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Counter telescopes for the study of (n, charged particle) reactions
NUCLEAR INSTRUMENTS AND METHODS 34 (I965) 40-44;
© NORTH-HOLLAND P U B L I S H I N G CO.
COUNTER TELESCOPES FOR THE STUDY O F (n, charged partide) REACTIONS G. PAI(~, I. ~LAUS and P. TOMA~
Institute "Ruder Bo~kovi~", Zagreb
Received 16 October 1964 Performances of the gas counter telescope and the semiconductor counter telescope are described and their advantages are discussed. 1. htroduction Fast neutron physics represents one of the most difficult fields in experimental nuclear physics, but the information which might be obtained from studies of the reactions induced by neutrons justifies an effort to build special experimental setups and to perform experiments which are much more time consuming than the ones where charged particle beams are used. For illustration one can say that a 3 day bombardment for a spectrum obtained with charged particle projectiles is called "a long run", while a 5-6 day bombardment (for similar or usually worse statistics) is called a "short experiment" in neutron physics. The mean irradiation time for an angular distribution is 1500 hour. This article is devoted to the description of an experimental technique used in the Institute "Ruder Bo[kovid" to study reactions induced by 14.4 MeV neutrons.
2. Detector The counters used to detect charged particles produced by neutron irradiation should satisfy all the requirements imposed on detectors in general. In addition these detectors should have a low cross section for (n, charged particle) processes in the energy region studied if these charged products cannot be reliably discriminated by special criteria (e.g. in ionographic emulsions one can specify that only the tracks beginning at the surface should be counted thus eliminating almost all reaction products induced by neutrons in the plate material). The absence of a similar possibility makes the use of plastic scintillators, owing to their large hydrogen content, inconvenient for the detection of charged particles in neutron environment. In the study of (n, charged particle) processes two experimental techniques for the detection and discrimination of charged particles have established their wide applicability: nuclear plates and counter telescopes. Several other techniques (cloud chambers, thin detectors, bubble chambers etc.) were successfully applied 40
to various specific problems. In this article we will be concerned with the counter telescope technique. 2.1. TELESCOPE TECHNIQUE A telescope consisting of two countersin coincidence, a thin one used to determine the energy loss of a particle which passed through it (this counter will be called A E counter) and a thick one in which the same particle is finally stopped (this counter is called E counter), represents a well-known general method for the identification of charged particles1). While in the work with charged particle beams the use of such a telescope is sufficient, an additional counter is required if the study of neutron induced reactions is intended. This is due to the neutron irradiation of detectors causing a high counting rate in both counters evenin the absence of a target. This results in a high random coincidence rate which disturbs the measurements. Also a non-negligible number of real coincidences is produced,each detector acting as target for the other one. A way of reducing the random coincidence rate is to insert a third counter in the telescope. This third counter should be thinner than the d E counter and then it is placed between the target and the A E counter. This counter in coincidence with the two others considerably reduces the rate of random coincidences as described in ref. 2). Furthermore this counter must be constructed so as to have a small yield of charged particles of interest, since at least a part of it acts as a target. Two types of telescopes will be described: 1. A counter telescope consisting of three CO2 proportional counters and a CsI(TI) scintillation counter 2) (in further text it will be called the gas telescope). This telescope has been in operation for several years and it was used in numerous measurements of spectra and angular distributions, e.g. ref. 3-6). 2. A telescope (in further text, the semiconductor telescope) consisting of three surface barrier deteetorsT). This type of telescope has been in operation for almost two years and was used in experiments described in s-lo).
COUNTER
TELESCOPES
FOR
TI-IE S T U D Y
AE 10
20
30
40
50
60
70
Fig. 1. The frequency distribution of E-AE2 pulses demonstrating the discrimination between protons and deuterons in a typical measurement. 2.2. GAS TELESCOPE In this telescope 2) the triple coincidence is accomplished between two gas proportional counters called ~lEt and / l E 2 and the scintillation E-counter where the particle finally stops and loses its residual energy. A n additional gas proportional counter is placed between the target and the front wall of the telescope. Its pulses are in anticoincidence with other counters thus eliminating the particles originated in the front wall of the telescope. The l i e t and A E 2 counters are not of equal size. The /lE2 counter is constructed to give an optimum resolution necessary for the discrimination of particles. In order to present the smallest possible target to incident neutrons, the A E 2 counter is quite small (its active length is 3 cm compared with 12.5 cm for the AE2 counter). Given the pressure of 7-9 cm Hg of CO2 in the counters the length of t h e / l E t counter is too small to give a satisfactory spectrum of energy losses (its resolution is around 40% for 12.8 MeV deuterons). The function of the ~lEt counter in the gas telescope is limited to trigger the triple coincidence, thus reducing the random coincidence rate and defining the direction of the particles. The qualities of the gas counter telescope are: 1. Good particle discrimination. T h e / l E 2 Spectrum of energy losses in the A E 2 counter with a resolution of ~ 25% for 12.8 MeV deuterons, permits to have a discrimination between protons and deuterons, as well as between deuterons and tritons of the order of 1 : 1000.
