- Email: [email protected]
A fast fission product monitor
NUCLEAR
INSTRUMENTS
AND
METHODS
33
(~965) 314-318; :O
NORTH-HOLLAND
PUBLISHING
CO.
A FAST FISSION P R O D U C T M O N I T O R O. S T R I N D E H A G
AB Atomenergi, Studsvik, Sweden
Received 20 October 1964 For monitoring of long-lived fission products in water a fast monitor with (~erenkov detector has been designed. The monitor responds in less than a second to yg! activities of 10 4/~c/cm3 and can be used for water flows up to about 60 I/rain. Detector construction and electronic equipment are described and the
selection of photomultiplier tube is discussed. The detector efficiency is given for some important isotopes as well as tile minimunl activity that can be detected. The performance of the monitor in an application connected with the handling of burned out fuel elements is also reported.
1. Introduction Need for a fast fission product monitor arose at the Studsvik R2 reactor in conjunction with the transportation of burned out fuel elements. The elements which are of MTR type are placed in shielded containers and shipped for reprocessing. Before the elements are placed in the containers, the end adapters are removed. At the bottom of a pool adjacent to the reactor the end adapters are cut off with a special saw operated from a point above the surface. Water depth is about nine meters. The fuel elements are placed in a fixture before sawing is started and optical inspection through the water is possible during operation. In the event of the saw accidentally coming in contact with the fuel itself, activities of several hundred curie can be sawn away in a few second. A fast response fission product detector for monitoring water samples taken near the saw is therefore desirable. For this purpose a monitor has been constructed that will respond in less than a second to activities just over pool water background. Background activity is mainly that due to Na 24 and amounts to about 1 0 - 4 / / c / c m 3. Irradiation time is 30 days for the elements and a decay time of about 90 days is chosen before sawing off the end adapters. This means that only long-lived fission products and their daughters are present for detection. Fission products that give a comparatively high contribution to the total activity a r e S r 89, y91, pr144 and Ba ~4°, all having beta end-point energies in the region 1-3 MeV. Beta particles of such high energies are easily detected with a Cerenkov detector.
is important when a fast response is required. Like most other beta detectors the sensitivity decreases rapidly with decreasing beta energy and falls to zero at the threshold for (~erenkov radiation in water, which for electrons amounts to 260 keV. A detector for the application in question should have a sample container big enough to give reasonable count rates at a beta activity level of 10 4Hc/cm3, but small enough to avoid water delay times in the container of more than roughly half a second. Since the water flow through the detector could be expected to lie between 20 and 60 l/min, a sample container volume of 200 cm 3 was chosen. When sawing through the end adapters, which consist mainly of aluminium, a great deal of sawdust of different particle sizes follows the water samples that are pumped up to the detector. Bigger particles are stopped in a filter placed ahead of the pump, whereas smaller particles pass the pump as well as detector. Because the aluminium alloy is somewhat radioactive, about 100/tc/g, sawdust could quickly increase the background if it gathers in the sample container. Thus the shape of the container, especially its inlet and outlet, had to be designed to prevent solid particles from getting stuck. With the chosen detector construction (fig. 1) there is only a small chance of solid particle contamination. One reason for this is the high water speed of several meters per second through the inlet and outlet tubes. In low level Cerenkov detectors good optical reflectors are required due to the very low light yield from beta particles in water. One finds in the energy region of interest that a 1 MeV electron gives about 30 photons and a 3 MeV electron about 230 photons when stopped in water, calculated for light emitted between the wavelength limits 3500 and 5500 A. The optical efficiency of a Cerenkov detector of this type can be estimated with a knowledge of reflection coefficient, photocathode area and total reflector area3). For the sample container in question the ratio cathode area to reflector
2. Detector construction For continuous monitoring of beta activities in water 12erenkov detectors are known to have a high sensitivity1'2). A large sample volume can be chosen for (2erenkov detectors without too great a degradation in efficiency. Thus a comparatively large number of counts is obtained for a given concentration of activity. This
314
A FAST FISSION P R O D U C T MONITOR
315
Fig. 1. Diagram of Cerenkov detector.
