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Weakly ionizing charged particle detectors with high efficiency using transitory electronic secondary emission of porous CsI
Nuclear Instruments and Methods in Physics Research A273 (1988) 245-256 North-Holland, Amsterdam
245
WEAKLY IONIZING CHARGED PARTICLE DETECTORS WITH HIGH EFFICIENCY USING TRANSITORY ELECTRONIC SECONDARY EMISSION OF POROUS CsI C. CHIANELLI, P. AGERON, J.P . BOUVET, M. KAROLAK, S. MARTIN and J.P . ROBERT Service de Physique Nucléaire - Moyenne Energie, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France
Received 18 April 1988
A very high efficiency detector for weakly ionizing particles is described. This very thin detector (2 mg/cm 2 ), using the transitory electronic secondary emission of porous CsI, has allowed us to detect protons of 540 MeV energy with a detection efficiency '7 >_ 95% and a time resolution of 450 ps within the SATURNE synchrotron (CEN Saclay).
1. Introduction
2. Study and realisation of prototype detector
Secondary electronic emission was discovered by Austin and Starke [1] in 1902, but it was not until 1950 that Alvarez and Cone [2] proposed to use this phenomenon as a means of detection within an accelerator. The secondary emission is a prompt surface phenomenon which, in principle, permits one to obtain good time resolution while inserting very little material into the particle beam being analysed . The signal obtained is proportional to the dE/dX of the incoming particles [3,4], This type of detector runs well for heavily ionizing particles (a, heavy ions) with low energy and gives timing information of high quality. When the energy of the particles increases, problems arise because the efficiency and time resolution of the device deteriorate drastically. These problems become critical when we reach energies approaching minimum ionization ; mainly, if we want to set this detector on the front part of a high resolution spectrometer in which the beam analysed must not be too disturbed. In the sixties, the highly emitting porous alkali halides appeared [5-71, giving a great improvement to the situation, but without giving, however, the very good efficiency obtained with highly ionizing particles. In the frame of those developments, we herewith report the study and the realization of a very high efficiency detector for weakly ionizing particles. This very thin detector (2 mg/cm2), using a not very well known and hardly defined characteristic of porous CsI [8], has given us the possibility of detecting protons having a 540 MeV energy with a detection efficiency rl >_ 95% and a time resolution of 450 ps in the SATURNE synchrotron (CEN Sacley).
2.1 . Generalities to structure of the detector
0168-9002/88/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
The prototype detector has two parts: one part concerns the detection of the primary particle by means of a principal layer; the second part, helped by the multiplier device, produces the multiplication of the electrons emitted by the principal layer. 2.1 .1 . Principal layer
The principal layer must meet two important criteria : (1) the efficiency for the production of secondary electrons must be high - a 100% efficiency corresponds to the emission of at least one secondary electron for each charged particle traversing this layer, and (2) the average (statistical) number of electrons emitted during the passage of weakly ionizing charged particles should be as large as possible . These conditions are best realized with an emitter dielectric material that has the form of a porous layer a few hundred microns thick. 2.1 .2 . Electron multiplication
The average number of electrons created by a weakly ionizing particle during its passage through the principal layer is low (= 15) ; amplification is necessary. The amplification system must if possible have the characteristics that it should neither degrade the response time nor perturb the beam. It is difficult to satisfy simultaneously these conditions . Thus, we were obliged to use different types of devices depending on the test necessary at each stage of development of our detector . For the tests in the laboratory, we used semiconductor detectors both as multiplier device and detection stage
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[91 in order to determine the principal layer characteristics . Then, for laboratory tests and tests m the beam, we replaced these by an assembly [10,11,131 of separate stages, one for the multiplication of electrons and associated multichannel plates for the detection of electrons. Further developments will allow us to replace the microchannel plates by a stronger device comprising a thin scintillator (10 gym) viewed via a vacuum light guide by a photomultiplier tube. This final system will best fulfil the operational needs of the nuclear and particle physics community.
Al (30nm) Mytar 15Pm or A1 20 3 100nm -~
Porous Cs I (th=120Pm, relative density = 25) /0
f--
EEXr
2 .2 . Study of the principal layer : transitory secondary electronic emission of porous Cs1 2.2 .1 . Choice of porous CsI
The detection of high energy particles is very difficult because the number of secondary electrons emitted is very small. Only the electrons produced at a few tens of nanometers from the device surface can go out. In order to enhance this number it is necessary to increase the apparent surface of the device. To obtain this effect we can use materials such as KCI, CsI or MgO obtained in porous form and having a density of some few percent of the bulk density [7,91. Among these, CsI has been chosen because it is a good compromise between high secondary emission in the steady state and low hygroscopic properties, which permits us to keep it in the atmosphere for short times during fabrication . As will be seen later, its high initial secondary emission at the time of switching on the prompt electric field is another reason for the choice of porous Cs1 ; this characteristic is not found in MgO [91 nor KCI m our experiments . 2 2 .2 . Description and production of principal layer (fig. 1)
The principal layer is made following a well known procedure: (1) a mylar foil (diameter = 40 mm, thickness = 1.5 l.Lm (230 wg/cm2 ) is glued onto a stainless steel ring ; (2) a conducting surface is formed on the mylar foil by the evaporation of aluminium in a vacuum (diameter = 40 mm, thickness = 30 rim (10 It g/cm2 )) ; and onto this conducting surface we evaporate CsI very slowly (30 min) in an (8 Torr) argon environment, which gives a porous CsI layer (diameter = 40 mm, thickness = 120 Win (1300 )rg/cm2 )) . The relative density of this layer is 2.5% of the bulk density. The structure of this layer is made of CsI whiskers having 10 to 50 rim diameter. Note that in order to decrease the weight of the material in the beam, we can, if necessary, replace the
* Supplier : E. Merck (RFA).
Fig. 1. Layout of principal layer made m porous Cs1.
mylar by a self-supporting alumina (A1 20,) foil of 100 rim thickness (45 13g/cm2) and 40 mm diameter . 2 .2 .3 . Experimental setup for the investigation of the principal layer
The device allows us to analyse the secondary emission owing to a particle of well-known energy when it passes through a thin electrode covered with a thin layer of porous Csi . 2.2 .3 .1 Method of measurement [9]
The experimental setup (fig . 2) allows the passage through the principal layer of a beam of particles emitted by a radioactive source . The secondary electrons emitted by the particles are transported by an electric field from the principal layer to the secondary electron detector, while the primary particle is detected after crossing the principal layer. The secondary electron detector signal is stored only if it meets two conditions : (1) it must be in coincidence with the primary particle detector signal, and (2) the analysed energy of the primary particle must be in a prescribed energy band . This allows us to know the number of secondary electrons emitted by a primary particle having a well-known energy. In fact, the signal registered by the semiconductor secondary electron detector is proportional to the number of electrons created by the primary particle :
C. Chianelh et al. / Weakly ionizing chargedparticle detectors
247
Test
Radioactive
source
chamber
Si(Li) detector for
secondary
electrons
Thin
Si(Li)
window
for
detector
primary
electrons
Fig.
2.
Probed
foil
Experimental setup for testing the principal layer
Let Vo be the potential difference between the principal layer and the detector . The electron energy will be : Wm - eVv ,
where e is the charge of the electron .
For n electrons W = ne Vo, which is the energy registered by the detector . If no electron is emitted, the system triggers on the detector noise and a W, signal is registered .