OF
(n, charged particle)
REACTIONS
41
The frequency distribution of E - . 4 E pulses demonstrating the quality of the discrimination is shown in fig. 1. 2. A relatively wide range of energies of detected particles is covered: e.g. from 16 MeV to 3 MeV for protons. 3. The evenness of the absorbers in front of the Ecounter. The possibility to change the thicknesses of the /lEt a n d / l E 2 counters in a limited range by changing the pressure of the gas. Successful operation was achieved in the range from 4 to 10 cm Hg of C O y 4. The low noise of proportional counters which is 5-10 keV after the preamplifier stage. This is a very important quality which enables one to obtain very good A E spectra even when energy losses are quite small. The energy lost by a proton of 14 MeV in our /lE2 counter is only ~ 60 keV. In fact the broadening of the spectra of energy loss for a particle of a given energy stems mostly from the statistical fluctuations in the number of created ion pairs, so that the influence of electric noise is negligible. 5. The practically unlimited life time of the scintillation counters in the neutron flux. 6. The size and thicknesses of either counter are not limited by technological considerations. 7. The low background of the arrangement when irradiated by neutrons. The inherent background of the telescope is due to those materials in the telescope which, in spite of the coincidence system, act as secondary targets. The main secondary target is the gas with which the AEt counter is filled. The use of the gas CO2 is very convenient owing to large negative Q values for neutron induced reactions on C 12 and 016 and to the reasonably small O16(n,d) and Ot~(n,p) cross sectionsS). As visible from table 1 only the low energy part of the spectra are hampered by greater background. The gas telescope in connection with a twodimensional multichannel analyzer has proved to be suitable for measuring cross sections of the order of 0.1 mb/sr when the energy of detected particles is more than 6 MeV. At lower energies the lowest detectable cross section both for protons and deuterons begins gradually to increase and reaches a value of 0.8 mb/sr at the energy of 4 MeV. If one intends to study the energy region below 5 MeV it is essential that the AEt counter is thin. 8. The low cost. Besides the good properties listed above the work with a gas telescope is limited by some disadvantages we would like to stress: 1). The variable pulse delay from the proportional
42
G. PAl6 et al.
counters due to the finite transit time necessary for electrons to reach the central wire. In our telescope this delay is of the order of 1 #s, which limits the time resolution of the coincidence unit to the microsecond region. The slow coincidence limits the maximum neutron flux one can use in measurements. The measurements are made with a yield of ~ 2 x 10 9 neutrons/sec with a neutron source-to-targct distance of 10 cm. 2). The relatively bad resolution of the scintillation crystal. It amounts for e.g. 12.8 MeV deuterons to ~ 3 ~ . 3). The relative position of the target and the counters is closely related with the performances wanted. This makes it difficult to change the geometry. 4). The dependence of the light output of the scintillation crystal on the kind of detected charged particles. 5). The sensitivity of the response of the proportional counters to slight variations of the applied high voltage and to small admixtures of impurities in the counter gas. The photomultiplier amplification is also sensitive to high voltage variations. This necessitates frequent checkings of the positions of the characteristic peaks in the AE 1, AE2 and E-counter. These checkings are made in 2-3 hour intervals replacing the studied target by a target convenient for normalization, e.g. targets containing deuterium or hydrogen and taking spectra of recoil particles (e.g. deuterons or protons) in each of the three coincident counters. 2.3. SEMICONDUCTOR T E ~ O P E
In the last few years semiconductor detectors have found a wide field of application in particle spectrometry owing to their good energy resolution and compactness. The necessity to improve the resolution of energy measurements in neutron physics suggested the use of semiconductor detectors instead of a scintillation counter in a gas telescope. This was made by Colli et al. 11) and gave satisfactory results. The next step was to make a telescope in which the AEI and AE2 counters would be replaced by silicon junction detectors. The telescope we used is described in detail in 7). The AE~ counter had a thickness of 25 # while the AE 2 counter was usually ~ 50 g thick. The E detector was made of a silicon wafer ~ 1 mm thick, depleted to ~ 800 ~t. All the detectors had a 1 cm 2 active area and were of the surface barrier type. The thin counters were made from 100 ~ . c m n-type material while the E-counter was made from 1000 f2. cm p-type material. During the past years a number of measurements have shown the advantages and disadvantages as well as the scope of applicability of the semiconductor counter telescope.