area is 0.11. A reflection coefficient of 0.95 gives then an optical efficiency of about 69 %, whereas a reflection coefficient of 0.80 gives an optical efficiency of about 35%. When taking into consideration the quantum yield of photomultiplier cathodes and the last figure for the optical efficiency, 1 MeV electrons give on the average about one photo-electron whereas 3 MeV electrons give about eight photo-electrons. Besides having good optical properties the reflector material should have low adsorption of activities from the water~). White paint of epoxy type was found to fulfil both these requirements quite well and also to have a satisfactory mechanical strength for this application. It is preferable not to have the photomultiplier in direct contact with water, especially when, as in this case, the container should withstand a certain overpressure. Therefore a plexiglas window of thickness 8 m m is inserted and optically coupled to the multiplier with silicone grease. Two neoprene O-rings are used to make a watertight seal. A photomultiplier tube with S l l response is utilized. I f a quartz window and a multiplier with S13 response had been chosen, a higher photo-electron yield could have been obtained due to the transmission of the ultraviolet component of the (~erenkov light. However, detector sensitivity was found to be quite satisfactory with the mentioned arrangement.
3. Choice of photomultiplier In low level (~erenkov detectors the photomultiplier tube determines to a great extent the performance of the whole detector. I m p o r t a n t photomultiplier parameters are spectral response, quantum yield and dark current. A great number of photomultiplier types are available, though practically none of them have sufficiently low dark current at r o o m temperature. When a
good energy discrimination is desired, for instance for suppression of some low energy background, the quantum efficiency should be as high as possible. In this case when more or less total activities are measured a high quantum efficiency is not quite so important. Separate tests of different photomultiplier tubes were performed before selecting one for the actual detector. Concentrating the investigation to 2 inch multipliers having Sll cathode, some EMI tubes of types 6097 B and 9514 B showed an acceptable dark current. A few 6097 S tubes were also tested but did not give lower dark current background than selected B-types. As known, the S-type cathode has lower thermal emission than the B-type. The tests were easily performed with a small plexiglas radiator which could be placed in optical contact with the multipliers inside a light-tight box. The radiator could be exposed to beta radiation from a Srg°-Y 9° source through a thin aluminium foil. Integral bias curves for the dark current pulses of two 6097 B tubes are shown in fig. 2. According to the data for these tubes supplied by the manufacturer the d.c. value of the dark current is equivalent to 5 x 10-1 z and 10-10 input lumen, respectively. The amplification was matched so that the same signal was obtained from the Srg°-Y 9° source in both cases. It should be remembered that a small part of the background pulses originate from background radiation which gives (~erenkov pulses in the plexiglas radiator and in the glass tube. When making the same measurements with photomultipliers having dark currents corresponding to near 10 - 9 input lumen, an increase in pulse count rates of the order to 100 times was observed compared with the best 6097 B tubes. The value 10 -9 input lumen is typical of many multiplier types. These tests show that the dark current d.c. value of the photomultiplier is a good measure of the expected background for (~erenkov
316
o. STRINDEHAG
detectors having relatively small radiators. Therefore tubes which are supplied with separate test data (covering dark current d.c. values) are preferable, because it is out of the question to use unselected photomultiplier tubes in low level detectors. This is due to the tremendous spread in dark current values for most types. For type 6097 B the equivalent dark current normally falls between the limits 2 × 10 12 and 2 x 10-1° input lumen.
