Nuclear Instruments and Methods in Physics Research A 405 (1998) 269-273
Fast, parallax-free, one-coordinate X-ray detector OD3
V.M. Aulchenko, M.A. Bukin, Yu.S. Velikzhanin, Ya.V. Gaponenko, M.S. Dubrovin, V.M. Titov,
A.I. Ancharov, Yu.A. Gaponov, O.V Evdokov, B.P. Tolochko, M.R. Sharafutdinov
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia
Institute of Solid State Chemistry, Novosibirsk 630090, Russia
Abstract
A second version of fast, parallax-free, one-coordinate X-ray detector OD3.2 destined for
X-ray diffraction experiments is presented. The construction, basic principles of operation, results
of the test with isotopic source 5SFe and X-ray beam test are being discussed.
1. Introduction
Since 1994 in Novosibirsk, BINP R&D work under the project of new one-coordinate X-ray
detector has been started. The detector is aimed for angular measurements in the diffraction
experiments at the synchrotron X-ray beam.
The distinctive features of the detector are as follows:
- the registration rate up to 10^7 photon/s;
- parallax-free in the angular range from 0° to 30°;
- variable focal distance from 40 cm to the infinity;
- photon coordinate resolution in transverse to the beam direction σ ~ 100 mkm (the angular
resolution depends on the focal distance).
The main idea of the detector with the above listed parameter was at first suggested in Ref.
[1]. The test of the first proportional chamber OD3.1 with focal distance 45 cm shows its good
reliability [2]. An electronics of the first detector was ready in the summer of 1995. After that, the
tests with X-ray tube and at the synchrotron radiation beam of the channel 5b of VEPP-3 storage
ring was started. The results of these tests were presented in the proceedings of the conference SRI95 [3]. The physical results, obtained in 1995-1996 with OD3.1 are presented in another report of
the present conference.
Since the autumn of 1995, the production of the second detector OD3.2 with a focal distance of 1.5
m has been started according to request of Japanese scientists for the Photon Factory of KEK.
Proportional chamber and preamplifiers of OD3.2 have essentially improved the design compared
with OD3.1. Some modification was also applied in electronics. At present, the detector is tested at
the beam of VEPP-3. In the report we describe the construction details of OD3.2 and the last test
results.
2. The basic principles of operation and construction
2.1. Proportional chamber
Photons scattered by the scanning object came over the Be window with a thickness of 0.8
mm and dimensions 260x25 mm2 into the drift gap of the proportional chamber (see Fig. 1) and
interact with atoms of the gas due to photoeffect. Appearing at photoabsorption secondary electrons
drift in the electric field to the anode wires where the avalanche multiplication takes place. A
coordinate of an avalanche is measured using charge distribution over the cathode strips. All
electrodes of the proportional chamber are placed in a leak-proof box, holding the pressure from 0
to 0.2 MPa. The materials for the chamber promotion are chosen specially for long term operation
without gas flow. The anode wires are joined into three groups which are provided by own anode
pre-amplifier for signal control.
It should be mentioned that a coordinate resolution and photon registration efficiency
depends on photon energy, gas mixture and its pressure.
Fig. 1. Schematic view of the OD3.2 proportional chamber and front-end electronics.
2.2. Electronics
Charge sensitive pre-amplifiers of signals from the cathode strips are located on the distance
of 5 cm from the strips outside the gas volume. They have a conversion coefficient k=10V/pC, and
dynamic range of output signal from -0.15V to +6.5 V. Analogous signals from pre-amplifiers are
transmitted via shielded twisted pairs with a length of 10 m into the crate with shapers and fast
analog-to-digital converters. A process of the event selection and fast co-ordinate definition was
described in detail in Refs. [2,3] and briefly as follows. The amplitudes of the signals from 54 strips
are digitized simultaneously by 8-bit FADCs with time discrete 30ns and transmitted into the buffer
for conveyer processing. An event selection scheme analyzes the buffer and searches for the signals
from separate photons. The photons with close location in space and in time are rejected by the
trigger logic. In spite of this, few photons separated by sufficient space (at least 3 strips) could be
registered simultaneously. The number of strips with maximal amplitude N, two amplitudes from
neighboring strips A(N-1), A(N+1) and time of the event with respect to the start of registration are
fixed for the selected events. The values of N, A(N-1), A(N+1) are used in coordinate processor
(see Fig. 2) for the photon coordinate (channel number) and energy definition. The tables, written
in RAMs, are used for all main conversions. The amplitude on the neighboring strips directly
depends on the avalanche coordinate, so, using these amplitudes as the addresses to read the data
from RAM1, there is a possibility to define the coordinate inside the strip with maximal amplitude.