The remarks to be enumerated emerge from the work in the detection of protons, deuterons and tritons but do not necessarily reflect all the aspects of the work in detecting alphas or heavier particles, Perhaps the best approach to the analysis of this system is to describe those properties of semiconductor counters which are favourable in the telescope operation under neutron irradiation. These are: 1. The r e s o l u t i o n - t h e energy resolutions obtainable with thick large area semiconductor detectors range to ~ 20 keV, which practically means that the E-counter resolution becomes an almost negligible factor in the overall energy resolution of the experimental arrangement. Thus the target thickness, the straggling in AE counters, the geometry and the energy spread in the neutron beam become prevailing factors. 2. Compactness of these detectors. Easy changes in the geometrical setup are possible. 3. Pulse height stability. Connecting the detectors to appropriate charge sensitive preamplifiers the pulse height does not depend upon small bias v a r i a t i o n s provided the depletion layer is thick enough to stop the detected particles. 4. Linear response as a function of energy (certainty in the energy region of interest) which is equal for all kinds of particles. 5. Fixed relations between the energy loss and the collected charge. This makes easy the summing of pulses of two or more detectors, contrary to the situation in proportional counters where the charge multiplication depends on several factors sometimes difficult to control. 6. The possibility to vary the depletion region within certain limits. The range is limited by two factors: i) the smallest bias necessary to have a good charge collection. The necessary fidd is ~ 2000 V/cm. ii) The highest applicable bias. 7. Fast charge collection. This makes the use of a fast coincidence circuit possible. 8. The low noise level of the E detector. Unconvenient properties of silicon counters in such arrangements can be summed as follows: 1). The noise level of these semiconductor detectors is much higher than that of gas proportional counters. The typical noise figure was 70 keV for the 25 # detector which was depleted to ~ 20 #. This noise is due partially to the thermal noise component which is proportional to the square root of the absolute temperature and the detector capacitance. The thermal noise energy equivalent for a 20 # detector at room temperature is about 35 keV. Other computable sources of noise give much lower values, so that one
C O U N T E R TELESCOPES FOR THE S T U D Y OF
can attribute a considerable fraction of the measured noise to the "excess noise" which can be assigned to the nature of electrodes or to the surface treatment which the detector has undergone12). One must bear in mind that not only the resolution is affected by the noise r.m.s, value but that the maximum noise amplitude is responsible for the triggering of the coincidence. For that reason one must allow the particles to lose more energy in the semiconductor E counters than in the proportional ones. In the case of the semiconductor telescope we used minimum energy losses which were ~ 4 times greater than those in the gas telescope. The resolution of the AE z counter coming from noise sources was measured to be ~ 50 keV. 2). Unconvenient Q values for (n, charged particle) reactions on silicon. Q values for (n,p) and (n,d) reactions on silicon are such that a larger energy region is obstructed by protons and deuterons from the AE1 counter than is the case when a CO2 counter is used as the AE1 counter. The cross sections and the Q values for (n,p), (n,d) and (n, alpha) reactions on O 16, C m and Si 2s are listed in table 1. In view of the very high cross section for the (n,p) reaction and the forward peaked angular distribution ~3) it is obvious that silicon is not a suitable material for the AE~ counter. TABLE 1
Element
Reaction
Q-value (MeV)
Si28 Si28 Si2a C12 Ct2 C12 O16 O16 O16
(n,p) (n,d) (n,~) (n,p) (n,d) (n,~) (n,p) (n,d) (n,u)
-3.857 -9.372 -2.656 -- 12.586 -13.731 - 5.704 -- 9.625 -9.886 - 2.215
] [
Ctotal
Ref.