PRIMARY PARTICLE CHANNEL
Si (Lil
PAC
SECONDARY EMISSION
PAC
A
CHANNEL
A
Fig. 3. Basic diagram for measuring the efficiency and average number of secondary electrons emitted from porous CsI. Legend : A: shaping amplifier; PAC: charge preamplifier ; ADC- analog-to-digital converter, LeCroy 3500 . DMA memory+microcomputer, Si(Li) : semiconductor detector .
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C Cluanelh et al. / Weakly ionizing charged particle detectors
For A coincidences, A is defined as the number of times when n electrons have been detected and n, the average number of electrons emitted, is calculated byN
n=
Y na
n=0 N
n=0
N
_
n=0
aVo A
a
'
N Y-
an
--J 100
Tl
2000
Y, n W
where N is the maximum number of electrons emitted. The primary layer efficiency rl, defined as the ratio between the times when there is at least the emission of one secondary electron and the whole number of coincidences is expressed as : n-1
n
A - ao
A
where a o represents the time when no electron has been detected, i.e . the signal stored for the electron detector corresponds to noise by the above defined coincidence . 2.2.3.2. Detectors
The detectors used are Si(Li) semiconductor detectors cooled with liquid nitrogen (77 K) and made to our laboratory . Their dimensions are: diameter = 28 mm, thickness = 2 mm for the secondary electron detector and 28 mm and 5 mm respectively for the primary particle detector . 2 2 3.3 . Electronics
The basic wiring diagram (fig . 3) includes two parts: secondary emission channel and primary particle channel. Each part includes a semiconductor detector, a charge preamplifier, a shaping amplifier and an analogto-digital converter whose contents are stored in a DMA memory LeCroy 3500 M microcomputer. This storage occurs only when the energy selection defined above is sufficient to initiate the analog-to-digital conversion . This allows us to store the spectrum of analysed particles and the secondary emission statistics of porous CsI. Then with the microcomputer we quickly calculate the efficiency and the average number of electrons emitted. With this device we can make measurements in two-minute periodes, including calculations. 2 2.3.4. Principal layer setting up to the device
Primarily, the principal layers are moved in argon between the bell jar and the experimental setup in order to avoid their deterioration by atmospheric moisture ; the layer is quickly installed under normal atmospheric conditions and then the setup is brought rapidly under vacuum .
E = 14 kV/cm
1000
50
T
0
30
I
60
90
I
120
t Imi n)
0
Fig. 4 . The efficiency r) and the average number of electrons n vs the time for a-particles (5 .5 MeV) 2 .2 .4 Experimental results
We will describe the results obtained in the various tests: (1) with a-particles coming from an 24 'Am radioactive source ; (2) with ß-rays having an energy higher than 1 MeV (from a y° Sr source). These results will be compared with those obtained by the only experimental group [8] that has described and tested a particle detector (with a source) using porous Csl and which show a time dependence of secondary electron emission . We have started again the tests made by these authors with a-particles placing emphasis on the study of weakly ionizing particles which are of particular interest to us . 2 .2 .4 .1 . Experiment s with highly ionizing particles ( 24 'Am) When suddenly applying an electric field E at t = 0
on a porous CsI layer, we observe that the number of secondary electrons emitted decreases with time (fig . 4) . After two or three hours this number is stabilized near an average value, n, of one hundred electrons. The number n obtained under steady state conditions is sufficiently high to ensure good secondary emission statistics which, when combined with the high signal to noise ratio of the semiconductor detector used in this experiment, yields a constant detection efficiency throughout the entire measurement of close to 100% . This phenomenon is reproducible with respect to time behaviour, n, and efficiency q . If we switch off the electric field E on the porous CsI layer and allow the device to relax for a sufficient period of time, we recover, after a new application of E, the same n and 71 with nearly the same time evolution. The time constant, T, for thus decay process is found to be about thirty minutes. 2 2 .4 .2. Tests with low-ionizing particles
We continued our tests with ß-rays (E > 1 MeV) and obtained results (fig . 5) for n which give the same variation as with a-particles . The levels obtained for n at t = 2 mm (n = 15) and t > 120 mm (n = 4) are much
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C. Chianelh et al / Weakly ionizing charged particle detectors
100 80
4
60 40
2 0
Fig. 5. The curves of the efficiency il and the average number of electrons n vs the time for ß-rays (E >- 1 MeV) .
lower. This is normal because, as has been demonstrated [3], secondary emission is directly related to the d E/dX of primary particles. We note that the decay constant T is nearly the same as that for a-particles, which suggests that we have the same physical phenomenon and that it does not depend on the type of particle . Because of the small number of electrons emitted by the ß-rays and their emission statistics, the detection efficiency also decreases in time when the phenomenon is stationary (t > 240 mm). We observed (fig. 6) a microstructure due to the emission statistics in porous CsI in that particular case . It is less favorable from the Poisson statistics, s, having the same n which governs the secondary emission of bulk materials. We also studied in this steady state (fig . 7) the influence of the electric field E on the efficiency and on n . We found that from 14 kV/cm we obtain a constant efficiency even though we have a continuous increase of n . The plateau indicates that all electrons emitted by secondary emission during the passage of a primary particle in CsI whiskers have been extracted. The increase of n corresponds to electron multiplication by shower [14] of the first secondary electrons created by collision with the CsI whiskers when they are extracted
20 8
12
10
14
E (kV/cm)
Fig. 7. The curves of the efficiency 71 and the average number of electrons n m steady state vs the electric field E for ß-rays (E >-I MeV) .
from the layer by the electric field. We regulated E at 14 kV/cm in all tests with constant electric field. 2.2 .4 .3 . Interpretatio n of the phenomenon
Two reasons can be found to explain the variation in time of secondary electronic emission of porous CsI: (1) A depolanzation phenomenon caused by the drift of ionic impurities in the fibrous structure of porous CsI. (2) An electrio-drift phenomenon of the Cs' ions on the surface of the CsI whiskers which diminishes the local surface density of Cs' and accumulates these ions in the porous structure traps. This redistribution of ions results in an average increase of the electron work function of the CsI whiskers, which leads to a decrease of the secondary-electron emission efficiency .
IE E REC
2
10 3
n=42
i 0
EESC
Electrons nbr 5
10
15
Fig. 6 . Secondary electron distribution of porous CsI in steady state (t >_ 4 h) for ß-rays
0
t,
tz T
Fig. 8. Periodic electric field, see text .
t (sec)
250
C Chianelli et al / WeaklY ionizing charged particle detectors EESC=
20r
14kV/cm
1
t, =05S t 2 =1,5 S
EREC =0 kV/cm
10
I 0
I 30
I 60
I 90
kV/cm E REC =4kV/cm
ERE( =5kV/cm EREC=4kV/cm
\
EESC =14
9%
120
EREC--OkV/cm EREC=O -----
50
ti=05S t2=1,55
7t2=0
Fig. 9. Average number of electrons vs time of different recovery fields E for ß-rays.
