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A solid state detector for charged particles at relativistic energies
NUCLEAR
INSTRUMENTS
AND
METHODS
7I
0969)
256-260;
©
NORTH-HOLLAND
PUBLISHING
CO.
A SOLID STATE D E T E C T O R FOR CHARGED PARTICLES AT RELATIVISTIC ENERGIES J.E. BATEMAN
Department of Natural Philosophy, Glasgow University, Glasgow W.2, Scotland Received 27 February 1969 Experiments are described in which a totally depleted silicon surface barrier detector is used to detect the passage of fast electrons (1.5-2.3 MeV and 150 MeV). A good signal to noise ratio is obtained and the output pulses have rise times of 7 ns
and adequate amplitude to drive tunnel diode logic circuits directly. The advantages of the detector over the photomultiplier-scintillator detector are discussed in relation to applications in high-energy physics.
1. Introduction
The detection of charged particles at relativistic energies is often done in conjunction with strong magnetic fields in order to define momentum. The sensitivity of the photomultiplier tube to such magnetic fields leads to complex light piping to remove the tube from the high field region in which the scintillator is situated. The silicon surface barrier detector is unaffected by strong magnetic fields and could, theoretically, offer a solution to detection problems of this type. The problems involved in the exploitation of this possibility are now listed:
The evaluation of various types of silicon devices as detectors for charged particles o f relativistic energies has been carried out by several investigators1"7). These tests have in general been carried out with two purposes in m i n d - t o test theoretical predictions of the distribution of the ionisation energy losses of the particles in silicon, and to try to find a solid state detector suitable for general use in high energy physics. This communication is concerned with the latter problem. The general conclusion of the earlier tests was that while for special purposes silicon detectors (of one sort or another) may have some advantages over other techniques, they did not look promising as general purpose detectors for relativistic charged particles2). Recently, however, more encouraging results have been reported by Aitken et al.8) with a 2 mm thick lithium drifted silicon detector. Satisfactory measurements of the ionisation energy loss of protons, pions, and electrons have been obtained and the viability of the detector in the environment of a high energy physics experiment demonstrated. However, if a detector is to be of general application it must have a response time comparable with that of a fast scintillator photomulfiplier detector. The low electric field and large charge carrier transit distances of a lithium drifted detector such as described above tend to lead to rather slow response times (of the order of tens of nsec). An overdepleted surface barrier detector of 0.5 mm thickness can, on the other hand, yield response times in the nsec region, while producing adequate signal amplitude to give an acceptable resolution of pulse height when a minimum ionising particle is detected. Two other advantages of the thin surface barrier detector are i m p o r t a n t - t h e high electric field and short charge collection distances obviate any trapping effects such as are described in 8) and the thinner detector causes less multiple scattering of transmitted particles.
a. The detector must be thick enough and the electronic noise of the detection system low enough to realise a satisfactory signal to noise ratio. b. The response time of the whole system (detector plus preamplifier) must be fast and of the same order of magnitude (nsec) as that of a plastic scintillator-photomultiplier detector. c. An adequate active area must be available. (There are several limitations on the area which a surface barrier diode can have, but the absolute limit has been raised to 4.5 cm ~ in recent years.) d. The device must be reliable and easy to handle. e. The associated preamplifier (it must always be mounted close beside the detector) must be of physical dimensions comparable to that of the detector itself for maximum usefulness. The way in which these requirements can be met are now summarised. Production techniques for surface barrier devices now allow requirements (a), (b) and (c) to be met simultaneously and guarantee requirement (d). For example, detectors of 0.5 mm thickness capable of total depletion can now be obtained with areas of up to 4.5 cm 2 and an associated noise width of 30 keV fwhm9). This combination of characteristics leads to a resolution of the Landau distribution for electrons better than that observed in fig. 4.