250
14)
35 15
15) 16)
The use of a silicon AE 1 counter increases the minimum detectable (n,p) cross section to a value of ~ 0.5 mb/sr in the region where cross sections of 0.1 mb/sr can be measured with a gas telescope. The minimum detectable (n,d) cross section is ~ 0.2 mb/sr. 3). The limited life time of the detectors in the neutron flux. After a dose of ~ 1013 neutrons the detectors rapidly lose their primary electrical characteristics, which requires the relatively frequent replacement of the detectors, especially of the E-detector. That is easy but expensive. 4). The overall quantity of the material of both the
(n, charged particle) R E A C T I O N S
43
A E counters is greater than that of their analogous in the gas telescope, which makes the d E counters have a greater counting rate, thus increasing the random coincidence rate. 5). The high cost. 3. Conclusion
The comparison between both types of telescopes offers the following conclusions: 1. The particle discrimination is about the same in both telescopes. 2. The energy resolution can be substantially improved using a semiconductor E detector confining the resolution of the telescope to depend mainly on the spread in energy caused by the target thickness, geometrical arrangement and statistical fluctuations in the energy losses in the absorbers. The best resolution obtained with our semiconductor telescope was ~ 450 keV but a resolution of ~ 300 keV could be easily obtained. This is to be compared with the 600-700 keV resolutions in the gas telescope counter. In view of the high noise level of thin AE detectors it is questionable whether the summing of the AE and E pulses can appreciably improve the final energy resolution. 3. Contrary to the situation in the gas telescope the AE~ counter in the semiconductor telescope can also be used for particle d i s c r i m i n a t i o n - t h e energy loss spectra obtained with the AE 1 counter being not much worse than those obtained with the AE2 counter. This property can be usefully applied in making a simultaneous three-dimensional analysis of the AE1, AE2 and E pulses. Since it is always possible to make the pulses from the AE1 and AE:~ counters equal, the criterion for AEt and AE2 pulses produced by the particle detected by the telescope is: AEt ~ AE 2. The "acceptable" pulses lie in the area between the two solid lines (fig. 2). The AE1 - AE2 pairs which fall outside this area should be rejected. These pulses are due to: i) random coincidences; ii) real coincidences produced by particles generated in the AE t counter where they lose a small part of energy while in the AE2 they lose a n o r m a l amount. iii) real coincidences produced by particles generated in the E-counter which travel backwards and stop in the AE 1 counter. The goodness of the criterion is demonstrated by the group of crosses which represent A E I - d E 2 pairs related to recoil tritons from the elastic n-T scattering.
o. PAirS et ak
44 ~0
counters combines some essential qualities o f both types o f the detectorsl7). The a u t h o r s express their t h a n k s to their colleagues V. Ajda6i6, B. Antolkovi~, M. Cerineo, B. Lalovi6, D. Rendi6, J. Tudori6 and V. Valkovi6 for m a n y stimulating discussions.
i ~
~
i
~
References
"
.
~"
_AE 2
Fig. 2. The dE1 -dE2 pairs of pulses. Solid lines determine the region of acceptable pulses. The crosses represent AE1-AE2 pairs related to recoil tritons from the elastic n-T scattering. It is visible that they fall within the limits o f the criterion. This criterion rejects ~ 1 0 ~ of pulses in the region o f interest. 4. The counter telescope consisting of a thin proportional counter and two semiconductor d E 2 and E
l) F. A. Aschenbrenner, Phys. Rev. 98 (1955) 657. 2) L. G. Kuo, M. Petravi~ and B. Turko, Nucl. Instr. and Meth. 13 (1961) 29. 3) K. Ilakovac, L. G. Kno, M. Petravi~, I. ~laus and P. Toma~[, Phys. Rev. Letters 6 (1961) 356. 4) M. Cerineo, K. Ilakovac, I. ~laus, P. Toma~ and V. Valkovi~ Phys. Rev. 133 (1964) B948. 5) G. Pai~, I. ~laus and P. Toma~, Physics Letters 9 (1964) 147. 6) K. Ilakovac, L. G. Kuo, M. Petravi~, I. ~laus, P. Toma~ and G. R. Satchler, Phys. Rev. 128 (1962) 2739. 7) B. I. Lalovi~ and V. S. Ajda6i6, Proc. Syrup. Nucl. Electronics, Paris (1963). s) V. Ajda~i6, M. Cerineo, B. Lalovi6, G. Pai~, I. ~laus and P. Toma~, to be published. 9) V. Ajda~i~, M. Cerineo, B. Lalovi6, G. Pai~, I. ~laus and P. Toma~, Congres Intern. de Phys. Nucl., Abstracts (1964) 21. 10) G. Pai~, D. Rendi~ and P. Toma~, to be published. 11) L. Colli, P. Forti and E. Gadioli, Nucl. Phys. 54(1964) 253. 12) G. Dearnaley and D. C. Northrop, Semiconductor counters for nuclear radiations (Ed. F. N. Spon, London 1964). 13) L. Colli, M. G. Marcazzan, F. Merzari, P. G. Sona and P. Toma~, Nuovo Cimento 20 (1961) 928. 14) D. G. Gardner, Nucl. Phys. 29 (1962) 373. 15) G. D. Joanou, F. Fenech, Reactor Sci. Techn. 17 (1963) 425. 16) A. B. Lillie, Phys. Rex,. 87 (1952) 716. 17) B. Antolkovi~, M. Cerineo, B. Lalovi~, G. Pai~, I. ~laus and P. Toma~, to be published.