Fig. 10 . Efficiency
2 .2 5 . Recovery of the initial conditions
field, some emitting foils of KCI and have not observed any variation of the secondary emission in contrast to ref. [8] : the secondary emission yield is sufficient to obtain a detection efficiency close to 100% . Tests on these KCI foils made with ß-rays (E > 1 MeV) under constant field have also given time independent results which are comparable to those of CsI under permanent field (71 = 60%) . Our results (better for both the CsI and the KCl under a permanent working field and for CA under a periodic field) suggest contamination problems or impurities in the formation of the emitting layers of ref. [8]. Our results tend to show that in CsI the electromigration of the Cs' ions is responsible for the decrease of the secondary emissions as a function of time in KCI the phenomenon does not occur. Further tests with other emitting compounds of cesium (CICs, BrCs) would permit us to confirm or reject this hypothesis
If the reduction of secondary emission is due to a depolarization phenomenon [8] and/or an electromigration, the application of a reverse electric field on the porous CsI would permit the recovery of the initial conditions . This is demonstrated by experiment . We applied on the porous CsI a periodic electric field (fig . 8) in which EESC is the electric field permitting the escape of the secondary electrons during the time t,, and EREC the field permitting to reduce the supposed causes of the secondary electronic emission decreasing during the time t, and setting the condition t i + 12 << T (fig . 4) . In figs . 9 and 10 we see, for particular values of t,, t2 and EESC, the important effect of the recovery electric field EREC on n and q . The reported tests were made with ß-rays (E > 1 MeV) . According to all measurements it is apparent that if we keep T= constant << T, RFsC-constant and allow t i , t 2 and ERrs to vary, we obtain an optimum efficiency when we are in agreement with the relationships shown by eq . (1): = constant
if
I ER,.C I x 1 2 > I
EESC
I x tl .
In accordance with these relations, we have obtained a constant efficiency in time whose value reaches 92% when the electric field EESC > 14 kV/cm. We also performed measurements with a-particles under the same experimental conditions . The improvement of n is also spectacular, but no efficiency increase is observed because the efficiency was already equal to 100% with a constant EESC
vs time and for different recovery electric fields E RE C for (3-rays.
h
2 3 . Realisation and tests of prototype detector 2 .3 .1 Principles of device
Our purpose is to instal near the SATURNE synchrotron a detector with good timing characteristics We
2 2 .6 . Discussion
If we compare our results to those of ref. [8], in which porous CA and KCI are used under a periodic field, but in a detector of different construction, it appears that the efficiency of our detector is much higher for the case of CsI, as we nearly set a 100% efficiency for 5 .5 MeV a-particles for the CA even without the help of a recovery field. We have also tested, for the same a-particles with a constant electric
EREC
EESC
Fig. 11 . Beam-field synchronization .
C Chianelh et al. / Weakly ionizing chargedparticle detectors
251
HT t= -12 5kV
BEAM
HT 3=-5 5kV
(g)
(h) Fig. 12 . Schematic drawing of the prototype detector . a anode; b: microchannel plates ; c, d, e: focusing electrodes, f. thin electrode of focalization, g: multiplier layer ; h emitter layer made of porous Csl, j : high voltage fast switch
have synchronised the periodic electric field ( EEsc, Eaec .) applied on the porous Csl layer with the time macrostructure of the accelerator (fig. 11). This allows us to realize a detector having an apparent duty cycle of 100% associated with high detection efficiency for weakly ionizing particles as shown in section 2.2 .5 . 2 3.2 . Description
of the
detector
2.3.2 .1 . Generalities
The detector has a principal layer and an electron multiplier device . In the laboratory for measuring the characteristics of the principal layer, the multiplier device, as seen before, comprises a semiconductor detector. In the beam this type of multiplier cannot generate a good "start" signal for the time-of-flight measurement . Thus, for a better time resolution and in order to avoid radiation damage we substituted for it a hybrid device as indicated in section 2 .1 .2 . 2.3.2 .2 . Electro n multiplier device
As shown in figs . 12 and 13 this device includes two parts:
(a) A pair of microchannel plates (b) with a useful diameter of 20 mm which give a high multiplication factor but cannot be placed in the primary beam . Then we have inserted between the porous CA layer (h) and the pair of microchannel plates, which are separated by 130 mm, a secondary electron focusing and transfer device . It is composed of cylindrical electrodes (c, d) and a square electrode (e) one side of which (f) the primary beam passes through. It is made of a mylar foil (thickness 1.5 l-.m) on which a 30 nm aluminium layer is evaporated. (b) In order to apply the periodic electric field (EEsc/EREC) on the porous CsI we have placed between it and the focusing system a very thin electrode (g). We took advantage of this electrode to create a transmission multiplier stage (cascade setup [10,11,15,161 for the secondary electrons that escaped from the porous CsI by forming a deposit of bulk CsI on the back of this electrode (fig . 14). This stage produces an amplification factor of 6 or 7 and enables us to overcome the inefficiency of the microchannel plates due to the low number of electrons created by the emitting sheet. This ultrathin electrode (fig . 14) is made of a self-supporting
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C Chianelh et al / Weakly ionizing charged particle detectors
Fig. 14 . Multiplier layer (with bulk density
Csl)
Fig. 13 View of the detector .
and upon which is evaporated under vacuum an aluminium electrode of 30 rim [16,21,22] and a bulk CsI layer of 60 rim acting as secondary emitter. A fast high voltage switch (1) permits us to apply a periodic electric field on the electrode which supports the porous CsI. This field has a reference voltage the potential of the multiplier (g) and is synchronized with the particle beam pulses being analysed .
alumina foil (AI z0 3 ) having a thickness of 100 rim (45 N,m/CM Z ) and a diameter of 60 mm realized in our laboratory by means of electrolytic anodization [17,18],
2.3 3. Experimental setup for testing the detector The experimental chamber is shown in fig. 15 . It permitted us to perform the tests of a prototype detec-
For beam tests
For laboratory tests
Radioactive source
Suntillator NE 102A 10 - 10 mm th = 5 mm
i
A
Photomuttiplier
Photomuitiplier
X, P 2020 , 2020 Scintillation counter # 2
Scintittator NE 102A 10 x 10 mm th=5mm
~ XP Scintillator counter
Thin windows (Kapton 10011)
1
Fig 15 The layout of the experimental setup of the prototype detector Alternate configurations for tests in the laboratory or in-beam of SATURNE are indicated.
C. Chianelh et al / Weakly ionizing charged particle detectors for to our laboratory with a ß-emitting source and in-beam near the SATURNE synchrotron . We installed a scintillation counter #1 at the exit of the chamber in order to detector the incoming particles passing through the detector. Thus counter is also used for the measurements of the time-of-flight of particles between itself and the probed detector . The beam tests were performed in a parasitic beam of protons scattered from a CD, target in order to prevent too high intensity in the detector . To make sure that the analysed particles were coming from the target, we added a second scintillation counter (#2) at the entrance of the chamber which, through a coincidence measurement, permitted us to define without ambiguity the trajectory and the origin of the particles passing through the detector . The entrance and the exit windows of this chamber were made of thin (100 win) kapton windows in order to prevent multiple scattering effects.
25 3
2 .3 .4. Associated electronics
The basic diagram is shown in fig. 16 . It can be divided into three parts. The first part determines, without random coincidences, the number n t of incoming particles passing through the detector during a period of time . This is realized by the coincidence measurement between the scintillation counters #1, # 2 and the beam gate . The scintillation counter # 1 is delayed by a delay line AT, to order to adjust its phase for the time-of-flight measurements . This scintillation counter is used alone for the laboratory tests and then the beam control is simulated by an adjustable local oscillator (OSC). The beam control makes it possible to apply the periodic field on the porous CsI by means of a fast high voltage switch (FHVS) developed in our laboratory . The second part of the setup is used to obtain the time resolution of the detector by using a time-to-ampli-
Beam gate Adjustable oscillator
PM1
For laboratory
[Fol Test in laboratory .1
PM2
Scaler 100 MHz
CF~p Tests in beam
Scaler 100 MHz
Scaler 100 MHz
Scaler 100 MHz
START
Multi channel analyser 4000 CX
Fig. 16 . Basic diagram for measuring the efficiency and time resolution in-beam and in the laboratory . FHVS : fast high voltage switch ; CFD: constant fraction discriminator, PM : photomultiplier, TAC: time-to-amplitude converter, OT : delay line ; C: coincidence unit .