4. Electronic equipment As fully transistorized electronic equipment was used, the complete monitor, containing detector, amplifiers, count rate meter, alarm unit, power supplies as well as recorder, could be built into a small standard cabinet with dimensions 62 x 53 x 38 cm. (~erenkov pulses from beta particles result, as mentioned above, in the emission of only one or a few photo-electrons per particle. Anyhow, these pulses do lO6
105
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104 u
o L -12
c o ¢3
10 3
102
10
I
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I
I
10
20
30
40
Discriminator
50
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Fig. 2. Background pulses from two 6097 B photomultipliers.
not fall in the noise region of the preamplifier. Using a photomultiplier with a current gain of 106 and an amplifier with a total input capacitance of 30 pF an input pulse of 5 mV is obtained for a single photoelectron. As single electrons give broad distorted pulse spectra at the amplifier input it is necessary to record pulses that are somewhat smaller than the average single electron output pulse when dealing with beta particles of low energy. Yet for amplifiers with bandwidths of the order of I MHz input noise is no problem. The preamplifier has a voltage gain of four and an input impedance of 20 k~. From this amplifier pulses are fed through a short cable to a main amplifier whose voltage gain can be varied over the range 5-50 times. The clipping time constant is 1 ps. The amplification and the high voltage of the photomultiplier are chosen to give signal amplitudes in the region 0.2-3 V which is the input range of the discriminator. The count rate meter has five linear ranges between 100 and 10000 cps and the time constant is 1 s during normal operation. For calibration the time constant can be increased to 5 s. Count rates are shown on a meter and also continuously recorded. If the count rate exceeds the alarm level an acoustic alarm is given. High voltage for the photomultiplier is derived from a transistor d.c. converter with a variable output voltage extending up to 1400 V. Output voltage is stabilized by means of a differential amplifier and a corona tube reference.
5. Measurements Before incorporating the (~erenkov detector in the actual monitor some initial experiments were performed. For these experiments commercially available instruments such as sealers, count rate meters and amplifiers were used. Calibrated solutions of long-lived fission products were utilized for determination of detector efficiency, sensitivity and energy resolution. It is of interest to compare the sensitivity of this simple (~erenkov detector with the sensitivity of other detectors of low level beta activities in water. In general low level measurements have been performed with Srg°-Y 9°. Typical values of the minimum activity concentration that can be detected are in the region 10- 8 _ 10 - 5 p c / c m 3 for G M tubes, scintillation and ~'erenkov detectors. For continuous monitoring figures down to 8.1 x 10-81lc/cm 3 s r g ° - Y 9° have been reported for Cerenkov detectors (measuring time = 5 min) ref. 1). When determining the sensitivity of low level detectors it is convenient to compare the signal (S) with the standard deviation of the background (B) for a given measuring time (often 5 or 10 min). Usually a S/~/B value of 3 is chosen for low level measurements. For
317
A FAST FISSION PRODUCT MONITOR 1oo
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Fig. 3. Detector efficiency for y90, y91 and Rb 8s.
monitors that should give alarm when activity increases, a value of 4-5 might be more realistic due to long-term background fluctuations. A closed loop containing water reservoir, detector (sample container) and p u m p was used for calibration of the detector. Solutions of measured activities of y90, y91 and Rb 88 were injected into the loop and the pulse height spectra studied. Energy resolution of low level Cerenkov detectors is in general very poor 4'5) and longtailed spectra are obtained. The resolution is mainly limited by the low number of photo-electrons. At the low energy end of a beta spectrum photomultiplier dark current makes measurements impossible. Integral pulse height spectra for y 9 o y 9 ! and Rb 88 are shown in fig. 3. The recorded pulse heights extend over two decades. Count rates have been converted to detector efficiency. The isotope y 9 o gives a
low contribution to the total fission product activity considered here (irradiation time 30 days, decay time 90 days) but is measured for the purpose of comparing the detector sensitivity with that obtained with other (~erenkov detectors. The limited volume of this sample container as well as the simple detector construction in general ought to result in a sensitivity far from the optimal. At the lowest discriminator level shown (rel. value 1) a detector efficiency of 2 5 . 1 ~ is found. It should be noted that optimal sensitivity is obtained at a somewhat higher discriminator level. Expressed in cpm/pc/cm 3 the detector efficiency in question amounts to 1.11 x 108cpm/pc/cm 3. At a water and detector temperature of 24 ° and without any additional shielding than the walls of the container a minimum detectable activity concentration of 5.5 × 10 -7 pc/cm 3 was obtained for a counting time of 10 min. This relatively good sensitivity can be realized due to the low dark current of the multiplier used. The dark current for the actual multiplier, i.e. 5 x 10-12 equivalent input lumen, is, however, not extreme. Thus by selecting a still better multiplier and by cooling and shielding the detector it should be possible to improve the sensitivity without going over to coincidence methods for background reduction. A multiplier with quartz window would also be advantageous as mentioned above. Measurements with y91 were performed because this isotope is expected to be important for the detector application in question. Its beta end-point energy of 1.55 MeV is also in the middle of the energy region of interest, y g t is therefore used for final calibration of the monitor. Under the same measuring conditions as above, activities of y91 can be detected down to a concentration of 1.1 x 10 -6 pc/cm 3. The optimal discriminator setting for highest sensitivity has the rel. value 1 or somewhat lower. Fig. 3 also shows detector I
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Fi B. 4, Minimum detectableactivity ]eve|s.