© NORTH-HOLLAND P U B L I S H I N G CO.
COUNTER TELESCOPES FOR THE STUDY O F (n, charged partide) REACTIONS G. PAI(~, I. ~LAUS and P. TOMA~
Institute "Ruder Bo~kovi~", Zagreb
Received 16 October 1964 Performances of the gas counter telescope and the semiconductor counter telescope are described and their advantages are discussed. 1. htroduction Fast neutron physics represents one of the most difficult fields in experimental nuclear physics, but the information which might be obtained from studies of the reactions induced by neutrons justifies an effort to build special experimental setups and to perform experiments which are much more time consuming than the ones where charged particle beams are used. For illustration one can say that a 3 day bombardment for a spectrum obtained with charged particle projectiles is called "a long run", while a 5-6 day bombardment (for similar or usually worse statistics) is called a "short experiment" in neutron physics. The mean irradiation time for an angular distribution is 1500 hour. This article is devoted to the description of an experimental technique used in the Institute "Ruder Bo[kovid" to study reactions induced by 14.4 MeV neutrons.
2. Detector The counters used to detect charged particles produced by neutron irradiation should satisfy all the requirements imposed on detectors in general. In addition these detectors should have a low cross section for (n, charged particle) processes in the energy region studied if these charged products cannot be reliably discriminated by special criteria (e.g. in ionographic emulsions one can specify that only the tracks beginning at the surface should be counted thus eliminating almost all reaction products induced by neutrons in the plate material). The absence of a similar possibility makes the use of plastic scintillators, owing to their large hydrogen content, inconvenient for the detection of charged particles in neutron environment. In the study of (n, charged particle) processes two experimental techniques for the detection and discrimination of charged particles have established their wide applicability: nuclear plates and counter telescopes. Several other techniques (cloud chambers, thin detectors, bubble chambers etc.) were successfully applied 40
to various specific problems. In this article we will be concerned with the counter telescope technique. 2.1. TELESCOPE TECHNIQUE A telescope consisting of two countersin coincidence, a thin one used to determine the energy loss of a particle which passed through it (this counter will be called A E counter) and a thick one in which the same particle is finally stopped (this counter is called E counter), represents a well-known general method for the identification of charged particles1). While in the work with charged particle beams the use of such a telescope is sufficient, an additional counter is required if the study of neutron induced reactions is intended. This is due to the neutron irradiation of detectors causing a high counting rate in both counters evenin the absence of a target. This results in a high random coincidence rate which disturbs the measurements. Also a non-negligible number of real coincidences is produced,each detector acting as target for the other one. A way of reducing the random coincidence rate is to insert a third counter in the telescope. This third counter should be thinner than the d E counter and then it is placed between the target and the A E counter. This counter in coincidence with the two others considerably reduces the rate of random coincidences as described in ref. 2). Furthermore this counter must be constructed so as to have a small yield of charged particles of interest, since at least a part of it acts as a target. Two types of telescopes will be described: 1. A counter telescope consisting of three CO2 proportional counters and a CsI(TI) scintillation counter 2) (in further text it will be called the gas telescope). This telescope has been in operation for several years and it was used in numerous measurements of spectra and angular distributions, e.g. ref. 3-6). 2. A telescope (in further text, the semiconductor telescope) consisting of three surface barrier deteetorsT). This type of telescope has been in operation for almost two years and was used in experiments described in s-lo).
COUNTER
TELESCOPES
FOR
TI-IE S T U D Y
AE 10
20
30
40
50
60
70
Fig. 1. The frequency distribution of E-AE2 pulses demonstrating the discrimination between protons and deuterons in a typical measurement. 2.2. GAS TELESCOPE In this telescope 2) the triple coincidence is accomplished between two gas proportional counters called ~lEt and / l E 2 and the scintillation E-counter where the particle finally stops and loses its residual energy. A n additional gas proportional counter is placed between the target and the front wall of the telescope. Its pulses are in anticoincidence with other counters thus eliminating the particles originated in the front wall of the telescope. The l i e t and A E 2 counters are not of equal size. The /lE2 counter is constructed to give an optimum resolution necessary for the discrimination of particles. In order to present the smallest possible target to incident neutrons, the A E 2 counter is quite small (its active length is 3 cm compared with 12.5 cm for the AE2 counter). Given the pressure of 7-9 cm Hg of CO2 in the counters the length of t h e / l E t counter is too small to give a satisfactory spectrum of energy losses (its resolution is around 40% for 12.8 MeV deuterons). The function of the ~lEt counter in the gas telescope is limited to trigger the triple coincidence, thus reducing the random coincidence rate and defining the direction of the particles. The qualities of the gas counter telescope are: 1. Good particle discrimination. T h e / l E 2 Spectrum of energy losses in the A E 2 counter with a resolution of ~ 25% for 12.8 MeV deuterons, permits to have a discrimination between protons and deuterons, as well as between deuterons and tritons of the order of 1 : 1000.