25 4
C Chianelli et al / Weakly ioni-ing charged particle detectors
tude converter to conjunction with a multichannel analyser which determines the time-of-flight and the time resolution of the incoming particles between the detector and the scintillation counter # 1 . This part also gives the number, n _ of coincidences during a period of time between the two scintillation counters, the detector and the beam gate . Because of the amplitude variations of the detector signals we had to use a constant fraction discriminator (CFD 3 ) in order to obtain a good time resolution . The last part of the system allows calculating the rate of random coincidences Tp. between the two photomultipliers and the rate TDET between the detector and the two scintillation counters . This is necessary because of the high single counting rate of the scintillation counters created by the background generated by the beam . These rates are determinated by eqs. (2) and (3): TPM
= 100( n 3 /n i ),
TDF-1
= 100(na/ni),
= 100
(3)
112-n4 . ni - n3
2 .3 .5 Experimental results 1n the laboratory
J
L
True secondary electrons
(2)
in which n 3 is the number of coincidences between scintillation counters #2, #1 (delayed by AT3 , the value of which is much greater than the duration of the allowed coincidence overlap) and the beam gate ; n 4 is the number of coincidences between the two scintillation counters and the detector (delayed by a duration ,T4 = AT,) and the beam gate. From these values we can determine the `"true" efficiency of our device, which is defined by eq . (4).
n
Transmitted primary electrons
The tests were made with ß-rays (E > 1 MeV) . The efficiency measurements gave values nearly equal to those obtained with the setup of the principal layer, i.e . nearly 92% . The average number of electrons emitted cannot be determined because of the use of the multiplier foil and the microchannel plates . Thus, during the rest of this paper we shall only describe the efficiency and time resolution measurements . If the threshold discriminator DFC3 was set sufficiently low, we observed two peaks with unequal size on the time-of-flight spectrum (fig . 17) : the taller peak corresponds to "true secondary electrons" created by the multiplier stage with a low energy of emission (a few eV) [16] which includes two types of secondary electrons : (1) those which come from the primary particle passing through the multiplier foil and (2) those which are created by transmission multiplication of secondary electrons coming from porous Csl [10,11,19] . These two electrons peaks are separated by 300 ps . This time is determined by the primary particle speed and the transit time of
Fig. 17 Time-of-flight spectrum of (3-rays from a e° Sr source (E >- 1 MeV) Time resolution FWHM - 600 ps
secondary electrons coming from porous CsI, but the two peaks are very unequal and are mixed because of the low efficiency of the multiplier foil for weakly charged ionizing particles (few percent) . The second peak corresponds to secondary electrons coming from porous CsI which have penetrated the bulk Csl and which cross it, creating the secondary emission of the first peak : these are the transmitted primary electrons [15,16] which have a remaining energy of some keV when exiting the multiplier So we have at the exit of the multiplier two electron peaks with different energies that do not take the same time to travel the 120 mm between the multiplier and the microchannel plates . These two electron peaks have two different times of flight (At =5 ns) which is in accordance with the value measured in fig. 17 . It is possible to observe the two pulses with a storage oscilloscope (Tektronix 7834). It must be noted that the two pulses are not always present together because sometimes the primary electrons even create secondary electrons which do exit from the multiplier . Conclusion : if we only are interested in the etficiency of the device, the values obtained with the microchannel plates are nearly 92-93% ; but if we want to suppress the second peaks in the time-of-flight spectrum, it is necessary to increase the threshold of discriminator DFC3. This will also eliminate in certain cases of low multiplication, due to statistics, some true secondary electrons. This will cause a decrease of the efficiency of 2-3% with a time resolution of nearly 500 ps (FWHM) .
C. Chianelh et al / Weakly ionizing charged particle detectors Transmitted primary electrons _7
F_
True secondary electrons
25 5
mentioned before, and in fig. 19 we can see the broadened spectrum of the true secondary electrons obtained after increasing the threshold of discriminator DFC, (fig. 16) in order to suppress the transmitted primary electron peak . In figs . 18 and 19 we note a tail on the right part of each peak which is due to the fact that the proton beam was not entirely monoenergetic . The characteristics of the detector remained stable during the entire period of our tests to the beam (5 d) . 3. Conclusions and developments
L
Fig. 18 . Time-of-flight spectrum of high energy protons ( E = 540 MeV) (200 ps/channel) . 2.3 .6 . Tests with proton beams near the SATURNE synchrotron These tests were done [20] using a 540 MeV proton beam with a 40% duty factor. The results obtained confirm the laboratory results, i.e . that the efficiencies measured, corrected for random coincidences, vary from 96% to 98% depending on whether, as discussed above, we take only into account the true secondary electron peak or both electron peaks. The time resolution measured on the time-of-flight between the detector and scintillation counter # 1 is nearly 450 ps . These results are higher than those obtained in the laboratory and can be explained by the fact that the dE/dX of protons of 540 MeV is 1 .5 time higher than the dE/dX of electrons of 1 MeV in Cs1 [21,22]. In the time-of-flight spectrum (fig . 18) we observe the two electron peaks
We have shown the feasibility of a low-ionizing particle detector based on the particular use of secondary electronic emission of porous CsI. This detector is only a prototype and it must be adapted to each experiment . We are studying a device to be used for time-of-flight that is to be mounted between the physical target and the entrance of the spectrometer SPES I near the SATURNE synchrotron . This type of detector is limited by the fact that it is very difficult to evaporate a regular, homogeneous, porous CsI layer with a diameter higher than 100 mm, and it is necessary, because of the electric field . that the vacuum be low (= 10 -7 Torr maximum) . As this last condition is not fulfilled in the spectrometer vacuum chamber, a local improved vacuum is to be realized . Acknowledgements The authors would like to acknowledge several colleagues who have at various steps contributed to this work : A. Gann for the preliminary tests of the laboratory ; B. Bonin, H. Fanet and M. Rouger for many fruitful discussions and advices; J.C . Lugol for his help during the tests on the accelerator beam ; J. Saudinos and G. Bruge for their constant interest and support . References
[3] [4] [5] [6] II~IIYIinIwWr.,~.. .