256
A SOLID
STATE DETECTOR
The problem of achieving a fast response time for the detection system falls into two parts: 1. The collecting field in the detector itself must be high enough to collect the ionisation produced by the incident particle in the order of a few nanoseconds. This requires over-depletion of the device without causing a serious increase in the detector noise width. This requirement can be met fairly easily up to active areas of ~ 4.5 c m 2. 2. The preamplifier must be fast enough to reproduce the rise time of the detector and at the same time preserve a small noise width. The development of the fast high gain field effect transistor (FET) (e.g. 2N4391) has led to charge sensitive preamplifier circuits which are adequate to the problem and which are also capable of miniaturisation as required in (e) above 1°' 12). The last important consideration is that of reliability. The preamplifier design is thoroughly tested and dependable. The reliability of the surface barrier detectors would depend very much on their manufact u r e r - the best commercially available devices show excellent stability. In the event of a failure replacement would be as simple as plugging in another diode since there is no feed for an elaborate housing as in the case of the photomultiplier. In long term operation the presence of an over-voltage takes care of fluctuations in the leakage current caused by variations in the ambient temperature and the gain of the system remains constant to a high degree. The general problems having been outlined, the
Magnetic Spectrometer ~ ~ l
Aluminiurn foil target (O-O0<~_SiZn Bremsstrahlung
I~
E~I
en
Preamplifier no.1
(:56 ~ : : ~
C76 r-~
beam
I
~
I Preamplifier ]-~
F-~
INS 601
' " L" "~sy ii_F_[Analyse r ~
3u s
Gate 4>LGeneratorJ Fig. 1. Experimental set-up for test o f the relativistic charged particle detector with a b e t m o f 150 MeV electrons.
FOR CHARGED
PARTICLES
257
specific system used in the present tests may now be described. 2. Constructional details
As indicated above the relativistic particle detector consists of two basic parts - a totally depleted surface barrier detector and a charge sensitive preamplifier. In its final form these two components will be contained in a c o m m o n enclosure of small dimensions (maximum dimension 10cm) similar to that shown~2). In the present case discrete components are used with the diode mounted on a standard preamplifier by means of a BNC connector. Since the charge sensitive preamplifier is an integrating device a further piece of electronics is added. This is indicated in fig. 1 as the "pickoff", which serves to differentiate the step function output of the preamplifier to produce a short pulse for logic purposes. The three items are now described separately. 2.1. T H E SURFACE BARRIER DETECTOR
The basic diode, C 56, is an aluminium on silicon surface barrier device made by the usual surface barrier techniques [ref. l j) for details]. The specification of the device is as follows: material:
8 k o h m • cm p-type silicon (ms carrier lifetimes), active area: 2 c m 2, thickness: 0.5 m m nominal (totally depleted), operating bias: 400 V (including 10% over-voltage), leakage current: 5/~A, electronic noise: 35 keV fwhm (at operating bias). The standard 2cm diameter slice is secured between pyrophyllite rings by means of epoxy resin and secured in the usual way with a brass ring 11). 2.2. THE CHARGE SENSITIVEPREAMPLIFIER A circuit diagram and detailed description of this circuit are given12). For present purposes the rise time and the noise width are the critical parameters. A 2 c m 2, 0.5 m m thick detector has a capacity of about 60 pF. With this source capacity the preamplifier has a rise time of 15-20 ns (depending on the FET) and a noise width of 10-15 keV fwhm. This rise time is that of the integrated charge pulse and, for logic purposes, it is differentiated to yield a pulse the rise and fall times of which are less than half this value. These pulses are shown in fig. 2a, b. The preamplifier noise is well below that of the diode and so is not a significant contribution to the energy resolution.
258 2.3.
J . E . BATEMAN THE LOGIC ELECTRONICS
Fig. 1 shows the block diagram of the circuitry used. The differentiating circuit used to generate a short current pulse from the output of the charge sensitive preamplifier is a simple two transistor operational amplifier fed by a capacitor. The bandwidth is sufficient to transmit the fastest pulse the preamplifier can supply and the best logic pulses obtained has a 7 ns rise and fall time. This circuit (called a pickoff in fig. 1) drives a tunnel diode discriminator directly and the 400 mV, 30 ns wide pulses from the latter are used for logic purposes. A description of the pickoff is given inla). Fig. 2b shows the output pulse derived from the pickoff.
3. The experimental tests In the first instance relativistic electrons from a radioactive source were used to test the detector. The counting rate was, however very low so a test was arranged in which pair produced electrons of 150 MeV
(b)
/i ....
i ....
i . /.
i .
.
.
.
i
.
.
.
.
i
.
.
.
.
i
derived from the bremsstrahlung beam of the Glasgow electron synchrotron were used. 3.1.