OF
(n, charged particle)
REACTIONS
41
The frequency distribution of E - . 4 E pulses demonstrating the quality of the discrimination is shown in fig. 1. 2. A relatively wide range of energies of detected particles is covered: e.g. from 16 MeV to 3 MeV for protons. 3. The evenness of the absorbers in front of the Ecounter. The possibility to change the thicknesses of the /lEt a n d / l E 2 counters in a limited range by changing the pressure of the gas. Successful operation was achieved in the range from 4 to 10 cm Hg of C O y 4. The low noise of proportional counters which is 5-10 keV after the preamplifier stage. This is a very important quality which enables one to obtain very good A E spectra even when energy losses are quite small. The energy lost by a proton of 14 MeV in our /lE2 counter is only ~ 60 keV. In fact the broadening of the spectra of energy loss for a particle of a given energy stems mostly from the statistical fluctuations in the number of created ion pairs, so that the influence of electric noise is negligible. 5. The practically unlimited life time of the scintillation counters in the neutron flux. 6. The size and thicknesses of either counter are not limited by technological considerations. 7. The low background of the arrangement when irradiated by neutrons. The inherent background of the telescope is due to those materials in the telescope which, in spite of the coincidence system, act as secondary targets. The main secondary target is the gas with which the AEt counter is filled. The use of the gas CO2 is very convenient owing to large negative Q values for neutron induced reactions on C 12 and 016 and to the reasonably small O16(n,d) and Ot~(n,p) cross sectionsS). As visible from table 1 only the low energy part of the spectra are hampered by greater background. The gas telescope in connection with a twodimensional multichannel analyzer has proved to be suitable for measuring cross sections of the order of 0.1 mb/sr when the energy of detected particles is more than 6 MeV. At lower energies the lowest detectable cross section both for protons and deuterons begins gradually to increase and reaches a value of 0.8 mb/sr at the energy of 4 MeV. If one intends to study the energy region below 5 MeV it is essential that the AEt counter is thin. 8. The low cost. Besides the good properties listed above the work with a gas telescope is limited by some disadvantages we would like to stress: 1). The variable pulse delay from the proportional
42
G. PAl6 et al.
counters due to the finite transit time necessary for electrons to reach the central wire. In our telescope this delay is of the order of 1 #s, which limits the time resolution of the coincidence unit to the microsecond region. The slow coincidence limits the maximum neutron flux one can use in measurements. The measurements are made with a yield of ~ 2 x 10 9 neutrons/sec with a neutron source-to-targct distance of 10 cm. 2). The relatively bad resolution of the scintillation crystal. It amounts for e.g. 12.8 MeV deuterons to ~ 3 ~ . 3). The relative position of the target and the counters is closely related with the performances wanted. This makes it difficult to change the geometry. 4). The dependence of the light output of the scintillation crystal on the kind of detected charged particles. 5). The sensitivity of the response of the proportional counters to slight variations of the applied high voltage and to small admixtures of impurities in the counter gas. The photomultiplier amplification is also sensitive to high voltage variations. This necessitates frequent checkings of the positions of the characteristic peaks in the AE 1, AE2 and E-counter. These checkings are made in 2-3 hour intervals replacing the studied target by a target convenient for normalization, e.g. targets containing deuterium or hydrogen and taking spectra of recoil particles (e.g. deuterons or protons) in each of the three coincident counters. 2.3. SEMICONDUCTOR T E ~ O P E
In the last few years semiconductor detectors have found a wide field of application in particle spectrometry owing to their good energy resolution and compactness. The necessity to improve the resolution of energy measurements in neutron physics suggested the use of semiconductor detectors instead of a scintillation counter in a gas telescope. This was made by Colli et al. 11) and gave satisfactory results. The next step was to make a telescope in which the AEI and AE2 counters would be replaced by silicon junction detectors. The telescope we used is described in detail in 7). The AE~ counter had a thickness of 25 # while the AE 2 counter was usually ~ 50 g thick. The E detector was made of a silicon wafer ~ 1 mm thick, depleted to ~ 800 ~t. All the detectors had a 1 cm 2 active area and were of the surface barrier type. The thin counters were made from 100 ~ . c m n-type material while the E-counter was made from 1000 f2. cm p-type material. During the past years a number of measurements have shown the advantages and disadvantages as well as the scope of applicability of the semiconductor counter telescope.