Fig. 19 . Time-of-flight spectrum of "true" secondary electrons of high energy protons (E = 540 MeV) (20 ps/channel) . FW HM = 450 ps
[7] [8]
L. Austin and H Starke, Ann. Phys . 9 (1902) 271 D.R. Cone, Univ . of California Radiation Lab., Report no . 1030 (1950) . EJ Sternglass, Phys Rev. 108 (1957) 1. H. Kanter, Phys . Rev 121 (1961) 677 G.W . Goetze, Advances in electronics and electron physics (Academic Press, New York, 1962) vol. 16, p. 145 E.J . Sternglass and G.W . Goetze, IRE Trans. Nucl . Sci NS-9 (1962) 97 . G.W Goetze, A.H Boerio and M Green, J. Appl. Phys . 35 (1964) 482 S.M . Gukasyan, R.L . Kavalov, M P. Lonkian and G.G . Petrosyan, Izvestiya Akad . Nauk SSSR, Ser . Fis. 44 (1980) 634
25 6
[10] [11] [12] [13] [14]
[15]
C Cheaneth et al / Weakly ionizing chargedparticle detectors P. Ageron, H Fanet, A Garni, J.P . Robert, M. Rouger and J. Saudinos, J. Phys. 39, suppl. no. 6 (1978) C3 D.L . Emberson, IRE Trans. Nucl Sci. NS-9 (1962) 107. W.L. Wilcock, D.L . Emberson and B. Weekley, IRE Trans. Nucl . Sci. NS-7 (1960) 126. F. Busch, W. Pfeffer, B. Kohlmeyer, D. Schüll and P. Piihlhoffer, Nucl . Instr. and Meth . 171 (1980) 71 . W Starzecke, A.M . Stefamm, S. Lunardh and C. Signornm, Nucl Instr. and Meth. 193 (1982) 499 M.P . Lonkyan, R.L . Kavalov, N N Trofmchuk. A.N . Arvanov and V.G . Gavalyan, Yerevan Physics Institute, Scientific Report EFI-131 (1975) . M. Hagino . S. Yoshizaki, M. Kmoshita and R. Nishida,
[16] [17] [18] [19] [20] [21] [22]
Advances m electronics and electron physics (Academic Press, New York, 1972) vol. 33A, p 469. J Llacer, SLAC Report no . 86 (1968) . L. Hams, J. Opt . Soc. Am . 45 (1955) 27 . G. Hass, J Opt. Soc. Am . 39 (1949) 532. C. Chianelli, Compte rendu d'activité du Département de Physique Nucléaire 1984-1985, note CEA-N-2470 (1986) C Chianelli, Compte rendu d'activité du Département de Physique Nucléaire 1985-1986, note CEA-N-2510 (1987). J.F . Jam, At Data Nucl Data Tables 27 (1982) no 2/3 . L. Pages, E Bertel, H. Joffra and L. Sklavemtis, At. Data 4 (1972) no . 1.
245
WEAKLY IONIZING CHARGED PARTICLE DETECTORS WITH HIGH EFFICIENCY USING TRANSITORY ELECTRONIC SECONDARY EMISSION OF POROUS CsI C. CHIANELLI, P. AGERON, J.P . BOUVET, M. KAROLAK, S. MARTIN and J.P . ROBERT Service de Physique Nucléaire - Moyenne Energie, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France
Received 18 April 1988
A very high efficiency detector for weakly ionizing particles is described. This very thin detector (2 mg/cm 2 ), using the transitory electronic secondary emission of porous CsI, has allowed us to detect protons of 540 MeV energy with a detection efficiency '7 >_ 95% and a time resolution of 450 ps within the SATURNE synchrotron (CEN Saclay).
1. Introduction
2. Study and realisation of prototype detector
Secondary electronic emission was discovered by Austin and Starke [1] in 1902, but it was not until 1950 that Alvarez and Cone [2] proposed to use this phenomenon as a means of detection within an accelerator. The secondary emission is a prompt surface phenomenon which, in principle, permits one to obtain good time resolution while inserting very little material into the particle beam being analysed . The signal obtained is proportional to the dE/dX of the incoming particles [3,4], This type of detector runs well for heavily ionizing particles (a, heavy ions) with low energy and gives timing information of high quality. When the energy of the particles increases, problems arise because the efficiency and time resolution of the device deteriorate drastically. These problems become critical when we reach energies approaching minimum ionization ; mainly, if we want to set this detector on the front part of a high resolution spectrometer in which the beam analysed must not be too disturbed. In the sixties, the highly emitting porous alkali halides appeared [5-71, giving a great improvement to the situation, but without giving, however, the very good efficiency obtained with highly ionizing particles. In the frame of those developments, we herewith report the study and the realization of a very high efficiency detector for weakly ionizing particles. This very thin detector (2 mg/cm2), using a not very well known and hardly defined characteristic of porous CsI [8], has given us the possibility of detecting protons having a 540 MeV energy with a detection efficiency rl >_ 95% and a time resolution of 450 ps in the SATURNE synchrotron (CEN Sacley).
2.1 . Generalities to structure of the detector
0168-9002/88/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
The prototype detector has two parts: one part concerns the detection of the primary particle by means of a principal layer; the second part, helped by the multiplier device, produces the multiplication of the electrons emitted by the principal layer. 2.1 .1 . Principal layer
The principal layer must meet two important criteria : (1) the efficiency for the production of secondary electrons must be high - a 100% efficiency corresponds to the emission of at least one secondary electron for each charged particle traversing this layer, and (2) the average (statistical) number of electrons emitted during the passage of weakly ionizing charged particles should be as large as possible . These conditions are best realized with an emitter dielectric material that has the form of a porous layer a few hundred microns thick. 2.1 .2 . Electron multiplication
The average number of electrons created by a weakly ionizing particle during its passage through the principal layer is low (= 15) ; amplification is necessary. The amplification system must if possible have the characteristics that it should neither degrade the response time nor perturb the beam. It is difficult to satisfy simultaneously these conditions . Thus, we were obliged to use different types of devices depending on the test necessary at each stage of development of our detector . For the tests in the laboratory, we used semiconductor detectors both as multiplier device and detection stage
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C Chianelh et al. / Weakly ionizing charged particle detectors
[91 in order to determine the principal layer characteristics . Then, for laboratory tests and tests m the beam, we replaced these by an assembly [10,11,131 of separate stages, one for the multiplication of electrons and associated multichannel plates for the detection of electrons. Further developments will allow us to replace the microchannel plates by a stronger device comprising a thin scintillator (10 gym) viewed via a vacuum light guide by a photomultiplier tube. This final system will best fulfil the operational needs of the nuclear and particle physics community.
Al (30nm) Mytar 15Pm or A1 20 3 100nm -~
Porous Cs I (th=120Pm, relative density = 25) /0
f--
EEXr
2 .2 . Study of the principal layer : transitory secondary electronic emission of porous Cs1 2.2 .1 . Choice of porous CsI
The detection of high energy particles is very difficult because the number of secondary electrons emitted is very small. Only the electrons produced at a few tens of nanometers from the device surface can go out. In order to enhance this number it is necessary to increase the apparent surface of the device. To obtain this effect we can use materials such as KCI, CsI or MgO obtained in porous form and having a density of some few percent of the bulk density [7,91. Among these, CsI has been chosen because it is a good compromise between high secondary emission in the steady state and low hygroscopic properties, which permits us to keep it in the atmosphere for short times during fabrication . As will be seen later, its high initial secondary emission at the time of switching on the prompt electric field is another reason for the choice of porous Cs1 ; this characteristic is not found in MgO [91 nor KCI m our experiments . 2 2 .2 . Description and production of principal layer (fig. 1)
The principal layer is made following a well known procedure: (1) a mylar foil (diameter = 40 mm, thickness = 1.5 l.Lm (230 wg/cm2 ) is glued onto a stainless steel ring ; (2) a conducting surface is formed on the mylar foil by the evaporation of aluminium in a vacuum (diameter = 40 mm, thickness = 30 rim (10 It g/cm2 )) ; and onto this conducting surface we evaporate CsI very slowly (30 min) in an (8 Torr) argon environment, which gives a porous CsI layer (diameter = 40 mm, thickness = 120 Win (1300 )rg/cm2 )) . The relative density of this layer is 2.5% of the bulk density. The structure of this layer is made of CsI whiskers having 10 to 50 rim diameter. Note that in order to decrease the weight of the material in the beam, we can, if necessary, replace the
* Supplier : E. Merck (RFA).
