influence the choice of detector can be considered as follows. Planar geometry is almost always used for energies below 100 keV and will give better energy resolution than coaxial configurations. Silicon and germanium form the basis of most detectors where crystal manufacture has been extensively developed for the semiconductor industry. It is essential that high resistivity material is used so that noise currents are minimised; this is achieved either by refining to hyperpure state or

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Fig. 4. Spectral response of Si and Ge detectors. III. SOLID STATE DEVICES

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J.S. Worgan / Energy dispersive detectors

by impurity compensation normally by a lithium diffusion process. Compensation is economically the simpler solution giving a high yield of units with closely matched performance characteristics and is the usual technique adopted for silicon detectors. The method has been used for germanium detectors in the past but the diffused structure is not Stable at room temperature and must therefore be always kept at liquid nitrogen temperature. As techniques have improved this detector has been superceded by the hyperpure germanium type. The relevant properties of Ge and Si are summarised as follows: Silicon (Li) + storage stability at room temperature + fewer calibration discontinuities at low energy + smaller performance variations + cheaper limited energy range (for given depth) Z :- 14 inferior resolution (c = 3.6, F - - 0.08) Germanium (hyperpure) + storage stability at room temperature + good energy range Z = 32 + superior resolution (c = 2.9, F = 0.05) K absorption edge at 11.1 keV expensive The spectral response of silicon and germanium detectors is shown in fig. 4.

8. A l t e r n a t i v e m a t e r i a l s

The most promising alternatives to silicon and germanium would appear to be cadmium telluride and mercuric iodiode [19]. These are both high Z materials allowing very thin detectors and have large enough band gap energies for possible operation at room temperature. However although they are becoming useful alternatives at higher energies the resolution is poor, typically 10% in the synchrotron radiation X-ray region. A further problem is the low carrier mobility of these devices which are approximately 100 times slower than germanium.

9. F u t u r e p r o s p e c t s

Recent advances in high speed analogue to digital conversion and digital data storage [20]

mean that the count rate bottleneck is, more than ever, concentrated in the analogue signal conditioning process. There is little prospect of removing this llimitation until a significant improvement in signal to noise ratio is achieved. As already stated noise in detectors and amplifiers has remained substantially the same over the last ten years, the best reported performance still coming from selected discrete components. Alternative semiconductor materials do not seem to have particular advantages in noise performance. It is possible however, assuming that the investment materialised from other market requirements, that custom built integrated circuits specifically matched to detectors could give superior performance and if cheap enough could introduce attractive possibilities for multi-detector systems. Another possibility would be to increase the detector signal by operation in the avalanche mode as a solid state proportional chamber. If a larger signal were available high speed current mode operation could be used or alternatively cheaper cooling systems could be employed. During the past few years promising work has been done in this area and signal gains greater than 100 have been reported [21]. However in other respects notably uniformity and reproducibility, performance is so far unacceptable at X-ray energies. Again the problems centre around the technology of extreme purity detector fabrication so the breakthrough will not occur without the market to stimulate substantial investment. The multielement or multi-chip detector is frequently quoted as an answer to the rate problem but is unlikely to gain wide acceptance simply on the grounds of cost since each element requires individual amplification at least to the A D C / c o m puter stage. Furthermore the geometrical restrictions compromise energy resolution and complications occur with cross-coupling particularly in optical charge restoration systems as used in the highest performance devices. The situation could change dramatically if cheap LSI systems become available. In the few applications where larger angle coverage of very weak fluxes is required clearly a single large area detector is more cost effective than a multi-element system. It should be noted however as a typical example than in substituting a 52 mm diameter detector (largest commercially available) for a standard 4 mm diameter unit, the cost is doubled

J.S. Worgan / Energy dispersive detectors

and resolution degraded from 150 eV to 800 eV. Commercial detectors are designed to work with conventional X-ray sources and consequently anticipate a requirement to work with relatively large apertures and collimation angles. Many synchrotron radiation applications require extremely small apertures and collimation so a marginal but useful improvement is possible by substituting the conventional geometry for a much smaller unit, reducing capacitance and surface leakage effects. Similarly, because of the fine collimation a coaxial geometry could be used to reduce detector transit time to less than 10 ns although this is of little consequence until the electronics can be correspondingly improved.

10. Conclusions

The application of energy dispersive solid state detectors to synchrotron radiation is attractive as a simple technique which has a broad spectral range from which events can be detected in a simultaneous mode. The energy resolution is at best only modest and the simultaneous mode cannot be adequately exploited due to the limited throughput rates possible. Significant improvements are likely to materialise only as a result of related research generated by large market forces, nevertheless the applications cited indicate the unique advantages of the method.

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References [1] H.W. Fulbright, Nucl. Instr. and Meth. 162 (1979) 21. [2] H. Sipil~i, Nucl. Instr. and Mete 133 (1976) 251. [3] H.E. Palmer and L.A. Braby, Nucl. Instr. and Meth. 116 (1974) 587. [4] A.J.P.L. Policarpo, Space Sci. and Instr. 3 (1977) 77. [5] D.E. Compstey and D.G. Voss, Nucl. Instr. and Meth. 167 (1979) 381. [6] F.S. Goulding and D.A. Landis, IEEE Trans. Nucl. Sci. NS-25 (1978) 2. [7] F.S. Goulding and Y. Stone, University of California Research Labs. Report, UCRL-19860 (1970). [8] Link Systems Limited, Pulse Processor Manual. [9] V. Radeka, IEEE Trans. Nucl. Sci. NS-15 (1968) 455. [10] N. Karlovac, IEEE Trans. Nucl. Sci. NS-22 (1975) 452. [11] D.A. Landis, N.W. Madden and F.S. Goulding, IEEE Trans. Nucl. Sci. NS-26 (1979) 428. [12] E. Fairstein, IEEE Trans. Nucl. Sci. NS-22 (1975) 463. [13] F.S. Goulding, Nucl. Instr. and Meth. 100 (1972) 541. [14] J. Bordas, I.H. Munro and A.M. Glazer, Nature, 262 No. 5569 (1976) 541. [15] J. Bordas and Sir J. Randall, 4th Int. Conf. on Small angle scattering of X-rays, Gatlinburg (1977); Daresbury Laboratory Preprint, D L / S R F / P 90 (1977). [16] A.M. Glazer, M. Midaka and J. Bordas, J. Appl. Cryst. 11 (1978) 165. [17] L. Gerward and B . Buras, Symp. on Crystallographic studies using energy-dispersive diffraction, A.C.A. Boston (1979). [18] C.J. Sparks, in: Synchrotron radiation research; eds. H. Winick and S. Doniach (New York, Plenum, 1980) p. 475. [19] M. Schieber (ed.) Int. Workshop on Mercuric iodide and cadmium telluride detectors, Nucl. Instr. and Meth. 150 (1978). [20] A. Berry, M.M. Przybyiski and I. Sumner, Daresbury Laboratory Preprint, D L / C S E / P 9 (1981) submitted to Nud. Instr. and Meth. [21] G.C. Huth, R.A. McKinney and R.J. Locker, IEEE Trans. Nucl. Sci. NS-15 (1968) 246.

III. SOLID STATE DEVICES