Towards Fast Transition Radiation Imaging Detectors *>*" with Csl

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WIS-91/78/Oct. PH

Towards Fast Transition Radiation Imaging Detectors *>*" with Csl Convertors

R. Chechik1, A. Breskin, A. Akkerman, A. Gibrekhterman, I. Frumkin 3 H. Aclander, and V. Elkind,

Department of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel

Presented at the IEEE Nuclear Science Symposium. Santc Fe, USA, November 1991

The Hettie H. Heinemann Research Fellow Also Soreq Research Ce Technion, Haifa, Israel.

Towards Fast Transition Radiation Imaging Detectors with Csl Convertors R. Chechik 1 , A. Breski.i, A. Akkerman, 2 A. Gibrekhtermaii, I. Frumkin 3 H. Aclander, and V. Elkind, Department of Physics, Weizmann I n s t i t u t e of Science, 70100 Rehovot, Israel

Abstract We present a new X-ray imaging method with a potential application to ultrarelativistic particle identification by Transition Radiation detection. It is based on the conversion of the X-ray photons in a thin layer of Csl and the amplification of secondary emitted electrons in a low-pressure multistep avalanche electron multiplier. The obvious advantages of the p 1 0 ' ) reached in these electron m u l t i p l i e r s ' - ' ' allow for high detection efficiency even of single electrons. Their high rate capability is derived from reduced space charge effects, due to fast ion removal and to a low charge density in the electron avalanche. The latter also reduces aging

effects. The fast amplification mechanism leads to a subnanusecond time resolution ' . Single electrons can be localized with an accuracy of the order of 200 /(in FYV1IM'). The method has been successfully applied to the detection of UV-photous with Osl photocathodes coupled to low-pressure avalanche electron multipliers 1 0 1 1 *.

A detector based on this principle, combining a C'sl convertor and a low-pressure electron imaging multiplier was tested with 6-60 keV X-rays and yielded very satisfactory results, as. reported in ref. 16-IK. Good localization properties (30-200 /JIII F W H M ) , subnanosecond time resolution, high X-ray detection efficiencies and very low sensitivity to particle background - of prime importance for particle identification in future colliders - were demonstrated and are summarized here. The response of Transition Radiation Detectors combining Li or Polypropylene (CH2) radiators and secondary emission Csl X-ray detectors was simulated and is presented.

Secondary emission X-ray detectors, for the detection of TR photons, based on a C-sI convert or coupled to a low-pressure gaseous electron multiplier, were proposed by Majewski K Csl seems to be the most efficient solid convertor for soft X-rays, and is widely used in combination with MOP electron multipliers '. [t has the same atomic number 2 . T h e D e t e c t o r (Z=54) as Xenon and therefore a similar X-ray abThe secondary emtssio). X-ray detector is shown sorption probability. ir< fig. 1. A converter foil is coupled to a multistage The primary electrons (photoelectrons, Auger avalanche electron multiplier, operating al low ga* and Compton electrons) of an energy of a few keV, pressure. Our prototype detector consists of several, 80 produced by the conversion of X-rays in the foil* lose their energy by a cascade of elastic and inelastic col- mm diameter, mesh and wire electrodes on G-IO lisions. This produces a large cloud of low-energy frames, mounted in a vacuum vessel. A thin win( E < 5 0 eV) secondary electrons, which can escape dow, made of 100 /jm Kapton, was used. The 40-800 the convertor surface. This process was studied in urn thick Csl convertor, is vacuum deposited on 20 detail, for various materials, by Henke et al ' and 11111 of Al to provide the potential. The substrate by Schwarz '. They demonstrated that I'sl has tin- is 2.5 //m thick mylar. The elections emitted from largest yield of secondary omitted electrons, up to the convertor are immediately amplified in a 3 mm 17 electrons per primary electron. Therefore Csl is wide parallel gap. The avalanche electrons are transa very good convertor for Tit photons, in the keV ferred directly, or through a transfer gap (not shown in the fig.), and further amplified in a M W P C Herange.

MIN mm? PARTII-IF PH0T0CATH00E • N SUPPORTING PLASTIC FOIL

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Fig.l A schematic view of the sr.canda.nj emission X-rinj detector. coupled to a low-pressure vudi.isUp secondary electron multiplier.

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A solid convertor

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ment, equipped with cathode delay-line readout. A detailed description of this element and the readout system is given elsewhere '. The detector was operated with 10-40 Torr of ethane, isobutane or dirnethylether (DME). These gases provide high gain, up to 10 for single electrons, even in a single parallel gap. The detector was tested with 6-60 keV x-rays, at normal incidence and at a grazing angle of 5" (0=85°), and with minimum ionizing electrons.