TEST WITH A 9 ° S t BETA PARTICLE SOURCE
Fig. 1 shows the experimental set-up for both tests: only the source of electrons is different in the two cases. C 56 is the 0.5 m m thick detector under test. C 76 is a geometry defining detector 1 m m thick (also capable of total depletion). The output of the preamplifier of C 56 is fed directly to the pulse height analyser through a 1 #s clipping line while the output from C 76 is passed through the differentiating circuit (pickoff) to the tunnel diode discriminator, the output of which drives the analyser gate circuit. This effects a coincidence adequate for the low counting rate used. In order to monochromate the beam of electrons from the radioactive source, the discriminator on C 76 was set at 1.5 MeV so that only electrons leaving at least that amount of energy in C 76 would trigger the analyser gate. The result is that the energy spectrum of pulses from C 56 is that produced by transmitted electrons in the 1.5-2.3 MeV range. (2.3 MeV is the end point of the beta spectrum.) Fig. 3 shows the observed analyser spectrum. It shows the characteristic Landau high energy "tail" but obviously suffers from very poor statistics. The most probable value of the energy loss agrees with the value calculated from the specific energy loss curve published in Ortec manuals to within a few percent. 3.2. TH~ TEST WITH 150 MeV ELECTRONS When the 150 MeV electrons from the magnetic spectrometer are detected, as shown in fig. 1, the discriminator level of the tunnel diode is set as low as possible since the electrons are also transmitted by C 76 and the pulse height obtained is only a few
(a)
6O u~ z 0 (940
SL~
EL 0
~t
Amplitude
= 166 KeY
LO O0
z
Vertical scale: 10mY/division Horizontal
scale:
19 Kov O
Fig. 2. a. Charge pulse waveform produced by a relativistic electron in the relativistic particle detector (output of charge sensitive preamplifier); b. Current pulse waveform given by pickoff circuit.
20
N 264Kev
¢I~
_ _
20ns/division
.40 CHANNEL
~ o_ .u u ~qlr~ ~
60
8Q
P-m
,
-
100
NUMBER
Fig. 3. Energy distribution produced in C 56 by electrons of energy between 1.5 and 2.3 MeV.
A SOLID STATE DETECTOR
FOR CHARGED
259
PARTICLES
1000
800 most
probable
energy
=
fwhm
U3 Z
loss
177KEY 48 ~eV
600
27%
0 (o
(detector
noise
= 35KeV
fwhm)
LL
0 13S
400
b3 ~D ~E
j
Z
200
0
J
0
20
193 KeY % i
40
i
60 80 CHANNEL
~
i
r
,
100 120 NUMBER
140
160
180
--
200
220
240
~
260
Fig. 4. Energy distribution p r o d u c e d in C56 by electrons of energy 150 MeV ( m o m e n t u m resolution ~, 2%). T h e solid curve is given by L a n d a u ' s theory.
hundred keV. C 76 provides a certain amount of m o m e n t u m definition and background suppression. However, as the m o m e n t u m is already well defined by the geometry of the experiment, the absence of the coincidence degrades the observed analyser spectrum only slightly. The m o m e n t u m resolution of the incident electrons is about 2%. Fig. 4 shows the analyser spectrum of the pulses observed in C 56. The most probable value of the energy loss shows good agreement with that obtained previously. (The slight increase is to be expected from the specific energy loss curve.) Since a good rate of electrons was obtainable from the spectrometer, fig. 4 shows good counting statistics, and a meaningful comparison with Landau's function 14) is made possible. 3.3. ASSESSMENT AND COMPARISON OF RESULTS The energy calibration is done in both cases by detecting the K and L conversion electron lines of a 2°3Hg source in C 56. The position of the two lines in the analyser spectra are indicated in figs. 3 and 4. A very poor counting rate was achieved with the radioactive source due to low source strength, restricted solid angle, and poor efficiency for total absorption of the energetic electrons in C 76. As a result fig. 3
gives a poorly defined Landau distribution. On the other hand, the statistics of fig. 4 are excellent (10 counts/sec were recorded) and an accurate value of 177 keV is obtained for the most probable energy loss. This agrees well with the slightly lower value of 166 keV indicated in fig. 3. The poor accuracy of fig. 3 does not really permit us to conclude that this comparison verifies the rise in the specific energy loss curve at high energies. Looking at fig. 4 we see that the general shape of the observed distribution of the ionisation losses of the 150 MeV electrons in the silicon of the detector is in fair agreement with L a n d a u ' s function (solid line). The most probable value of the energy loss, Ao, is observed to be 177 keV. This is not in agreement with the value of 215keV calculated from Landau's formula when the thickness of the detector is assumed to be 0.5 m m and the density of silicon 2.33 g/cm 3. This reflects the uncertainty of the figure for the thickness of the detector and the presence of the density effect. (The figure of 0.5 m m is the nominal thickness of the original slice from which the device was fabricated.) Thus in deriving the theoretical distribution the experimental value of A o was used. Landau's expression for the scale factor was, however, found to give a good
260
J.E.
