Fast response amplitude scintillation detector for X-ray synchrotron radiation

Nuclear Instruments anti Methods in Physics Research A246 (1986) 549-551 North-Holland, Amsterdam

FAST RESPONSE

AMPLITUDE

SCINTILLATION

549

DETECTOR

F O R X-RAY S Y N C H R O T R O N R A D I A T I O N E.N. DEMENTYEV,

M.A. SHEROMOV

a n d A.S. S O K O L O V

Institute of Nuclear Physics, 630090 Novosibirsk, USSR

The present paper describes a scintillation detector for X-ray synchrotron radiation. This detector has been created on the basis of a scintillator and a photoelectron multiplier (FEU-130) and its construction allows one to use the specific features of the time characteristics of synchrotron radiation from the electron storage ring. In a given range of amplitudes, the detector electronics makes a 64-channel amplitude analysis of the FEU-130 signal strobed by the revolution frequency of an electron bunch in the storage ring (f0 = 818 kHz). There is the possibility of operating the detector at high intensities of the monochromatic radiation incident on the scintillator. Such a possibility is directly provided by the time structure of SR and is not realizable with the use of other X-ray sources. The detector will find wide application in studies on X-ray structural analysis, transmission and fluorescent EXAFS- and XANES-spectroscopy, transmission scanning microscopy and microtomography, calibration of X-ray detectors and as a monitor on SR beams from the storage ring VEPP-4.

A m o n g the variety of unique advantages of synchrotron radiation from electron storage rings (high intensity, small angular divergence, extended continuous spectrum) there is one more important property: emission of radiation quanta from an effective SR source are rigidly correlated with the time of arrival of an electron bunch at the radiation point. Therefore, this peculiarity of SR ensures the possibility to build up detectors whose operation is synchronized with the moment of arrival of the SR quanta. The first detector of this type, designed at the Novosibirsk Institute of Nuclear Physics for work on synchrotron radiation from the VEPP-4 storage ring, is a multichannel one-coordinate scintillation detector for difference angiography [1]. At present, this detector is in successful operation. A great variety of other experimental work with X-ray synchrotron radiation, which is performed at the VEPP-4 storage ring, has stimulated the creation of a multipurpose detector with a fast amplitude analysis strobed by the time of arrival of the SR quanta at the detector. The requirements which the detector must satisfy were: a high efficiency in the X-ray range, compactness, high amplitude resolution, low noise parameters. In addition, its speed of response must be not less than the revolution frequency of the storage ring VEPP-4 (f0 = 818 kHz). In recent years, as to the amplitude resolution and the noise characteristics, the best Soviet industrial photoelectron multiplier is the FEU-130 whose distinctive feature is a high (15-20) amplification factor of the first dynode with GaP covering. With respect to the coefficient of light yield (hence, the amplitude resolution) and the efficiency, one of the best scintillators is a NaI(T1) crystal. Our detector has a metallic body and its 0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

overall dimensions are 104 × 70 x 60 mm (fig. 1). In the body there is a high-voltage divider (for FEU-130) packed in epoxy resin with a pocket intended to place FEU-130 with a NaI(T1) scintillator (the latter has a Be-window and a light emitting diode to adjust the detector without the SR beam) and an electron track for a fast amplitude-digit conversion of the photomultiplier signal. The electron track is schematically shown in fig. 2. It comprises an input strobed integrator, an analogto-digital converter (ADC) and a logic buffer. The time diagram of electronics operation is shown in fig. 3. The switch T1 of the integrator is open during the integration of the PM current. The integration period lasts - 300 ns and is mainly determined by the time of scintillator glow. With the integration process completed, the synchronizing circuit produces the starting signal for the A D C and the integrator switch is closed thereby discharging the integrating capacitance. The

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Fig. 1. General view of the detector. III(c). DETECTORS

