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Development of a high sensitivity liquid scintillation-NaI(Tl) coincidence detector for X-γ emitters in lunar rocks
NUCLEAR
INSTRUMENTS
AND
METHODS
99
(I972)
349-354;
©
NORTH-HOLLAND
PUBLISHING
CO.
D E V E L O P M E N T OF A HIGH SENSITIVITY LIQUID SCINTILLATION-NaI(TI) COINCIDENCE DETECTOR FOR X-? E M I T T E R S I N L U N A R R O C K S J.S. FRUCHTER*
University of California, San Diego, La Jolla, California 92037, U.S.A. Received 8 N o v e m b e r 1971 A new type o f high sensitivity counter e m p l o y i n g a liquid scintillation cell as a coincidence gate for a 3" NaI(T1) well crystal has been developed for the detection o f small a m o u n t s of X - 7 emitting nuclides. This c o u n t e r has approximately 10 times the efficiency o f a c o m p a r a b l e detector e m p l o y i n g a conventional gas proportional c o u n t e r as the coincidence gate. T h e b a c k g r o u n d
o f the existing system u n d e r the full width at half m a x i m u m o f the 54Mn p h o t o p e a k is 0 . 0 2 c p m . M e a s u r e m e n t s to the 15% precision level of as little as 0.25 d p m o f 54Mn in l u n a r rock samples are reported. T h e applicability o f the counter to X - t, emitters other t h a n 54Mn is discussed.
1. Introduction
2. Counter design and operation
X - ? coincidence counters have been used successfully by a number of investigators in order to detect very small amounts of those radionuclides which decay by electron capture to an excited state [cf., Bhandaril)]. An example of such a nuclide is the 303 d 54Mn which emits an 834 keV g a m m a in coincidence with 5.41 keV C.r K X-ray. Measurements of this nuclide in lunar rocks were desired as a monitor for solar proton emission during time scales of the order of 1-2 y, corresponding to the mean life of 5+Mn. Calculations showed that ordinary X-7 counters using a conventional gas X-ray counter as the coincidence gate would not be suitable for this application, because although they achieve very low backgrounds, they do so only by sacrificing a great deal of efficiency. This efficiency loss is, due to the low efficiency of small gas counters for X-rays coupled with the loss of Auger electrons in solid samples. These combined factors generally yield in the order of a 3-5% efficiency for the emitted X-rays. Coupled with the 11% efficiency of a 3" Na[(TI) well detector for the 834 keV photon, the instrument has a coincidence efficiency of only 0.6%. Because of the rather small absolute activity present in our lunar samples ( < 0.5 dpm) at the time of counting, a detector with such low efficiency could not produce a measurement of acceptable statistical precision in a reasonable length of time. To alleviate this problem, a new type of counter, described below, was developed.
The counter employed a liquid scintillation cell coupled to a 2" photomultiplier tube as a coincidence gate for a 3" NaI(TI) well clystal. The 8 ml quartz liquid scintillation vial was placed in the well and the entire assembly was coupled together in a light-tight container (see fig. 1). The counter was then placed inside an iron shield surrounded by a Geiger ring. In this configuration, the counter had a weightless efficiency of 6.1% (fwhm) and a working background of 0.025 epm. The sample was dissolved as MnCI2 in a commercially available xylene-napthalene based solvent*. As much as 50 mg of Mn could be solubilized in the 8 ml solution before an appreciable loss of efficiency occurred. (At 50 mg the efficiency was still 87% of the weightless
* Present address: Center for Volcanology, Oregon, Eugene, O r e g o n 97403, U.S.A.
University
t Marketed u n d e r the trade n a m e A q u a s o l by New E n g l a n d Nuclear Corp.
~
C O N E T R A I
of Fig. 1. Liquid scintillation-NaI(Tl) coincidence counter.
349
350
J.S. FRUCHTER IAquid Scintillation Counter PM
Ceil
NaI [ Ill
]
Preamp
]
RC-Shaping Ampiifier ]
Single Channel Analyzer
Spectroscopy [ Arnplifie r [ and Logic Pulse Shaper ! I Logic 1 Pulse
Linear Pulse Li~ ea r
F Fast Coincidence > | L e a d i n g Edge Overlap el. 5 ~.sec Resolving Tithe r Geiger ~ Guard Ring I
Slo\~ Anticoincidence I 200 j s e c Blanking Time Coincidence Pulse Multic hannel Analyze r
<
Fig. 2. Electronics and logic for liquid scintillation-Nal(TI) coincidence counter.