BATEMAN
description of the width of the observed distribution. If we consider the fwhm produced by the ionisation processes and the fwhm due to the detector's electronic noise to be added in quadrature, to give the observed width of 48 keV, then the electronic noise width can be unfolded to yield the fwhm attributable solely to the ionisation loss process. This is found to be 34 keV, in good agreement with the fwhm of the theoretical curve shown in fig. 4 ( ~ 35 keV). Thus we can conclude that the multichannel analyser spectrum shown in fig. 4 accords fairly well with Landau's theory in respect of the width and shape of the observed distributions of pulse heights produced in C 56 by the 150 MeV electrons. When the detector is used as a logical device (as in general it would be) an important requirement is that there be an adequate separation between pulses due to detected electrons and those due to electronic noise. Fig. 4 shows the result of a 30-min-exposure so that the valley counting rate at (say) channel 40 on the analyser is observed to be low (1 per channel/rain). This would seem to be an acceptable background rate when a fast coincidence is used. With a detector better than C 56 the separation would be complete. As already indicated, the speed of response of the detector is limited at present by the charge sensitive preamplifier. However, the presently obtainable logic pulse rise times of ~ 7 ns and base width of 15-20 ns are useful for many purposes. There is considerable potential for development in this area and it is probably safe to predict that a rise time of 3-4 ns and a base width o f 10 ns will be achieved, without sacrificing the noise resolution of the preamplifier. 4. Conclusion
The tests just described have shown three important properties of the relativistic electron detector under test: 1. A satisfactory signal to noise ratio can be Obtained (fig. 4). 2. Fast pulses can be obtained for logic purposes (fig. 2b). 3. Simple fast logic circuits can be made to trigger on the low signal levels given by the device. For a practical application to high energy physics the present apparatus would require to be modified in two ways: the detector made a more suitable size and shape (i.e. 4-5 c m 2 in area with a rectangular instead of a circular shape) and the electronics miniaturised. These developments are well within the state of the art as can be seen from 9) (large area detector) and 12)
(miniaturised preamplifier). It is hoped to build such a device and to evaluate its performance in the experimental environment of a high energy electron accelerator. In addition to simplifying the problem of light piping, the silicon device has several other agreeable properties recommending it for use as a momentum selector in intense particle backgrounds: a. 0.5 mm of silicon is considerably thinner than the 5-10 mm thickness of scintillator normally used for momentum selection. This helps to decrease multiple scattering uncertainties. b. The silicon detector is remarkably insensitive to fast neutrons and gamma rays. This can be a great benefit and probably allows the slower response time currently attainable to be adequate for most situations in which a photomultiplier could be used. The major limitation of the device would seem to be the small active area attainable (5 cm2). However, the fact that the detector can be put anywhere e.g. in the centre of a magnet, means that it can be placed in regions of high focussing to compensate for this restriction. My thanks are due to Dr. W . R . Hogg of this department for his help and encouragement, to Dr. W. MacFarlane and the synchrotron team for their cooperation and to Professor P . I . Dee for financial support throughout this work. References 1) C. Amsel, D. Bonaskas and C. Zajde, L.A.L. Report 1102 Laboratoire de l'Acc616rateur Lin6aire, Orsay, France. 2) G . L . Miller, B. M. Foreman, L. C. Yuan, P. F. Donovan and W. M. Gibson, IRE Trans. Nucl. Sci. NS-8 (1961) 73. 3) D. van Putten and J. C. van der Velde, IRE Trans. Nucl. Sci. NS-8 (1961) 124. l) j. M. MacKenzie and G . T . Ewan, IRE Trans. Nucl. Sci. NS-8 0961) 50. 5) G. W. Grew, IEEE Trans. Nucl. Sci. NS-12 (1965) 308. 6) M . R . Raju, H. Aceto and C. Richman, Nucl. Instr. and Meth. 37 (1965) 152. 7) H. D. Macabee and M. R. Raju, Nucl. Instr. and Meth. 37 (1965) 176. 8) D . W . Aitken, D . W . Emerson and H. R. Zulliger, IEEE Trans. Nucl. Sci. NS-15 (1968) 456. 9) ORTEC Inc., Surface barrier detector catalogue. 10) F . S . Goulding, D . A . Landis, J. Cerny and R . H . Pehl, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 514. 11) j. E. Bateman, Nucl. Instr. and Meth. 67 (1969) 93. 12) j. E. Bateman, Nucl. Instr. and Meth. 71 (1969) 261. 13) j . B . Mundell, An electronic system for processsing the output of a new position sensitive detector, Intern. Symp. Nuclear Electronics (Versailles, 1968). 14) L. Landau, J. Phys. (U.S.S.R.) 8 (1944) 201.