E.N. Dementyev et al. / Fast response amplitude scintillation detector

550

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commutation noise compensating network allows the signals to be measured in the range from 1 mV to 3 V at C = 20 pF. The A D C consists of 64 comparators operating in parallel with a special device which converts their signals to the 6-bit code. The time of conversion is - 30 ns. The threshold voltages of the comparators are given by a resistive divider whose limiting voltages are, in turn, determined by the digital-to-analog converter (DAC). At the A D C output, an analysis is made with respect to the overflow (code = 63) or the absence of the signal (code = 0). If the case when the signal exists and ils magnitude satisfies the conditions UI < Uc< U2,

the pulses are written in the A D C code in the increment address of the memory device (MD) as well as the TTL pulses in the TTL-counter. Thus, the operation of the

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electronic track results in obtaining the spectral distribution of the signal in 64 words of the increment M D and the integral count in the threshold-given amplitude window in the TTL-counter. The detector can operate in the following three different modes: in the mode of low loads .N << 1, in the mode of average loads N - 1 and in the mode of high loads ~V>> 1 where JV is the average amount of the radiation quanta reaching the detector during one flight of the electron beam in the storage ring through the radiation point. In the lowqoad mode of operation (~V<< 1), the controlled thresholds make it possible to set the amplitude window by means of the observed spectral distribution in order to separate the quanta of definite energy. Hence, it becomes possible to discriminate the radiation harmonics, which usually occur, and to reduce the background load of the detector. In the average-load mode of operation ( N - 1 ) , because of a high enough probability that two and more radiation quanta arrive simultaneously in the same SR bunch. For the Poisson distribution, where the probability P, of arrival of n quanta is defined as P'=7

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one can work with the monochromatic radiation alone. One of the methods to measure the absolute intensity of incident quanta under such loads is to determine the integral PM counting, f, simultaneously with the number of the revolutions of the electrons in the storage ring, f0, with N defined as

Fig. 4 illustrates the load characteristic of the detector

E.N. Dementyev et al. / Fast response amplitude scintillation detector K

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Fig. 4. Curve of the load characteristic of the detector: K - the ratio of the measured intensity to that incident on the detector,

in this mode of operation, which has been obtained at a quantum energy E v = 18 keV using the flight ionization chamber as a monitor operated in the regime of ionization current measurement. One can see that the deviation from the linearity constitutes 8.6% under a load = 4.3 (this corresponds to ~Vf0= 3.5 M H z and 37% at = 8.2 (Nf0 = 6.7 MHz). A monotonic growth of the deviation with increasing load may account for an increase in the after-glow of the NaI(T1) crystal under large loads, which causes a shift of the spectrum towards large amplitudes and, correspondingly, an increase in the integral count of the PM. In the other method with the average-loaded detector, there is the possibility of analysing the 64-channel spectral distribution with several concrete channel windows corresponding to 1Ev, 2 E v, 3Ev etc. energies of the quanta (in

551

other words, corresponding to the arrival of one, two, three and more quanta in the SR bunch). The sum of the integrals in these spectral windows with the weight, corresponding to their energy, gives the total amount of quanta of energy Ev arrived at the detector. In the high-load mode of operation ( N > > 1), the intensity of the incident monochromatic radiation is determined by the centre of gravity of the spectral distribution of the PM signal, the position of this centre dependents linearly on N. The accuracy of the measurements in such a regime depends subtantially on the stability of the PM amplification and on the linearity of the A D C electron track. The above indicated possible modes of operation of the detector enable a wide application of it in experiments with X-ray synchrotron radiation. This detector is being employed already in the calibration of X-ray detectors, in studies on X-rays structural analysis~ transmission and fluorescent EXAFS- and XANES-spectroscopy [2], transmission scanning microscopy and microtomography [3] as well as the monitor [1] on the SR beams from the storage ring VEPP-4.

References [1] E.N. Dementyev et al., these Proceedings (Synchrotron Radiation Instrumentation, Stanford, 1985) Nucl. Instr. and Meth. A246 (1986) 726. [2] A.S. Sokolov et al., these Proceedings (Synchrotron Radiation Instrumentation, Stanford, 1985) Nucl. Instr. and Meth. A246 (1986) 360. [3] Yu.I. Borodin et al., these Proceedings (Synchrotron Radiation Instrumentation, Stanford, 1985) Nucl. Instr. and Meth. A246 (1986) 649.

III(c). DETECTORS