The remarks to be enumerated emerge from the work in the detection of protons, deuterons and tritons but do not necessarily reflect all the aspects of the work in detecting alphas or heavier particles, Perhaps the best approach to the analysis of this system is to describe those properties of semiconductor counters which are favourable in the telescope operation under neutron irradiation. These are: 1. The r e s o l u t i o n - t h e energy resolutions obtainable with thick large area semiconductor detectors range to ~ 20 keV, which practically means that the E-counter resolution becomes an almost negligible factor in the overall energy resolution of the experimental arrangement. Thus the target thickness, the straggling in AE counters, the geometry and the energy spread in the neutron beam become prevailing factors. 2. Compactness of these detectors. Easy changes in the geometrical setup are possible. 3. Pulse height stability. Connecting the detectors to appropriate charge sensitive preamplifiers the pulse height does not depend upon small bias v a r i a t i o n s provided the depletion layer is thick enough to stop the detected particles. 4. Linear response as a function of energy (certainty in the energy region of interest) which is equal for all kinds of particles. 5. Fixed relations between the energy loss and the collected charge. This makes easy the summing of pulses of two or more detectors, contrary to the situation in proportional counters where the charge multiplication depends on several factors sometimes difficult to control. 6. The possibility to vary the depletion region within certain limits. The range is limited by two factors: i) the smallest bias necessary to have a good charge collection. The necessary fidd is ~ 2000 V/cm. ii) The highest applicable bias. 7. Fast charge collection. This makes the use of a fast coincidence circuit possible. 8. The low noise level of the E detector. Unconvenient properties of silicon counters in such arrangements can be summed as follows: 1). The noise level of these semiconductor detectors is much higher than that of gas proportional counters. The typical noise figure was 70 keV for the 25 # detector which was depleted to ~ 20 #. This noise is due partially to the thermal noise component which is proportional to the square root of the absolute temperature and the detector capacitance. The thermal noise energy equivalent for a 20 # detector at room temperature is about 35 keV. Other computable sources of noise give much lower values, so that one
C O U N T E R TELESCOPES FOR THE S T U D Y OF
can attribute a considerable fraction of the measured noise to the "excess noise" which can be assigned to the nature of electrodes or to the surface treatment which the detector has undergone12). One must bear in mind that not only the resolution is affected by the noise r.m.s, value but that the maximum noise amplitude is responsible for the triggering of the coincidence. For that reason one must allow the particles to lose more energy in the semiconductor E counters than in the proportional ones. In the case of the semiconductor telescope we used minimum energy losses which were ~ 4 times greater than those in the gas telescope. The resolution of the AE z counter coming from noise sources was measured to be ~ 50 keV. 2). Unconvenient Q values for (n, charged particle) reactions on silicon. Q values for (n,p) and (n,d) reactions on silicon are such that a larger energy region is obstructed by protons and deuterons from the AE1 counter than is the case when a CO2 counter is used as the AE1 counter. The cross sections and the Q values for (n,p), (n,d) and (n, alpha) reactions on O 16, C m and Si 2s are listed in table 1. In view of the very high cross section for the (n,p) reaction and the forward peaked angular distribution ~3) it is obvious that silicon is not a suitable material for the AE~ counter. TABLE 1
Element
Reaction
Q-value (MeV)
Si28 Si28 Si2a C12 Ct2 C12 O16 O16 O16
(n,p) (n,d) (n,~) (n,p) (n,d) (n,~) (n,p) (n,d) (n,u)
-3.857 -9.372 -2.656 -- 12.586 -13.731 - 5.704 -- 9.625 -9.886 - 2.215
] [
Ctotal
Ref.