INSTRUMENTS
AND
METHODS
7I
0969)
256-260;
©
NORTH-HOLLAND
PUBLISHING
CO.
A SOLID STATE D E T E C T O R FOR CHARGED PARTICLES AT RELATIVISTIC ENERGIES J.E. BATEMAN
Department of Natural Philosophy, Glasgow University, Glasgow W.2, Scotland Received 27 February 1969 Experiments are described in which a totally depleted silicon surface barrier detector is used to detect the passage of fast electrons (1.5-2.3 MeV and 150 MeV). A good signal to noise ratio is obtained and the output pulses have rise times of 7 ns
and adequate amplitude to drive tunnel diode logic circuits directly. The advantages of the detector over the photomultiplier-scintillator detector are discussed in relation to applications in high-energy physics.
1. Introduction
The detection of charged particles at relativistic energies is often done in conjunction with strong magnetic fields in order to define momentum. The sensitivity of the photomultiplier tube to such magnetic fields leads to complex light piping to remove the tube from the high field region in which the scintillator is situated. The silicon surface barrier detector is unaffected by strong magnetic fields and could, theoretically, offer a solution to detection problems of this type. The problems involved in the exploitation of this possibility are now listed:
The evaluation of various types of silicon devices as detectors for charged particles o f relativistic energies has been carried out by several investigators1"7). These tests have in general been carried out with two purposes in m i n d - t o test theoretical predictions of the distribution of the ionisation energy losses of the particles in silicon, and to try to find a solid state detector suitable for general use in high energy physics. This communication is concerned with the latter problem. The general conclusion of the earlier tests was that while for special purposes silicon detectors (of one sort or another) may have some advantages over other techniques, they did not look promising as general purpose detectors for relativistic charged particles2). Recently, however, more encouraging results have been reported by Aitken et al.8) with a 2 mm thick lithium drifted silicon detector. Satisfactory measurements of the ionisation energy loss of protons, pions, and electrons have been obtained and the viability of the detector in the environment of a high energy physics experiment demonstrated. However, if a detector is to be of general application it must have a response time comparable with that of a fast scintillator photomulfiplier detector. The low electric field and large charge carrier transit distances of a lithium drifted detector such as described above tend to lead to rather slow response times (of the order of tens of nsec). An overdepleted surface barrier detector of 0.5 mm thickness can, on the other hand, yield response times in the nsec region, while producing adequate signal amplitude to give an acceptable resolution of pulse height when a minimum ionising particle is detected. Two other advantages of the thin surface barrier detector are i m p o r t a n t - t h e high electric field and short charge collection distances obviate any trapping effects such as are described in 8) and the thinner detector causes less multiple scattering of transmitted particles.
a. The detector must be thick enough and the electronic noise of the detection system low enough to realise a satisfactory signal to noise ratio. b. The response time of the whole system (detector plus preamplifier) must be fast and of the same order of magnitude (nsec) as that of a plastic scintillator-photomultiplier detector. c. An adequate active area must be available. (There are several limitations on the area which a surface barrier diode can have, but the absolute limit has been raised to 4.5 cm ~ in recent years.) d. The device must be reliable and easy to handle. e. The associated preamplifier (it must always be mounted close beside the detector) must be of physical dimensions comparable to that of the detector itself for maximum usefulness. The way in which these requirements can be met are now summarised. Production techniques for surface barrier devices now allow requirements (a), (b) and (c) to be met simultaneously and guarantee requirement (d). For example, detectors of 0.5 mm thickness capable of total depletion can now be obtained with areas of up to 4.5 cm 2 and an associated noise width of 30 keV fwhm9). This combination of characteristics leads to a resolution of the Landau distribution for electrons better than that observed in fig. 4.