I 6
318
o. STRINDEHAG
efficiency for the isotope Rb 88. A possible application for this detector is for monitoring reactor coolant water. Rb 88 is then one of the fission products that has a beta energy in excess of the neutron activated nonfission products also present in the coolant water. The half-life of this isotope is also long enough (17.8 rain) to make calibrations possible. The variation of detector sensitivity with energy is shown in fig. 4 where the minimum activity level that can be detected is given as a function of beta end-point energy. The sensitivity drops for the 12erenkov detector at a higher beta energy than, for instance, for a beta sensitive G M tube6). 6. Monitor performance Calibration of the monitor was carried out with a y 9 1 solution and the discriminator level was set to give 100cps for an activity of 10-4pc/cm 3. An iron shield with thickness 5 cm was placed around the detector at the final installation. This shield is regarded as sufficient when the monitor is in operation in the R2 reactor hall. Normal background radiation amounts here to 0.1 0.2 mr/h. With the sample container filled with de-ionized water a total background of 90 cps was measured. The R C time constant of one second is equivalent to a measuring time of two second. Thus a signal count rate of 30 cps is about four times the standard deviation of the background. Accordingly activities of y91 down to 3 x 10-Spc/cm 3 can be regarded as possible to detect with this monitor, and for activities of 10-4pc/cm 3 or more the time to alarm is less than a second. The detector response is checked with a 10 pc Co 6° source that can be placed inside the shielding. When pool water was flowing through the sample container a background increase of up to 200 cps
was measured. This background is mainly due to Na 24 activity in the water. The first experience with the saw in operation showed that the sawing of the aluminium end adapters gave a considerable background increase. Count rate increases varied somewhat from element to element but were normally of the order of 700-800 cps. An alarm setting which is about 500 cps higher than the background is now chosen. This corresponds to 5 x 10-4pc/cm 3 o f Y 91. It is hoped to lower this level when a better knowledge of background variations has been obtained. 7. Conclusion A (~erenkov detector utilized for fast monitoring of fission products in water has the advantage of having a high detector efficiency even for relatively large sample volumes. The detector construction is simple and it is possible to operate with high water pressures. I f a good photomultiplier tube is selected conventional electronic circuits can be used. The author wishes to thank the staff at the R2 reactor for helpful assistance during the experiments. Thanks are also due to Mr. L. Marbfick for constructing the electronic equipment.
References 1) S. Yamada and H. Yamamoto, J.A.E. Soc. Japan 4 (1962) 607. 2) R. and G. Ducros, Proc. Conf. Nucl. Electronics, Belgrade, 1 (1961) 55. 3) J. V. Jelley, Cerenkov radiation and its applications (Pergamon Press, London, 1958). 4) S. E. Rippon, A.E.I. Res. Report No. A 1091 (1960). 5) C. M. Gordon, J. H. Miller and R. E. Larson, NRL Quarterly on Nucl. Sci. and Techn. (Oct. I, 1962) 46. o) H. Gebauer and S. Mfiller, Die Atomwirtschaft (1962) 487.
INSTRUMENTS
AND
METHODS
33
(~965) 314-318; :O
NORTH-HOLLAND
PUBLISHING
CO.