250
14)
35 15
15) 16)
The use of a silicon AE 1 counter increases the minimum detectable (n,p) cross section to a value of ~ 0.5 mb/sr in the region where cross sections of 0.1 mb/sr can be measured with a gas telescope. The minimum detectable (n,d) cross section is ~ 0.2 mb/sr. 3). The limited life time of the detectors in the neutron flux. After a dose of ~ 1013 neutrons the detectors rapidly lose their primary electrical characteristics, which requires the relatively frequent replacement of the detectors, especially of the E-detector. That is easy but expensive. 4). The overall quantity of the material of both the
(n, charged particle) R E A C T I O N S
43
A E counters is greater than that of their analogous in the gas telescope, which makes the d E counters have a greater counting rate, thus increasing the random coincidence rate. 5). The high cost. 3. Conclusion
The comparison between both types of telescopes offers the following conclusions: 1. The particle discrimination is about the same in both telescopes. 2. The energy resolution can be substantially improved using a semiconductor E detector confining the resolution of the telescope to depend mainly on the spread in energy caused by the target thickness, geometrical arrangement and statistical fluctuations in the energy losses in the absorbers. The best resolution obtained with our semiconductor telescope was ~ 450 keV but a resolution of ~ 300 keV could be easily obtained. This is to be compared with the 600-700 keV resolutions in the gas telescope counter. In view of the high noise level of thin AE detectors it is questionable whether the summing of the AE and E pulses can appreciably improve the final energy resolution. 3. Contrary to the situation in the gas telescope the AE~ counter in the semiconductor telescope can also be used for particle d i s c r i m i n a t i o n - t h e energy loss spectra obtained with the AE 1 counter being not much worse than those obtained with the AE2 counter. This property can be usefully applied in making a simultaneous three-dimensional analysis of the AE1, AE2 and E pulses. Since it is always possible to make the pulses from the AE1 and AE:~ counters equal, the criterion for AEt and AE2 pulses produced by the particle detected by the telescope is: AEt ~ AE 2. The "acceptable" pulses lie in the area between the two solid lines (fig. 2). The AE1 - AE2 pairs which fall outside this area should be rejected. These pulses are due to: i) random coincidences; ii) real coincidences produced by particles generated in the AE t counter where they lose a small part of energy while in the AE2 they lose a n o r m a l amount. iii) real coincidences produced by particles generated in the E-counter which travel backwards and stop in the AE 1 counter. The goodness of the criterion is demonstrated by the group of crosses which represent A E I - d E 2 pairs related to recoil tritons from the elastic n-T scattering.
o. PAirS et ak
44 ~0
counters combines some essential qualities o f both types o f the detectorsl7). The a u t h o r s express their t h a n k s to their colleagues V. Ajda6i6, B. Antolkovi~, M. Cerineo, B. Lalovi6, D. Rendi6, J. Tudori6 and V. Valkovi6 for m a n y stimulating discussions.
i ~
~
i
~
References
"
.
~"
_AE 2
Fig. 2. The dE1 -dE2 pairs of pulses. Solid lines determine the region of acceptable pulses. The crosses represent AE1-AE2 pairs related to recoil tritons from the elastic n-T scattering. It is visible that they fall within the limits o f the criterion. This criterion rejects ~ 1 0 ~ of pulses in the region o f interest. 4. The counter telescope consisting of a thin proportional counter and two semiconductor d E 2 and E
l) F. A. Aschenbrenner, Phys. Rev. 98 (1955) 657. 2) L. G. Kuo, M. Petravi~ and B. Turko, Nucl. Instr. and Meth. 13 (1961) 29. 3) K. Ilakovac, L. G. Kno, M. Petravi~, I. ~laus and P. Toma~[, Phys. Rev. Letters 6 (1961) 356. 4) M. Cerineo, K. Ilakovac, I. ~laus, P. Toma~ and V. Valkovi~ Phys. Rev. 133 (1964) B948. 5) G. Pai~, I. ~laus and P. Toma~, Physics Letters 9 (1964) 147. 6) K. Ilakovac, L. G. Kuo, M. Petravi~, I. ~laus, P. Toma~ and G. R. Satchler, Phys. Rev. 128 (1962) 2739. 7) B. I. Lalovi~ and V. S. Ajda6i6, Proc. Syrup. Nucl. Electronics, Paris (1963). s) V. Ajda~i6, M. Cerineo, B. Lalovi6, G. Pai~, I. ~laus and P. Toma~, to be published. 9) V. Ajda~i~, M. Cerineo, B. Lalovi6, G. Pai~, I. ~laus and P. Toma~, Congres Intern. de Phys. Nucl., Abstracts (1964) 21. 10) G. Pai~, D. Rendi~ and P. Toma~, to be published. 11) L. Colli, P. Forti and E. Gadioli, Nucl. Phys. 54(1964) 253. 12) G. Dearnaley and D. C. Northrop, Semiconductor counters for nuclear radiations (Ed. F. N. Spon, London 1964). 13) L. Colli, M. G. Marcazzan, F. Merzari, P. G. Sona and P. Toma~, Nuovo Cimento 20 (1961) 928. 14) D. G. Gardner, Nucl. Phys. 29 (1962) 373. 15) G. D. Joanou, F. Fenech, Reactor Sci. Techn. 17 (1963) 425. 16) A. B. Lillie, Phys. Rex,. 87 (1952) 716. 17) B. Antolkovi~, M. Cerineo, B. Lalovi~, G. Pai~, I. ~laus and P. Toma~, to be published.