256
A SOLID
STATE DETECTOR
The problem of achieving a fast response time for the detection system falls into two parts: 1. The collecting field in the detector itself must be high enough to collect the ionisation produced by the incident particle in the order of a few nanoseconds. This requires over-depletion of the device without causing a serious increase in the detector noise width. This requirement can be met fairly easily up to active areas of ~ 4.5 c m 2. 2. The preamplifier must be fast enough to reproduce the rise time of the detector and at the same time preserve a small noise width. The development of the fast high gain field effect transistor (FET) (e.g. 2N4391) has led to charge sensitive preamplifier circuits which are adequate to the problem and which are also capable of miniaturisation as required in (e) above 1°' 12). The last important consideration is that of reliability. The preamplifier design is thoroughly tested and dependable. The reliability of the surface barrier detectors would depend very much on their manufact u r e r - the best commercially available devices show excellent stability. In the event of a failure replacement would be as simple as plugging in another diode since there is no feed for an elaborate housing as in the case of the photomultiplier. In long term operation the presence of an over-voltage takes care of fluctuations in the leakage current caused by variations in the ambient temperature and the gain of the system remains constant to a high degree. The general problems having been outlined, the
Magnetic Spectrometer ~ ~ l
Aluminiurn foil target (O-O0<~_SiZn Bremsstrahlung
I~
E~I
en
Preamplifier no.1
(:56 ~ : : ~
C76 r-~
beam
I
~
I Preamplifier ]-~
F-~
INS 601
' " L" "~sy ii_F_[Analyse r ~
3u s
Gate 4>LGeneratorJ Fig. 1. Experimental set-up for test o f the relativistic charged particle detector with a b e t m o f 150 MeV electrons.
FOR CHARGED
PARTICLES
257
specific system used in the present tests may now be described. 2. Constructional details
As indicated above the relativistic particle detector consists of two basic parts - a totally depleted surface barrier detector and a charge sensitive preamplifier. In its final form these two components will be contained in a c o m m o n enclosure of small dimensions (maximum dimension 10cm) similar to that shown~2). In the present case discrete components are used with the diode mounted on a standard preamplifier by means of a BNC connector. Since the charge sensitive preamplifier is an integrating device a further piece of electronics is added. This is indicated in fig. 1 as the "pickoff", which serves to differentiate the step function output of the preamplifier to produce a short pulse for logic purposes. The three items are now described separately. 2.1. T H E SURFACE BARRIER DETECTOR
The basic diode, C 56, is an aluminium on silicon surface barrier device made by the usual surface barrier techniques [ref. l j) for details]. The specification of the device is as follows: material:
8 k o h m • cm p-type silicon (ms carrier lifetimes), active area: 2 c m 2, thickness: 0.5 m m nominal (totally depleted), operating bias: 400 V (including 10% over-voltage), leakage current: 5/~A, electronic noise: 35 keV fwhm (at operating bias). The standard 2cm diameter slice is secured between pyrophyllite rings by means of epoxy resin and secured in the usual way with a brass ring 11). 2.2. THE CHARGE SENSITIVEPREAMPLIFIER A circuit diagram and detailed description of this circuit are given12). For present purposes the rise time and the noise width are the critical parameters. A 2 c m 2, 0.5 m m thick detector has a capacity of about 60 pF. With this source capacity the preamplifier has a rise time of 15-20 ns (depending on the FET) and a noise width of 10-15 keV fwhm. This rise time is that of the integrated charge pulse and, for logic purposes, it is differentiated to yield a pulse the rise and fall times of which are less than half this value. These pulses are shown in fig. 2a, b. The preamplifier noise is well below that of the diode and so is not a significant contribution to the energy resolution.
258 2.3.
J . E . BATEMAN THE LOGIC ELECTRONICS
Fig. 1 shows the block diagram of the circuitry used. The differentiating circuit used to generate a short current pulse from the output of the charge sensitive preamplifier is a simple two transistor operational amplifier fed by a capacitor. The bandwidth is sufficient to transmit the fastest pulse the preamplifier can supply and the best logic pulses obtained has a 7 ns rise and fall time. This circuit (called a pickoff in fig. 1) drives a tunnel diode discriminator directly and the 400 mV, 30 ns wide pulses from the latter are used for logic purposes. A description of the pickoff is given inla). Fig. 2b shows the output pulse derived from the pickoff.
3. The experimental tests In the first instance relativistic electrons from a radioactive source were used to test the detector. The counting rate was, however very low so a test was arranged in which pair produced electrons of 150 MeV
(b)
/i ....
i ....
i . /.
i .
.
.
.
i
.
.
.
.
i
.
.
.
.
i
derived from the bremsstrahlung beam of the Glasgow electron synchrotron were used. 3.1.