Fig. 1. Layout of principal layer made m porous Cs1.
mylar by a self-supporting alumina (A1 20,) foil of 100 rim thickness (45 13g/cm2) and 40 mm diameter . 2 .2 .3 . Experimental setup for the investigation of the principal layer
The device allows us to analyse the secondary emission owing to a particle of well-known energy when it passes through a thin electrode covered with a thin layer of porous Csi . 2.2 .3 .1 Method of measurement [9]
The experimental setup (fig . 2) allows the passage through the principal layer of a beam of particles emitted by a radioactive source . The secondary electrons emitted by the particles are transported by an electric field from the principal layer to the secondary electron detector, while the primary particle is detected after crossing the principal layer. The secondary electron detector signal is stored only if it meets two conditions : (1) it must be in coincidence with the primary particle detector signal, and (2) the analysed energy of the primary particle must be in a prescribed energy band . This allows us to know the number of secondary electrons emitted by a primary particle having a well-known energy. In fact, the signal registered by the semiconductor secondary electron detector is proportional to the number of electrons created by the primary particle :
C. Chianelh et al. / Weakly ionizing chargedparticle detectors
247
Test
Radioactive
source
chamber
Si(Li) detector for
secondary
electrons
Thin
Si(Li)
window
for
detector
primary
electrons
Fig.
2.
Probed
foil
Experimental setup for testing the principal layer
Let Vo be the potential difference between the principal layer and the detector . The electron energy will be : Wm - eVv ,
where e is the charge of the electron .
For n electrons W = ne Vo, which is the energy registered by the detector . If no electron is emitted, the system triggers on the detector noise and a W, signal is registered .
PRIMARY PARTICLE CHANNEL
Si (Lil
PAC
SECONDARY EMISSION
PAC
A
CHANNEL
A
Fig. 3. Basic diagram for measuring the efficiency and average number of secondary electrons emitted from porous CsI. Legend : A: shaping amplifier; PAC: charge preamplifier ; ADC- analog-to-digital converter, LeCroy 3500 . DMA memory+microcomputer, Si(Li) : semiconductor detector .
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C Cluanelh et al. / Weakly ionizing charged particle detectors
For A coincidences, A is defined as the number of times when n electrons have been detected and n, the average number of electrons emitted, is calculated byN
n=
Y na
n=0 N
n=0
N
_
n=0
aVo A
a
'
N Y-
an
--J 100
Tl
2000
Y, n W
where N is the maximum number of electrons emitted. The primary layer efficiency rl, defined as the ratio between the times when there is at least the emission of one secondary electron and the whole number of coincidences is expressed as : n-1
n
A - ao
A
where a o represents the time when no electron has been detected, i.e . the signal stored for the electron detector corresponds to noise by the above defined coincidence . 2.2.3.2. Detectors
The detectors used are Si(Li) semiconductor detectors cooled with liquid nitrogen (77 K) and made to our laboratory . Their dimensions are: diameter = 28 mm, thickness = 2 mm for the secondary electron detector and 28 mm and 5 mm respectively for the primary particle detector . 2 2 3.3 . Electronics
The basic wiring diagram (fig . 3) includes two parts: secondary emission channel and primary particle channel. Each part includes a semiconductor detector, a charge preamplifier, a shaping amplifier and an analogto-digital converter whose contents are stored in a DMA memory LeCroy 3500 M microcomputer. This storage occurs only when the energy selection defined above is sufficient to initiate the analog-to-digital conversion . This allows us to store the spectrum of analysed particles and the secondary emission statistics of porous CsI. Then with the microcomputer we quickly calculate the efficiency and the average number of electrons emitted. With this device we can make measurements in two-minute periodes, including calculations. 2 2.3.4. Principal layer setting up to the device
Primarily, the principal layers are moved in argon between the bell jar and the experimental setup in order to avoid their deterioration by atmospheric moisture ; the layer is quickly installed under normal atmospheric conditions and then the setup is brought rapidly under vacuum .
E = 14 kV/cm
1000
50
T
0
30
I
60
90
I
120
t Imi n)
0
Fig. 4 . The efficiency r) and the average number of electrons n vs the time for a-particles (5 .5 MeV) 2 .2 .4 Experimental results
We will describe the results obtained in the various tests: (1) with a-particles coming from an 24 'Am radioactive source ; (2) with ß-rays having an energy higher than 1 MeV (from a y° Sr source). These results will be compared with those obtained by the only experimental group [8] that has described and tested a particle detector (with a source) using porous Csl and which show a time dependence of secondary electron emission . We have started again the tests made by these authors with a-particles placing emphasis on the study of weakly ionizing particles which are of particular interest to us . 2 .2 .4 .1 . Experiment s with highly ionizing particles ( 24 'Am) When suddenly applying an electric field E at t = 0
on a porous CsI layer, we observe that the number of secondary electrons emitted decreases with time (fig . 4) . After two or three hours this number is stabilized near an average value, n, of one hundred electrons. The number n obtained under steady state conditions is sufficiently high to ensure good secondary emission statistics which, when combined with the high signal to noise ratio of the semiconductor detector used in this experiment, yields a constant detection efficiency throughout the entire measurement of close to 100% . This phenomenon is reproducible with respect to time behaviour, n, and efficiency q . If we switch off the electric field E on the porous CsI layer and allow the device to relax for a sufficient period of time, we recover, after a new application of E, the same n and 71 with nearly the same time evolution. The time constant, T, for thus decay process is found to be about thirty minutes. 2 2 .4 .2. Tests with low-ionizing particles
We continued our tests with ß-rays (E > 1 MeV) and obtained results (fig . 5) for n which give the same variation as with a-particles . The levels obtained for n at t = 2 mm (n = 15) and t > 120 mm (n = 4) are much
249
C. Chianelh et al / Weakly ionizing charged particle detectors
100 80
4
60 40
2 0
Fig. 5. The curves of the efficiency il and the average number of electrons n vs the time for ß-rays (E >- 1 MeV) .
lower. This is normal because, as has been demonstrated [3], secondary emission is directly related to the d E/dX of primary particles. We note that the decay constant T is nearly the same as that for a-particles, which suggests that we have the same physical phenomenon and that it does not depend on the type of particle . Because of the small number of electrons emitted by the ß-rays and their emission statistics, the detection efficiency also decreases in time when the phenomenon is stationary (t > 240 mm). We observed (fig. 6) a microstructure due to the emission statistics in porous CsI in that particular case . It is less favorable from the Poisson statistics, s, having the same n which governs the secondary emission of bulk materials. We also studied in this steady state (fig . 7) the influence of the electric field E on the efficiency and on n . We found that from 14 kV/cm we obtain a constant efficiency even though we have a continuous increase of n . The plateau indicates that all electrons emitted by secondary emission during the passage of a primary particle in CsI whiskers have been extracted. The increase of n corresponds to electron multiplication by shower [14] of the first secondary electrons created by collision with the CsI whiskers when they are extracted
20 8
12
10
14
E (kV/cm)
Fig. 7. The curves of the efficiency 71 and the average number of electrons n m steady state vs the electric field E for ß-rays (E >-I MeV) .
from the layer by the electric field. We regulated E at 14 kV/cm in all tests with constant electric field. 2.2 .4 .3 . Interpretatio n of the phenomenon
Two reasons can be found to explain the variation in time of secondary electronic emission of porous CsI: (1) A depolanzation phenomenon caused by the drift of ionic impurities in the fibrous structure of porous CsI. (2) An electrio-drift phenomenon of the Cs' ions on the surface of the CsI whiskers which diminishes the local surface density of Cs' and accumulates these ions in the porous structure traps. This redistribution of ions results in an average increase of the electron work function of the CsI whiskers, which leads to a decrease of the secondary-electron emission efficiency .