This coordinate together with a strip number N are used as addresses for RAM2 to define the
general coordinate in the device range.
Fig. 2. A scheme of the coordinate reconstruction algorithm in OD3.2
A coordinate information is accumulated in the increment RAM with capacity of 64K 32-bit
words. There arc several modes of data accumulation in 16, 32, 64, 128, 256 or 512 frames with
size of 4K, 2K, IK, 0.5K, 0.25K or 0.125K channels correspondingly. The frame size is defined by
monitoring program and it could contain the all device scale, any part of it or several parts. The
frame duration could be chosen by program or according to the external timer signals. The minimal
frame duration is 1 mks.
An electronics of the detector is located in two CAMAC crates. One of them contains a set
of special modules with shapers, FADC and event selection logic joined together in one supermodule by the fast "pin-channel" bus and via the interface connects to the second crate, which
contains standard CAMAC modules and controller. We used two types of controllers for
connection with PC-486 and SUN-station.
2.3. Software
At the present time there are two main program packages for the detector control using PC486 and SUN-station. In both cases the window interface is exploited for the CAMAC modules
management and presentation of an amplitude, coordinate and service information in the graphic
and digital form [4].
3. The results of the detector tests
3.1. Test of the proportional chamber
The reliability of the proportional chamber was tested with gas mixture Ar-CO2 10% at
pressure of 1 and 2 atm. Counting rate characteristics with and without isotopic source Fe55 for one
of the wire groups are presented in Fig. 3 and show that PC has its own noise ~ 10 Hz and wide
range ~ 500 V of working voltage. The dependences of an average amplitude, corresponding to the
photon energy 5.9 keV, as a function of anode and drift voltage are shown in Fig. 4. A drift voltage
variation above 2000V does not influence on the signal amplitude while the dependence of
amplitude on the anode voltage has an exponential behavior. At gas pressure of 2 atm (0.2 MPa)
the working voltage increased ~ 600V compared with 1 atm case.
Fig. 3. Counting rate characteristics of the chamber with and without isotopic source Fe55.
Fig. 4. Anode signal amplitude, corresponding to the 5.9 keV energy deposition, as a function of
the anode and drift voltage.
The amplitude conversion coefficients in the different shaper channels were adjusted
uniform with precision better than 1%. After that the spectral measurements have been done.
In Fig. 5a a spectrum of sum amplitude from three nearest to the avalanche strips is shown.
It was obtained at irradiation of the chamber volume by photons from isotopic source Fe55 without
any collimation. A resolution of 5.9 keV peak on FWHM is 31%. An amplitude of the peak is
changing less than 5% at movement of the isotopic source in front of the window. All of these
confirm good geometrical quality of the proportional chamber. The same condition was used to
obtain the spectrum from the strip located at left of the strip with maximal amplitude (Fig. 5b). This
figure shows, that amplitude of the signal on the side strip is variated ~ 2.5 times depending on the
avalanche position under the central strip. The noise signals spectra (Fig. 5c) was accumulated
during 5 minutes. It has an exponential decrease with amplitude, that confirms normal chamber
mode of operation.
Fig. 5. Cathode spectra: (a) sum of amplitude from the three strips nearest to the avalanche
produced by photons from isotopic source S5Fc; (b) amplitude of the side strip with respect
to the strip with maximal amplitude; (c) sum amplitude of the noise signals; (d) sum
amplitude from the synchrotron radiated photons with energy of 8.3 keV.
3.2. Beam test of the detector OD3.2
In July 1996 detector OD3.2 was tested at synchrotron radiation channel 5b of the VEPP-3
storage ring. The detector was installed on the goniometer and horizontal (in transverse to the beam
direction) translation stage, which together provide the rotation with respect to the focal point,
located at 1.5m in front of the chamber. Photons with energy 8.3 keV were selected by the crystal
monochromator. The beam divergence and its size were controlled by the set of horizontal and
vertical slits before the detector.