TEST WITH A 9 ° S t BETA PARTICLE SOURCE
Fig. 1 shows the experimental set-up for both tests: only the source of electrons is different in the two cases. C 56 is the 0.5 m m thick detector under test. C 76 is a geometry defining detector 1 m m thick (also capable of total depletion). The output of the preamplifier of C 56 is fed directly to the pulse height analyser through a 1 #s clipping line while the output from C 76 is passed through the differentiating circuit (pickoff) to the tunnel diode discriminator, the output of which drives the analyser gate circuit. This effects a coincidence adequate for the low counting rate used. In order to monochromate the beam of electrons from the radioactive source, the discriminator on C 76 was set at 1.5 MeV so that only electrons leaving at least that amount of energy in C 76 would trigger the analyser gate. The result is that the energy spectrum of pulses from C 56 is that produced by transmitted electrons in the 1.5-2.3 MeV range. (2.3 MeV is the end point of the beta spectrum.) Fig. 3 shows the observed analyser spectrum. It shows the characteristic Landau high energy "tail" but obviously suffers from very poor statistics. The most probable value of the energy loss agrees with the value calculated from the specific energy loss curve published in Ortec manuals to within a few percent. 3.2. TH~ TEST WITH 150 MeV ELECTRONS When the 150 MeV electrons from the magnetic spectrometer are detected, as shown in fig. 1, the discriminator level of the tunnel diode is set as low as possible since the electrons are also transmitted by C 76 and the pulse height obtained is only a few
(a)
6O u~ z 0 (940
SL~
EL 0
~t
Amplitude
= 166 KeY
LO O0
z
Vertical scale: 10mY/division Horizontal
scale:
19 Kov O
Fig. 2. a. Charge pulse waveform produced by a relativistic electron in the relativistic particle detector (output of charge sensitive preamplifier); b. Current pulse waveform given by pickoff circuit.
20
N 264Kev
¢I~
_ _
20ns/division
.40 CHANNEL
~ o_ .u u ~qlr~ ~
60
8Q
P-m
,
-
100
NUMBER
Fig. 3. Energy distribution produced in C 56 by electrons of energy between 1.5 and 2.3 MeV.
A SOLID STATE DETECTOR
FOR CHARGED
259
PARTICLES
1000
800 most
probable
energy
=
fwhm
U3 Z
loss
177KEY 48 ~eV
600
27%
0 (o
(detector
noise
= 35KeV
fwhm)
LL
0 13S
400
b3 ~D ~E
j
Z
200
0
J
0
20
193 KeY % i
40
i
60 80 CHANNEL
~
i
r
,
100 120 NUMBER
140
160
180
--
200
220
240
~
260
Fig. 4. Energy distribution p r o d u c e d in C56 by electrons of energy 150 MeV ( m o m e n t u m resolution ~, 2%). T h e solid curve is given by L a n d a u ' s theory.
hundred keV. C 76 provides a certain amount of m o m e n t u m definition and background suppression. However, as the m o m e n t u m is already well defined by the geometry of the experiment, the absence of the coincidence degrades the observed analyser spectrum only slightly. The m o m e n t u m resolution of the incident electrons is about 2%. Fig. 4 shows the analyser spectrum of the pulses observed in C 56. The most probable value of the energy loss shows good agreement with that obtained previously. (The slight increase is to be expected from the specific energy loss curve.) Since a good rate of electrons was obtainable from the spectrometer, fig. 4 shows good counting statistics, and a meaningful comparison with Landau's function 14) is made possible. 3.3. ASSESSMENT AND COMPARISON OF RESULTS The energy calibration is done in both cases by detecting the K and L conversion electron lines of a 2°3Hg source in C 56. The position of the two lines in the analyser spectra are indicated in figs. 3 and 4. A very poor counting rate was achieved with the radioactive source due to low source strength, restricted solid angle, and poor efficiency for total absorption of the energetic electrons in C 76. As a result fig. 3
gives a poorly defined Landau distribution. On the other hand, the statistics of fig. 4 are excellent (10 counts/sec were recorded) and an accurate value of 177 keV is obtained for the most probable energy loss. This agrees well with the slightly lower value of 166 keV indicated in fig. 3. The poor accuracy of fig. 3 does not really permit us to conclude that this comparison verifies the rise in the specific energy loss curve at high energies. Looking at fig. 4 we see that the general shape of the observed distribution of the ionisation losses of the 150 MeV electrons in the silicon of the detector is in fair agreement with L a n d a u ' s function (solid line). The most probable value of the energy loss, Ao, is observed to be 177 keV. This is not in agreement with the value of 215keV calculated from Landau's formula when the thickness of the detector is assumed to be 0.5 m m and the density of silicon 2.33 g/cm 3. This reflects the uncertainty of the figure for the thickness of the detector and the presence of the density effect. (The figure of 0.5 m m is the nominal thickness of the original slice from which the device was fabricated.) Thus in deriving the theoretical distribution the experimental value of A o was used. Landau's expression for the scale factor was, however, found to give a good