IE E REC
2
10 3
n=42
i 0
EESC
Electrons nbr 5
10
15
Fig. 6 . Secondary electron distribution of porous CsI in steady state (t >_ 4 h) for ß-rays
0
t,
tz T
Fig. 8. Periodic electric field, see text .
t (sec)
250
C Chianelli et al / WeaklY ionizing charged particle detectors EESC=
20r
14kV/cm
1
t, =05S t 2 =1,5 S
EREC =0 kV/cm
10
I 0
I 30
I 60
I 90
kV/cm E REC =4kV/cm
ERE( =5kV/cm EREC=4kV/cm
\
EESC =14
9%
120
EREC--OkV/cm EREC=O -----
50
ti=05S t2=1,55
7t2=0
Fig. 9. Average number of electrons vs time of different recovery fields E for ß-rays.
Fig. 10 . Efficiency
2 .2 5 . Recovery of the initial conditions
field, some emitting foils of KCI and have not observed any variation of the secondary emission in contrast to ref. [8] : the secondary emission yield is sufficient to obtain a detection efficiency close to 100% . Tests on these KCI foils made with ß-rays (E > 1 MeV) under constant field have also given time independent results which are comparable to those of CsI under permanent field (71 = 60%) . Our results (better for both the CsI and the KCl under a permanent working field and for CA under a periodic field) suggest contamination problems or impurities in the formation of the emitting layers of ref. [8]. Our results tend to show that in CsI the electromigration of the Cs' ions is responsible for the decrease of the secondary emissions as a function of time in KCI the phenomenon does not occur. Further tests with other emitting compounds of cesium (CICs, BrCs) would permit us to confirm or reject this hypothesis
If the reduction of secondary emission is due to a depolarization phenomenon [8] and/or an electromigration, the application of a reverse electric field on the porous CsI would permit the recovery of the initial conditions . This is demonstrated by experiment . We applied on the porous CsI a periodic electric field (fig . 8) in which EESC is the electric field permitting the escape of the secondary electrons during the time t,, and EREC the field permitting to reduce the supposed causes of the secondary electronic emission decreasing during the time t, and setting the condition t i + 12 << T (fig . 4) . In figs . 9 and 10 we see, for particular values of t,, t2 and EESC, the important effect of the recovery electric field EREC on n and q . The reported tests were made with ß-rays (E > 1 MeV) . According to all measurements it is apparent that if we keep T= constant << T, RFsC-constant and allow t i , t 2 and ERrs to vary, we obtain an optimum efficiency when we are in agreement with the relationships shown by eq . (1): = constant
if
I ER,.C I x 1 2 > I
EESC
I x tl .
In accordance with these relations, we have obtained a constant efficiency in time whose value reaches 92% when the electric field EESC > 14 kV/cm. We also performed measurements with a-particles under the same experimental conditions . The improvement of n is also spectacular, but no efficiency increase is observed because the efficiency was already equal to 100% with a constant EESC
vs time and for different recovery electric fields E RE C for (3-rays.
h
2 3 . Realisation and tests of prototype detector 2 .3 .1 Principles of device
Our purpose is to instal near the SATURNE synchrotron a detector with good timing characteristics We
2 2 .6 . Discussion
If we compare our results to those of ref. [8], in which porous CA and KCI are used under a periodic field, but in a detector of different construction, it appears that the efficiency of our detector is much higher for the case of CsI, as we nearly set a 100% efficiency for 5 .5 MeV a-particles for the CA even without the help of a recovery field. We have also tested, for the same a-particles with a constant electric
EREC
EESC
Fig. 11 . Beam-field synchronization .
C Chianelh et al. / Weakly ionizing chargedparticle detectors
251
HT t= -12 5kV
BEAM
HT 3=-5 5kV
(g)
(h) Fig. 12 . Schematic drawing of the prototype detector . a anode; b: microchannel plates ; c, d, e: focusing electrodes, f. thin electrode of focalization, g: multiplier layer ; h emitter layer made of porous Csl, j : high voltage fast switch
have synchronised the periodic electric field ( EEsc, Eaec .) applied on the porous Csl layer with the time macrostructure of the accelerator (fig. 11). This allows us to realize a detector having an apparent duty cycle of 100% associated with high detection efficiency for weakly ionizing particles as shown in section 2.2 .5 . 2 3.2 . Description
of the
detector
2.3.2 .1 . Generalities
The detector has a principal layer and an electron multiplier device . In the laboratory for measuring the characteristics of the principal layer, the multiplier device, as seen before, comprises a semiconductor detector. In the beam this type of multiplier cannot generate a good "start" signal for the time-of-flight measurement . Thus, for a better time resolution and in order to avoid radiation damage we substituted for it a hybrid device as indicated in section 2 .1 .2 . 2.3.2 .2 . Electro n multiplier device
As shown in figs . 12 and 13 this device includes two parts:
(a) A pair of microchannel plates (b) with a useful diameter of 20 mm which give a high multiplication factor but cannot be placed in the primary beam . Then we have inserted between the porous CA layer (h) and the pair of microchannel plates, which are separated by 130 mm, a secondary electron focusing and transfer device . It is composed of cylindrical electrodes (c, d) and a square electrode (e) one side of which (f) the primary beam passes through. It is made of a mylar foil (thickness 1.5 l-.m) on which a 30 nm aluminium layer is evaporated. (b) In order to apply the periodic electric field (EEsc/EREC) on the porous CsI we have placed between it and the focusing system a very thin electrode (g). We took advantage of this electrode to create a transmission multiplier stage (cascade setup [10,11,15,161 for the secondary electrons that escaped from the porous CsI by forming a deposit of bulk CsI on the back of this electrode (fig . 14). This stage produces an amplification factor of 6 or 7 and enables us to overcome the inefficiency of the microchannel plates due to the low number of electrons created by the emitting sheet. This ultrathin electrode (fig . 14) is made of a self-supporting
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C Chianelh et al / Weakly ionizing charged particle detectors
Fig. 14 . Multiplier layer (with bulk density
Csl)
Fig. 13 View of the detector .
and upon which is evaporated under vacuum an aluminium electrode of 30 rim [16,21,22] and a bulk CsI layer of 60 rim acting as secondary emitter. A fast high voltage switch (1) permits us to apply a periodic electric field on the electrode which supports the porous CsI. This field has a reference voltage the potential of the multiplier (g) and is synchronized with the particle beam pulses being analysed .
alumina foil (AI z0 3 ) having a thickness of 100 rim (45 N,m/CM Z ) and a diameter of 60 mm realized in our laboratory by means of electrolytic anodization [17,18],
2.3 3. Experimental setup for testing the detector The experimental chamber is shown in fig. 15 . It permitted us to perform the tests of a prototype detec-
For beam tests
For laboratory tests
Radioactive source
Suntillator NE 102A 10 - 10 mm th = 5 mm
i
A
Photomuttiplier
Photomuitiplier
X, P 2020 , 2020 Scintillation counter # 2
Scintittator NE 102A 10 x 10 mm th=5mm
~ XP Scintillator counter
Thin windows (Kapton 10011)
1
Fig 15 The layout of the experimental setup of the prototype detector Alternate configurations for tests in the laboratory or in-beam of SATURNE are indicated.
C. Chianelh et al / Weakly ionizing charged particle detectors for to our laboratory with a ß-emitting source and in-beam near the SATURNE synchrotron . We installed a scintillation counter #1 at the exit of the chamber in order to detector the incoming particles passing through the detector. Thus counter is also used for the measurements of the time-of-flight of particles between itself and the probed detector . The beam tests were performed in a parasitic beam of protons scattered from a CD, target in order to prevent too high intensity in the detector . To make sure that the analysed particles were coming from the target, we added a second scintillation counter (#2) at the entrance of the chamber which, through a coincidence measurement, permitted us to define without ambiguity the trajectory and the origin of the particles passing through the detector . The entrance and the exit windows of this chamber were made of thin (100 win) kapton windows in order to prevent multiple scattering effects.