In Fig. 5d a spectrum of sum amplitude from three strips is shown. It was accumulated at
the photon beam width of 0.2mm and observed counting rate of ~ 233 kHz/channel, that is higher
than maximum project frequency 200kHz/ channel. Obtained resolution of the main peak FWHM is
28%. A prolongation of the spectral distribution to the large amplitude region could be explained
by the photons with higher energy in the beam. It was proved by suppression of the photons of
main harmonic using the absorber.
Before the coordinate measurements, a calibration of the detector was done. This procedure
was described in detail in Ref. [3]. At calibration, the horizontal stage moved the detector with
constant speed in transverse to the beam direction. A distribution of values ln(A(N+1)/A(N-1)) is
accumulated in memory. Using this information, the calibration arrays are calculated in suggestion
of uniform events distribution on coordinate. Then these arrays are used to fill RAM1 and RAM2.
Two histograms in the top of Fig. 6 show the coordinate distribution, obtained just after
calibration at back movement of the translation stage. Differential non-linearity, obtained from this
distribution is r.m.s. 2.5% (statistics contribution is 0.9%) over all scale. The histogram in the
bottom part of Fig. 6, was obtained for the discrete horizontal shifts of the detector with steep of 0.5
mm. This figure shows the detector resolution of the close, narrow peaks.
Fig. 6. Two top histograms show about 2/3 of the detector range with a coordinate
distribution obtained at movement of the detector with constant speed in transverse to the
beam direction. The bottom histogram, was obtained for discrete horizontal translation of
the detector with steep of 0.5 mm.
Fig. 7. A coordinate distribution is obtained for detector rotation around the focal point with
steep of 0.1°. Two top histograms show about 2/3 of the detector range. Two bottom
histograms are the extended view of the distribution edges.
A coordinate distribution in Fig. 7 was obtained at detector rotation around the focal point
with steep of 0.1°. It is clear, that the coordinate resolution does not depend on the photon angle
(parallax-free) and equal to 5 channel (FWHM) or σz=150 mkm.
A rate performance of the detector is also tested. The region, corresponding to the one
cathode strip, was irradiated by the synchrotron photon beam. An intensity of the photons
controlled by the number of absorber layers at fixed width of the slit gap and slowly decreasing
current in the storage ring. The total rate of the signals from the anode wires f_mode was measured
due to additional scalers; an average amplitude from 8.3 keV photons was fixed using spectrum; the
rate of events reading by the cathode electronics f_read was also stored. The anode rate as a
function of the absorber layer number is shown in Fig. 8a. At low photon flux (thick absorber) the
anode rate should coincide with a rate of the photon conversion f_γ in the chamber drift gap.
Extrapolating this dependence for the thin absorber and taking in to account the value of current in
the storage ring, one could calculate the real photon conversion rate f_γ for any photon flux. In Fig.
8b-d the dependences of the peak amplitude, event reading rate, and mode signal rate as a function
of the photon conversion rate are shown. It is clear, that even at four times frequency overload of
the detector (800 KHz/strip instead of 200 KHz/strip) the amplitude drop is 20%; about 15% of
events are loosed at reading while the anode rate is steel coincide with photon conversion rate. The
similar graphics in Fig. 9 but for much more overload are shown. These histograms demonstrate
that the saturation rate of the channel is about 2 MHz. However, the normal operation with correct
amplitude measurements is possible at much lower frequency, due to the strong signals overlapping
at high rate.
Fig. 8. Rate performance of the OD3.2 for the 4-times overload of the channel: (a) anode
signals rate as a function of absorber layer number; dependence of (b) pulse height, (c) rate
of event reading (d) anode rate versus the rate of photon interaction in the chamber drift
volume.
Fig. 9. The same as that in Fig. 8, but for larger overload of channel.
4. Conclusion
Described tests are the small part of all, that was doing with detector. They confirm that
parameters of the detector OD3.2 are close to the expected one. The coordinate resolution σ = 150
mkm was obtained for the photon energy 8.3 keV and gas mixture Ar CO2 10% at normal
condition. The detector is completely parallax-free for the focal distance 1.5 m. Narrow peaks in
the coordinate distribution could be distinguished at the distance of 0.5mm. The r.m.s. differential
non-linearity is 2.5% if the anode rate does not exceed 200 kHz/channel.
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