efficiency. When 60 mg of Mn was solubilized the efficiency fell to 61% of the weightless efficiency.) The Photomultiplier tube used was an E M ! 9635QB. It is necessary to use a low noise tube of this nature in order to keep chance coincidences from contributing appreciably to the counter background. Since many of the X-ray events produce ultimately only one electron at the photocathode, it is necessary either to set the electronic discriminators quite low or the gain quite high in order to avoid a serious loss of efficiency. The problem is compounded by the fact that it is desirable to operate the counter at room temperature. In the system described, the chance coincidence rate contributed around 10% of the total background. The 9635QB also has a high efficiency photocathode (25% quantum efficiency at 3800 A), which is desirable since the light pulses that the X-rays produce in the scintillator are small in amplitude. As mentioned previously, the optimum overall efficiency obtained for this particular system was 6.1% (weightless fwhm for the 835 keV 54Mn peak). Since the efficiency of the NaI(TI) crystal for this geometry was found to be almost exactly 11%, the liquid scintillation counter had a 55% efficiency for the Cr X - r a y s - Auger electrons. This efficiency is
10-20 times greater than that of small X-ray counters, and since there is no self-absorption correction, the efficiency increase can be even larger for large samples. This efficiency was stable over a period of eight months during which the counter was operated. The counter's electronic system is shown diagrammatically in fig. 2. The NaI(Tl) crystal was operated at 1050 V. A preamplifier was used with the particular crystal since it seemed to improve its resolution. The full width at half maximum of the 54Mn photopeak was 9.0%. The liquid scintillation counter in this particular system was operated at about 1200 V. The voltage is of course dependent on the individual tube and the amplifier gain setting. The single channel analyzer was run with a window found experimentally to cover the energy range of 0.5-14MeV. This window seems rather wide, but is to be expected in part due to the poor resolution of the liquid scintillation system at such low energies. If the upper-level discriminator is lowered the efficiency falls off rather slowly, whereas it falls off quickly as the lower-level discriminator is raised, showing that the bulk of the pulses come at low photoelectron numbers. Fig. 3 shows the response spectrum produced by the liquid scintillation counter for 54Mn when operated in coincidence with the NaI(T1) crystal. Leaving the SCA with as wide a window as possible was found to give the best efficiency to background ratio. Using the SCA with the lower-level discriminator only, however, caused an enormous increase in background (factor of 10) with only a small increase in efficiency (factor of 1.2). The coincidence module used was a Canberra Model 1446 which is of the leading edge overlap type with a i
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,
i
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i
i
3O0
2OO
o
~00
lO0
200
300
CHANNEL, FULL SCALE :
400 [0
500
VOLTS
Fig. 3. CrK X-ray response spectrum produced in liquid scintillation counter by ~4Mn.
LIQUID SCINTILLATION-Na[(T1)
variable resolving time from 0.5-3.0 #sec. A resolving time of this order is necessary, again to avoid the chance-coincidence problem. The analyzer was an N D 1100, and the single-channel analyzer and amplifiers were also Nuclear Data modules.
3. Background The best background during development of the counter was 0.025 cpm (fwhm for the 835 keV 54Mn peak). The background was also quite stable during the eight months of actual operation. As pointed out previously, approximately 10% of this background was due to chance coincidences due to P.M. tube noise. This percentage was determined by measuring the P.M. tube noise rate and then applying this count rate to the circuit with a pulser. The P.M. tube itself could not be used because of a more significant effect, namely the sensitivity of the P.M. tubes themselves to cosmic rays. This effect has been reported previously and is described by Schram2). It is difficult to eliminate using an anticoincidence counter in the usual ring configuration due to the geometry of the two P.M. tubes facing one another. As a result of this sensitivity, the background of the counter without the liquid scintillation cell in the system was found to be 0.013 cpm or about 60% of the total background with the liquid scintillation cell included. The remainder of the background is presumably due to events in the liquid scintillator itself. As many of the standard practices of low level counting as possible, considering limitations imposed by cost and existing facilities, were utilized in order to minimize the background. The counter was tested in both a very clean lead shield (4" St. Joe Lead) and in a clean iron shield (6" Fe). The counter was found to have a slightly lower background in the 54Mn photopeak region in the Fe shield, 0.033 instead of 0.039. This