A FAST FISSION P R O D U C T M O N I T O R O. S T R I N D E H A G
AB Atomenergi, Studsvik, Sweden
Received 20 October 1964 For monitoring of long-lived fission products in water a fast monitor with (~erenkov detector has been designed. The monitor responds in less than a second to yg! activities of 10 4/~c/cm3 and can be used for water flows up to about 60 I/rain. Detector construction and electronic equipment are described and the
selection of photomultiplier tube is discussed. The detector efficiency is given for some important isotopes as well as tile minimunl activity that can be detected. The performance of the monitor in an application connected with the handling of burned out fuel elements is also reported.
1. Introduction Need for a fast fission product monitor arose at the Studsvik R2 reactor in conjunction with the transportation of burned out fuel elements. The elements which are of MTR type are placed in shielded containers and shipped for reprocessing. Before the elements are placed in the containers, the end adapters are removed. At the bottom of a pool adjacent to the reactor the end adapters are cut off with a special saw operated from a point above the surface. Water depth is about nine meters. The fuel elements are placed in a fixture before sawing is started and optical inspection through the water is possible during operation. In the event of the saw accidentally coming in contact with the fuel itself, activities of several hundred curie can be sawn away in a few second. A fast response fission product detector for monitoring water samples taken near the saw is therefore desirable. For this purpose a monitor has been constructed that will respond in less than a second to activities just over pool water background. Background activity is mainly that due to Na 24 and amounts to about 1 0 - 4 / / c / c m 3. Irradiation time is 30 days for the elements and a decay time of about 90 days is chosen before sawing off the end adapters. This means that only long-lived fission products and their daughters are present for detection. Fission products that give a comparatively high contribution to the total activity a r e S r 89, y91, pr144 and Ba ~4°, all having beta end-point energies in the region 1-3 MeV. Beta particles of such high energies are easily detected with a Cerenkov detector.
is important when a fast response is required. Like most other beta detectors the sensitivity decreases rapidly with decreasing beta energy and falls to zero at the threshold for (~erenkov radiation in water, which for electrons amounts to 260 keV. A detector for the application in question should have a sample container big enough to give reasonable count rates at a beta activity level of 10 4Hc/cm3, but small enough to avoid water delay times in the container of more than roughly half a second. Since the water flow through the detector could be expected to lie between 20 and 60 l/min, a sample container volume of 200 cm 3 was chosen. When sawing through the end adapters, which consist mainly of aluminium, a great deal of sawdust of different particle sizes follows the water samples that are pumped up to the detector. Bigger particles are stopped in a filter placed ahead of the pump, whereas smaller particles pass the pump as well as detector. Because the aluminium alloy is somewhat radioactive, about 100/tc/g, sawdust could quickly increase the background if it gathers in the sample container. Thus the shape of the container, especially its inlet and outlet, had to be designed to prevent solid particles from getting stuck. With the chosen detector construction (fig. 1) there is only a small chance of solid particle contamination. One reason for this is the high water speed of several meters per second through the inlet and outlet tubes. In low level Cerenkov detectors good optical reflectors are required due to the very low light yield from beta particles in water. One finds in the energy region of interest that a 1 MeV electron gives about 30 photons and a 3 MeV electron about 230 photons when stopped in water, calculated for light emitted between the wavelength limits 3500 and 5500 A. The optical efficiency of a Cerenkov detector of this type can be estimated with a knowledge of reflection coefficient, photocathode area and total reflector area3). For the sample container in question the ratio cathode area to reflector
2. Detector construction For continuous monitoring of beta activities in water 12erenkov detectors are known to have a high sensitivity1'2). A large sample volume can be chosen for (2erenkov detectors without too great a degradation in efficiency. Thus a comparatively large number of counts is obtained for a given concentration of activity. This
314
A FAST FISSION P R O D U C T MONITOR
315
Fig. 1. Diagram of Cerenkov detector.