25 3
2 .3 .4. Associated electronics
The basic diagram is shown in fig. 16 . It can be divided into three parts. The first part determines, without random coincidences, the number n t of incoming particles passing through the detector during a period of time . This is realized by the coincidence measurement between the scintillation counters #1, # 2 and the beam gate . The scintillation counter # 1 is delayed by a delay line AT, to order to adjust its phase for the time-of-flight measurements . This scintillation counter is used alone for the laboratory tests and then the beam control is simulated by an adjustable local oscillator (OSC). The beam control makes it possible to apply the periodic field on the porous CsI by means of a fast high voltage switch (FHVS) developed in our laboratory . The second part of the setup is used to obtain the time resolution of the detector by using a time-to-ampli-
Beam gate Adjustable oscillator
PM1
For laboratory
[Fol Test in laboratory .1
PM2
Scaler 100 MHz
CF~p Tests in beam
Scaler 100 MHz
Scaler 100 MHz
Scaler 100 MHz
START
Multi channel analyser 4000 CX
Fig. 16 . Basic diagram for measuring the efficiency and time resolution in-beam and in the laboratory . FHVS : fast high voltage switch ; CFD: constant fraction discriminator, PM : photomultiplier, TAC: time-to-amplitude converter, OT : delay line ; C: coincidence unit .
25 4
C Chianelli et al / Weakly ioni-ing charged particle detectors
tude converter to conjunction with a multichannel analyser which determines the time-of-flight and the time resolution of the incoming particles between the detector and the scintillation counter # 1 . This part also gives the number, n _ of coincidences during a period of time between the two scintillation counters, the detector and the beam gate . Because of the amplitude variations of the detector signals we had to use a constant fraction discriminator (CFD 3 ) in order to obtain a good time resolution . The last part of the system allows calculating the rate of random coincidences Tp. between the two photomultipliers and the rate TDET between the detector and the two scintillation counters . This is necessary because of the high single counting rate of the scintillation counters created by the background generated by the beam . These rates are determinated by eqs. (2) and (3): TPM
= 100( n 3 /n i ),
TDF-1
= 100(na/ni),
= 100
(3)
112-n4 . ni - n3
2 .3 .5 Experimental results 1n the laboratory
J
L
True secondary electrons
(2)
in which n 3 is the number of coincidences between scintillation counters #2, #1 (delayed by AT3 , the value of which is much greater than the duration of the allowed coincidence overlap) and the beam gate ; n 4 is the number of coincidences between the two scintillation counters and the detector (delayed by a duration ,T4 = AT,) and the beam gate. From these values we can determine the `"true" efficiency of our device, which is defined by eq . (4).
n
Transmitted primary electrons
The tests were made with ß-rays (E > 1 MeV) . The efficiency measurements gave values nearly equal to those obtained with the setup of the principal layer, i.e . nearly 92% . The average number of electrons emitted cannot be determined because of the use of the multiplier foil and the microchannel plates . Thus, during the rest of this paper we shall only describe the efficiency and time resolution measurements . If the threshold discriminator DFC3 was set sufficiently low, we observed two peaks with unequal size on the time-of-flight spectrum (fig . 17) : the taller peak corresponds to "true secondary electrons" created by the multiplier stage with a low energy of emission (a few eV) [16] which includes two types of secondary electrons : (1) those which come from the primary particle passing through the multiplier foil and (2) those which are created by transmission multiplication of secondary electrons coming from porous Csl [10,11,19] . These two electrons peaks are separated by 300 ps . This time is determined by the primary particle speed and the transit time of
Fig. 17 Time-of-flight spectrum of (3-rays from a e° Sr source (E >- 1 MeV) Time resolution FWHM - 600 ps
secondary electrons coming from porous CsI, but the two peaks are very unequal and are mixed because of the low efficiency of the multiplier foil for weakly charged ionizing particles (few percent) . The second peak corresponds to secondary electrons coming from porous CsI which have penetrated the bulk Csl and which cross it, creating the secondary emission of the first peak : these are the transmitted primary electrons [15,16] which have a remaining energy of some keV when exiting the multiplier So we have at the exit of the multiplier two electron peaks with different energies that do not take the same time to travel the 120 mm between the multiplier and the microchannel plates . These two electron peaks have two different times of flight (At =5 ns) which is in accordance with the value measured in fig. 17 . It is possible to observe the two pulses with a storage oscilloscope (Tektronix 7834). It must be noted that the two pulses are not always present together because sometimes the primary electrons even create secondary electrons which do exit from the multiplier . Conclusion : if we only are interested in the etficiency of the device, the values obtained with the microchannel plates are nearly 92-93% ; but if we want to suppress the second peaks in the time-of-flight spectrum, it is necessary to increase the threshold of discriminator DFC3. This will also eliminate in certain cases of low multiplication, due to statistics, some true secondary electrons. This will cause a decrease of the efficiency of 2-3% with a time resolution of nearly 500 ps (FWHM) .
C. Chianelh et al / Weakly ionizing charged particle detectors Transmitted primary electrons _7
F_
True secondary electrons
25 5
mentioned before, and in fig. 19 we can see the broadened spectrum of the true secondary electrons obtained after increasing the threshold of discriminator DFC, (fig. 16) in order to suppress the transmitted primary electron peak . In figs . 18 and 19 we note a tail on the right part of each peak which is due to the fact that the proton beam was not entirely monoenergetic . The characteristics of the detector remained stable during the entire period of our tests to the beam (5 d) . 3. Conclusions and developments
L
Fig. 18 . Time-of-flight spectrum of high energy protons ( E = 540 MeV) (200 ps/channel) . 2.3 .6 . Tests with proton beams near the SATURNE synchrotron These tests were done [20] using a 540 MeV proton beam with a 40% duty factor. The results obtained confirm the laboratory results, i.e . that the efficiencies measured, corrected for random coincidences, vary from 96% to 98% depending on whether, as discussed above, we take only into account the true secondary electron peak or both electron peaks. The time resolution measured on the time-of-flight between the detector and scintillation counter # 1 is nearly 450 ps . These results are higher than those obtained in the laboratory and can be explained by the fact that the dE/dX of protons of 540 MeV is 1 .5 time higher than the dE/dX of electrons of 1 MeV in Cs1 [21,22]. In the time-of-flight spectrum (fig . 18) we observe the two electron peaks
We have shown the feasibility of a low-ionizing particle detector based on the particular use of secondary electronic emission of porous CsI. This detector is only a prototype and it must be adapted to each experiment . We are studying a device to be used for time-of-flight that is to be mounted between the physical target and the entrance of the spectrometer SPES I near the SATURNE synchrotron . This type of detector is limited by the fact that it is very difficult to evaporate a regular, homogeneous, porous CsI layer with a diameter higher than 100 mm, and it is necessary, because of the electric field . that the vacuum be low (= 10 -7 Torr maximum) . As this last condition is not fulfilled in the spectrometer vacuum chamber, a local improved vacuum is to be realized . Acknowledgements The authors would like to acknowledge several colleagues who have at various steps contributed to this work : A. Gann for the preliminary tests of the laboratory ; B. Bonin, H. Fanet and M. Rouger for many fruitful discussions and advices; J.C . Lugol for his help during the tests on the accelerator beam ; J. Saudinos and G. Bruge for their constant interest and support . References
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Fig. 19 . Time-of-flight spectrum of "true" secondary electrons of high energy protons (E = 540 MeV) (20 ps/channel) . FW HM = 450 ps
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