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description of the width of the observed distribution. If we consider the fwhm produced by the ionisation processes and the fwhm due to the detector's electronic noise to be added in quadrature, to give the observed width of 48 keV, then the electronic noise width can be unfolded to yield the fwhm attributable solely to the ionisation loss process. This is found to be 34 keV, in good agreement with the fwhm of the theoretical curve shown in fig. 4 ( ~ 35 keV). Thus we can conclude that the multichannel analyser spectrum shown in fig. 4 accords fairly well with Landau's theory in respect of the width and shape of the observed distributions of pulse heights produced in C 56 by the 150 MeV electrons. When the detector is used as a logical device (as in general it would be) an important requirement is that there be an adequate separation between pulses due to detected electrons and those due to electronic noise. Fig. 4 shows the result of a 30-min-exposure so that the valley counting rate at (say) channel 40 on the analyser is observed to be low (1 per channel/rain). This would seem to be an acceptable background rate when a fast coincidence is used. With a detector better than C 56 the separation would be complete. As already indicated, the speed of response of the detector is limited at present by the charge sensitive preamplifier. However, the presently obtainable logic pulse rise times of ~ 7 ns and base width of 15-20 ns are useful for many purposes. There is considerable potential for development in this area and it is probably safe to predict that a rise time of 3-4 ns and a base width o f 10 ns will be achieved, without sacrificing the noise resolution of the preamplifier. 4. Conclusion
The tests just described have shown three important properties of the relativistic electron detector under test: 1. A satisfactory signal to noise ratio can be Obtained (fig. 4). 2. Fast pulses can be obtained for logic purposes (fig. 2b). 3. Simple fast logic circuits can be made to trigger on the low signal levels given by the device. For a practical application to high energy physics the present apparatus would require to be modified in two ways: the detector made a more suitable size and shape (i.e. 4-5 c m 2 in area with a rectangular instead of a circular shape) and the electronics miniaturised. These developments are well within the state of the art as can be seen from 9) (large area detector) and 12)
(miniaturised preamplifier). It is hoped to build such a device and to evaluate its performance in the experimental environment of a high energy electron accelerator. In addition to simplifying the problem of light piping, the silicon device has several other agreeable properties recommending it for use as a momentum selector in intense particle backgrounds: a. 0.5 mm of silicon is considerably thinner than the 5-10 mm thickness of scintillator normally used for momentum selection. This helps to decrease multiple scattering uncertainties. b. The silicon detector is remarkably insensitive to fast neutrons and gamma rays. This can be a great benefit and probably allows the slower response time currently attainable to be adequate for most situations in which a photomultiplier could be used. The major limitation of the device would seem to be the small active area attainable (5 cm2). However, the fact that the detector can be put anywhere e.g. in the centre of a magnet, means that it can be placed in regions of high focussing to compensate for this restriction. My thanks are due to Dr. W . R . Hogg of this department for his help and encouragement, to Dr. W. MacFarlane and the synchrotron team for their cooperation and to Professor P . I . Dee for financial support throughout this work. References 1) C. Amsel, D. Bonaskas and C. Zajde, L.A.L. Report 1102 Laboratoire de l'Acc616rateur Lin6aire, Orsay, France. 2) G . L . Miller, B. M. Foreman, L. C. Yuan, P. F. Donovan and W. M. Gibson, IRE Trans. Nucl. Sci. NS-8 (1961) 73. 3) D. van Putten and J. C. van der Velde, IRE Trans. Nucl. Sci. NS-8 (1961) 124. l) j. M. MacKenzie and G . T . Ewan, IRE Trans. Nucl. Sci. NS-8 0961) 50. 5) G. W. Grew, IEEE Trans. Nucl. Sci. NS-12 (1965) 308. 6) M . R . Raju, H. Aceto and C. Richman, Nucl. Instr. and Meth. 37 (1965) 152. 7) H. D. Macabee and M. R. Raju, Nucl. Instr. and Meth. 37 (1965) 176. 8) D . W . Aitken, D . W . Emerson and H. R. Zulliger, IEEE Trans. Nucl. Sci. NS-15 (1968) 456. 9) ORTEC Inc., Surface barrier detector catalogue. 10) F . S . Goulding, D . A . Landis, J. Cerny and R . H . Pehl, IEEE Trans. Nucl. Sci. NS-13, no. 3 (1966) 514. 11) j. E. Bateman, Nucl. Instr. and Meth. 67 (1969) 93. 12) j. E. Bateman, Nucl. Instr. and Meth. 71 (1969) 261. 13) j . B . Mundell, An electronic system for processsing the output of a new position sensitive detector, Intern. Symp. Nuclear Electronics (Versailles, 1968). 14) L. Landau, J. Phys. (U.S.S.R.) 8 (1944) 201.