area is 0.11. A reflection coefficient of 0.95 gives then an optical efficiency of about 69 %, whereas a reflection coefficient of 0.80 gives an optical efficiency of about 35%. When taking into consideration the quantum yield of photomultiplier cathodes and the last figure for the optical efficiency, 1 MeV electrons give on the average about one photo-electron whereas 3 MeV electrons give about eight photo-electrons. Besides having good optical properties the reflector material should have low adsorption of activities from the water~). White paint of epoxy type was found to fulfil both these requirements quite well and also to have a satisfactory mechanical strength for this application. It is preferable not to have the photomultiplier in direct contact with water, especially when, as in this case, the container should withstand a certain overpressure. Therefore a plexiglas window of thickness 8 m m is inserted and optically coupled to the multiplier with silicone grease. Two neoprene O-rings are used to make a watertight seal. A photomultiplier tube with S l l response is utilized. I f a quartz window and a multiplier with S13 response had been chosen, a higher photo-electron yield could have been obtained due to the transmission of the ultraviolet component of the (~erenkov light. However, detector sensitivity was found to be quite satisfactory with the mentioned arrangement.
3. Choice of photomultiplier In low level (~erenkov detectors the photomultiplier tube determines to a great extent the performance of the whole detector. I m p o r t a n t photomultiplier parameters are spectral response, quantum yield and dark current. A great number of photomultiplier types are available, though practically none of them have sufficiently low dark current at r o o m temperature. When a
good energy discrimination is desired, for instance for suppression of some low energy background, the quantum efficiency should be as high as possible. In this case when more or less total activities are measured a high quantum efficiency is not quite so important. Separate tests of different photomultiplier tubes were performed before selecting one for the actual detector. Concentrating the investigation to 2 inch multipliers having Sll cathode, some EMI tubes of types 6097 B and 9514 B showed an acceptable dark current. A few 6097 S tubes were also tested but did not give lower dark current background than selected B-types. As known, the S-type cathode has lower thermal emission than the B-type. The tests were easily performed with a small plexiglas radiator which could be placed in optical contact with the multipliers inside a light-tight box. The radiator could be exposed to beta radiation from a Srg°-Y 9° source through a thin aluminium foil. Integral bias curves for the dark current pulses of two 6097 B tubes are shown in fig. 2. According to the data for these tubes supplied by the manufacturer the d.c. value of the dark current is equivalent to 5 x 10-1 z and 10-10 input lumen, respectively. The amplification was matched so that the same signal was obtained from the Srg°-Y 9° source in both cases. It should be remembered that a small part of the background pulses originate from background radiation which gives (~erenkov pulses in the plexiglas radiator and in the glass tube. When making the same measurements with photomultipliers having dark currents corresponding to near 10 - 9 input lumen, an increase in pulse count rates of the order to 100 times was observed compared with the best 6097 B tubes. The value 10 -9 input lumen is typical of many multiplier types. These tests show that the dark current d.c. value of the photomultiplier is a good measure of the expected background for (~erenkov
316
o. STRINDEHAG
detectors having relatively small radiators. Therefore tubes which are supplied with separate test data (covering dark current d.c. values) are preferable, because it is out of the question to use unselected photomultiplier tubes in low level detectors. This is due to the tremendous spread in dark current values for most types. For type 6097 B the equivalent dark current normally falls between the limits 2 × 10 12 and 2 x 10-1° input lumen.
4. Electronic equipment As fully transistorized electronic equipment was used, the complete monitor, containing detector, amplifiers, count rate meter, alarm unit, power supplies as well as recorder, could be built into a small standard cabinet with dimensions 62 x 53 x 38 cm. (~erenkov pulses from beta particles result, as mentioned above, in the emission of only one or a few photo-electrons per particle. Anyhow, these pulses do lO6
105
Id~16 I° L
104 u
o L -12
c o ¢3
10 3
102
10
I
I
I
I
10
20
30
40
Discriminator
50
voltoge
Fig. 2. Background pulses from two 6097 B photomultipliers.