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3. Detector performance 3.1 response to X-ray photons (6-60 keV) Fig. 2 shows our calculated values of conversion probability in a 200 nni Csl layer. According to our calculations, which are consistent with the data of Henke ', this is about the optimal thickness for Xrays of 1-10 keV, balancing secondary electrons production and self-absorption. An obvious way to increase the effective convertor thickness without increasing the electron self-absorption is to operate the detector in a grazing geometry. For example, the absorption probability of 8 keV X-rays at a grazing angle o f l O o ( 0 = 8 O " ) i s 2 4 % (fig. 2). Fig. 3 presents our experimental values of detection efficiencies, measured with 8 keV photons, for various Csl layer thicknesses at normal incidence. The measured efficiency at a grazing angle of 5° is 22%, for a 200 run Csl converter, about 8 times larger than at normal incidence. The measured efficiencies are about 10%- lower than the calculated absorption values. Fig. 4 presents II) and 2D localization images, measured with an 8 keV collimated photon beam. (The slit image on the Csl convertor is 140 jum wide). At normal incidence ( l a ) the intrinsic resolution is of about 200 ;im FWHM. At a grazing angle a the resolution is expected to improve by a factor of sina. The localization images in Fig. 4b were measured at a grazing angle of 5°; the intrinsic resolution is estimated to be 50//m FWHM, not corrected for electronic noise or X-ray scattering. At higher photon energies the resolution deteriorates, due to an increase in the fraction of high-energy electrons escaping the convertor surface and ionizing the low-pressure gas. At normal incidence we measured resolutions of 280 /mi and 500 /(in FWHM for 17.5 and for 60 keV respectively. The time resolution of the detector was measured with a pulsed 1'V-photon source of tunable shows the time resolution intensity 1.16) Fig. 4

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Fig.2 The calculated conversion probability of Xray photons in a 200 mn Csl layer, for various incidence angles.

2000 4000 6000 |)liol(K:allioilc thickness. A

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Fig.S Experiviental detection efficiency of the secondary emission X-ray detector with S KeV photons, for various Csl thicknesses at normal incidence.

200 urn

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95 nm (intrinsic: - 50 um)

L Fig.4 Two-dimensional images and their •projections, obtained by irradiating the detector with 8 KeV photons through a narrow slit: a) normal incidence, intrinsic FWHM ^200 pra; b) grazing angle of 5°, intrinsic FWHM ^50fim. Jsobutane at 18 Torr, Csl thickness 200 nm.

electrons is expected ' , and thus the time resolution should be of the order of a few hundred picoseconds.

Fig.5 The time response of the detector to about S simultaneous secondary electrons. Peak separation is 4 7w, the FWHM is 600 ps. CH\ at 10 Torr. Signals measured on a 64 cm active area. measured with light pulses emitting about 8 simultaneous photoelcctrons from the Csl photocatliode. The FWHM is G50 ps. The time resolution is in the range of U.:J5-0.5 MeV), respectively. Fig. 6c corresponds to spontaneous single electron emission (dark current). It is our understanding that the higher pail of the spectrum of 6a is due to avalanches of multiple secondary-electrons induced by each X-ray photon. The lower part of the spectrum corresponds to avalanches initiated by single electrons, which are mainly produced by fast, primary, electrons ionizing

tin 1 ftii.s. T l i t ' U»v\cr pint of i In- s p e i t i t i m is similai to the single electron spectrum tit in . I'his iuterpre l a t i o n of i h e /)ijKe height -.peel i n in i- ^u j»jn>i I eit l i \ the data of l ' i ^ . 7l>. when- half of i h e l M p h o i n c a t h ode was covered l>\ a "uini A l la,\ei. Consequently

I ay or induce signals hi I In- |;as. I he pulse height Mist r i h u l ion associated w i l l i i h e i i u p . n l til' r e l a l n i s l i c electrons ( T i n . ). i* douiinantlv tine i n avalanches i u i l i a l e i l hy a small i i u m l i e i of eh-i-ti.ui- I 1 01 _M. O u r e x p e r i m e n t a l values fur the deiectUm etli

the high a u i p l i t u t i e pulses, i n i t i a l e d l>> i h e *luw seco n d a r v electrons ( 7 a ) . are strongly suppressed, and mostly fast p r i m a r y electrons which penetrate the A l

ciency of M i l ' s are present e. I in l i t : , v U»i ( \ | Hiii k ness of 200-MHI n m . T l i e ellii-ieiu \ seems to I M W veiv l i t t l e dependence on I lie •electrons of emission /.*mIn the prcanlYD// nm Csl.

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Fig.6 The pulse height distribution, induced by: at t> KcV photons, bj energy >0.5 MeV. c} spoiitaiit tins gle electrons). Tin rhetne jield pllficallon gap is I SI) I' c.o.-Torr. Isobutanc ill 17 Tore.

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Fig.S Experimental detection efficiencies of i electrons jE>U.j MeV,. :c:lh carious Csl eoncertor thicknesses. .\'o threshold applied (sec te.'ti. li'.-obutune at I'l Ton-

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energies. Based on such models we have calculated the mean free path for production of ^-electrons of various energies, by MIP's of 1-2000 MeV in Csl. as shown in Fig. 9. It is evident that the probability of producing a (^-electron of a few keV, which will develop a large cascade of secondary electrons, is very small - a fraction of a percent for a 200 nm Csl foil. The probability of producing a ^-electron of low energy (