not fall in the noise region of the preamplifier. Using a photomultiplier with a current gain of 106 and an amplifier with a total input capacitance of 30 pF an input pulse of 5 mV is obtained for a single photoelectron. As single electrons give broad distorted pulse spectra at the amplifier input it is necessary to record pulses that are somewhat smaller than the average single electron output pulse when dealing with beta particles of low energy. Yet for amplifiers with bandwidths of the order of I MHz input noise is no problem. The preamplifier has a voltage gain of four and an input impedance of 20 k~. From this amplifier pulses are fed through a short cable to a main amplifier whose voltage gain can be varied over the range 5-50 times. The clipping time constant is 1 ps. The amplification and the high voltage of the photomultiplier are chosen to give signal amplitudes in the region 0.2-3 V which is the input range of the discriminator. The count rate meter has five linear ranges between 100 and 10000 cps and the time constant is 1 s during normal operation. For calibration the time constant can be increased to 5 s. Count rates are shown on a meter and also continuously recorded. If the count rate exceeds the alarm level an acoustic alarm is given. High voltage for the photomultiplier is derived from a transistor d.c. converter with a variable output voltage extending up to 1400 V. Output voltage is stabilized by means of a differential amplifier and a corona tube reference.
5. Measurements Before incorporating the (~erenkov detector in the actual monitor some initial experiments were performed. For these experiments commercially available instruments such as sealers, count rate meters and amplifiers were used. Calibrated solutions of long-lived fission products were utilized for determination of detector efficiency, sensitivity and energy resolution. It is of interest to compare the sensitivity of this simple (~erenkov detector with the sensitivity of other detectors of low level beta activities in water. In general low level measurements have been performed with Srg°-Y 9°. Typical values of the minimum activity concentration that can be detected are in the region 10- 8 _ 10 - 5 p c / c m 3 for G M tubes, scintillation and ~'erenkov detectors. For continuous monitoring figures down to 8.1 x 10-81lc/cm 3 s r g ° - Y 9° have been reported for Cerenkov detectors (measuring time = 5 min) ref. 1). When determining the sensitivity of low level detectors it is convenient to compare the signal (S) with the standard deviation of the background (B) for a given measuring time (often 5 or 10 min). Usually a S/~/B value of 3 is chosen for low level measurements. For
317
A FAST FISSION PRODUCT MONITOR 1oo
,
,,,I
,
,
,
,
,
,,,I
,
,
,
,
,
,,,~,
,
,
,
,
,
,,
,
,
,
t
,
,
10-
==
c}
0,01
i i li
I 1
10
Relative
discriminator
, , 100
level
Fig. 3. Detector efficiency for y90, y91 and Rb 8s.
monitors that should give alarm when activity increases, a value of 4-5 might be more realistic due to long-term background fluctuations. A closed loop containing water reservoir, detector (sample container) and p u m p was used for calibration of the detector. Solutions of measured activities of y90, y91 and Rb 88 were injected into the loop and the pulse height spectra studied. Energy resolution of low level Cerenkov detectors is in general very poor 4'5) and longtailed spectra are obtained. The resolution is mainly limited by the low number of photo-electrons. At the low energy end of a beta spectrum photomultiplier dark current makes measurements impossible. Integral pulse height spectra for y 9 o y 9 ! and Rb 88 are shown in fig. 3. The recorded pulse heights extend over two decades. Count rates have been converted to detector efficiency. The isotope y 9 o gives a
low contribution to the total fission product activity considered here (irradiation time 30 days, decay time 90 days) but is measured for the purpose of comparing the detector sensitivity with that obtained with other (~erenkov detectors. The limited volume of this sample container as well as the simple detector construction in general ought to result in a sensitivity far from the optimal. At the lowest discriminator level shown (rel. value 1) a detector efficiency of 2 5 . 1 ~ is found. It should be noted that optimal sensitivity is obtained at a somewhat higher discriminator level. Expressed in cpm/pc/cm 3 the detector efficiency in question amounts to 1.11 x 108cpm/pc/cm 3. At a water and detector temperature of 24 ° and without any additional shielding than the walls of the container a minimum detectable activity concentration of 5.5 × 10 -7 pc/cm 3 was obtained for a counting time of 10 min. This relatively good sensitivity can be realized due to the low dark current of the multiplier used. The dark current for the actual multiplier, i.e. 5 x 10-12 equivalent input lumen, is, however, not extreme. Thus by selecting a still better multiplier and by cooling and shielding the detector it should be possible to improve the sensitivity without going over to coincidence methods for background reduction. A multiplier with quartz window would also be advantageous as mentioned above. Measurements with y91 were performed because this isotope is expected to be important for the detector application in question. Its beta end-point energy of 1.55 MeV is also in the middle of the energy region of interest, y g t is therefore used for final calibration of the monitor. Under the same measuring conditions as above, activities of y91 can be detected down to a concentration of 1.1 x 10 -6 pc/cm 3. The optimal discriminator setting for highest sensitivity has the rel. value 1 or somewhat lower. Fig. 3 also shows detector I
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Fi B. 4, Minimum detectableactivity ]eve|s.
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318
o. STRINDEHAG
efficiency for the isotope Rb 88. A possible application for this detector is for monitoring reactor coolant water. Rb 88 is then one of the fission products that has a beta energy in excess of the neutron activated nonfission products also present in the coolant water. The half-life of this isotope is also long enough (17.8 rain) to make calibrations possible. The variation of detector sensitivity with energy is shown in fig. 4 where the minimum activity level that can be detected is given as a function of beta end-point energy. The sensitivity drops for the 12erenkov detector at a higher beta energy than, for instance, for a beta sensitive G M tube6). 6. Monitor performance Calibration of the monitor was carried out with a y 9 1 solution and the discriminator level was set to give 100cps for an activity of 10-4pc/cm 3. An iron shield with thickness 5 cm was placed around the detector at the final installation. This shield is regarded as sufficient when the monitor is in operation in the R2 reactor hall. Normal background radiation amounts here to 0.1 0.2 mr/h. With the sample container filled with de-ionized water a total background of 90 cps was measured. The R C time constant of one second is equivalent to a measuring time of two second. Thus a signal count rate of 30 cps is about four times the standard deviation of the background. Accordingly activities of y91 down to 3 x 10-Spc/cm 3 can be regarded as possible to detect with this monitor, and for activities of 10-4pc/cm 3 or more the time to alarm is less than a second. The detector response is checked with a 10 pc Co 6° source that can be placed inside the shielding. When pool water was flowing through the sample container a background increase of up to 200 cps
was measured. This background is mainly due to Na 24 activity in the water. The first experience with the saw in operation showed that the sawing of the aluminium end adapters gave a considerable background increase. Count rate increases varied somewhat from element to element but were normally of the order of 700-800 cps. An alarm setting which is about 500 cps higher than the background is now chosen. This corresponds to 5 x 10-4pc/cm 3 o f Y 91. It is hoped to lower this level when a better knowledge of background variations has been obtained. 7. Conclusion A (~erenkov detector utilized for fast monitoring of fission products in water has the advantage of having a high detector efficiency even for relatively large sample volumes. The detector construction is simple and it is possible to operate with high water pressures. I f a good photomultiplier tube is selected conventional electronic circuits can be used. The author wishes to thank the staff at the R2 reactor for helpful assistance during the experiments. Thanks are also due to Mr. L. Marbfick for constructing the electronic equipment.
References 1) S. Yamada and H. Yamamoto, J.A.E. Soc. Japan 4 (1962) 607. 2) R. and G. Ducros, Proc. Conf. Nucl. Electronics, Belgrade, 1 (1961) 55. 3) J. V. Jelley, Cerenkov radiation and its applications (Pergamon Press, London, 1958). 4) S. E. Rippon, A.E.I. Res. Report No. A 1091 (1960). 5) C. M. Gordon, J. H. Miller and R. E. Larson, NRL Quarterly on Nucl. Sci. and Techn. (Oct. I, 1962) 46. o) H. Gebauer and S. Mfiller, Die Atomwirtschaft (1962) 487.