reduction is presumably due to a reduction in the amount of cosmic-ray produced bremsstrahlung in the lowm Z shield material. Addition of a Geiger guard ring ultimately reduced the background to 0.025 cpm under the 54Mn peak. A NaI(TI) or plastic scintillation anticoincidence mantle would probably have been superior to the Geiger ring, but a suitable scintillation device was not available during development of the counter. It is hoped that one may be acquired in the near future for testing. The NaI(T1) of the crystal contained less than 1 p p m K, also in order to reduce the background, and the liquid scintillation vial was made of quartz for the same reason. The body of the counter was constructed of PVC (poly vinyl chloride) which had been previously checked for g a m m a contamination and the reflector was built of shim steel which
COINCIDENCE
DETECTOR
351
had been checked for fl contamination. AI foil was also tried as a reflector, but since it gave, in this case, no noticeable increase in sensitivity, it was rejected because of a slight increase in the background as well as its inferior mechanical qualities. The background of the NaI(T1) crystal in the iron shield with the Geiger guard ring but without the liquid scintillation coincidence was 1.98 cpm under the 54Mn peak so that it can be seen that a background reduction of around 100 is achieved with an efficiency reduction of less than a factor of 2. Small peaks of the radium daughters 214pb and z 14Bi are present in the background. Since these peaks lie below the 54Mn energy and cause no difficulty, their source was not isolated. A typical coincidence background is shown in fig. 4. The 51Cr internal standard was always included in background measurements to make sure that any contaminants it might contain did not contribute to the 54Mn peak. The background is fairly flat and featureless above 650 keV. Table 1 shows a comparison of characteristics of this counter with other methods for counting 54Mn samples.
4. Quenching One of the most troublesome problems in a liquid scintillation system is the scintillation quenching effect exerted by dissolved 02 as well as other dissolved impurities. In order to eliminate atmospheric 02, a stream of pure dry N2 was bubbled through the dissolved sample for 10 rain prior to counting. In the case of 54Mn a check for further quenching effects was possible by dissolving a known amount of 51Cr along with the sample or standard. 51Cr w a s chosen because the 4.9 keV X-ray is close to the 54Mn X-ray energy, it ] i i i i l i , l l l , t l r
Cr 5t 300
200
~÷
I00
Bi 214
Mn54
K
ooJ O0
I
50
~
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/00
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~50
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r
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Fig. 4. Background of liquid scintillation-Nal(T1) counter.
i
?~0
352
J.S. F R U C H T E R TABLE 1
C o m p a r i s o n o f liquid scintillation NaI(TI) coincidence detector with other m e t h o d s o f counting 54Mn in a 15 g lunar sample (30 m g Mn).
Detector
Radiation detected (mode)
Efficiency
Self-absorption correction
Efficiency corrected for selfabsorption
Background (cpm)
S2/B (relative)
Signal/ background
Specificity
Interferences
S/B (relative)
LS-NaI(3") Nal(3") only Gas-proportional (small) Gas-proportional Gas-proportionalN a l (3") Ge(Li) 40 cm '3
X ?~ 7e X
6.3% 12% 4.3%
None None 0.60
6.3% 12.0% 2.6%
0.025 2.0 0.010
1.00 0.045 0.42
1.00 0.024 1.05
Good Good Fair
X
4.5%
0.90
4.1%
0.041
0.26
0.40
Fair
X- T ~
0.6% 0.4%
0.60 None
0.4% 0.4%
0.002 0.020
0.050 0.0050
0.80 0.08
Good Excellent
is a pure K-capture isotope and the 320 keV gamma falls below the 54Mn gamma. It is somewhat inconvenient because of its short half-life (28 d) and because the gamma is only a 9% branch. Of course, if one wished to count a low energy gamma such as 57Co, it would be necessary to use a calibration source external to the liquid scintillation vial so that it could be removed during the actual counting of the sample. 57Co was itself not chosen as the internal standard because of the presence of a 14 keV gamma ray in addition to the X-ray in coincidence with the main gamma ray. It was found in the case of two samples that quenching did occur, at least in the case of 51Cr" If quenching occurs which cannot be remedied by further degassing with N2, then it is necessary to recover the sample as described below and start over again.
The basic procedure for recovering an inorganic salt from the organic scintillation liquid is wet washing with HNO3. It is advisable at least in the case of the Aquasol mixture, to evaporate the liquid first. This evaporation must be done carefully in a large container because of the fact that when the xylene naphthalenewater emulsion is heated, separation into aqueous and organic phases occurs. Since the liquid phase with the lower boiling point (aqueous) is on the bottom, there will inevitably be some splattering as evaporation occurs. If, however, the process is carried out in a sufficiently large container, then none of the sample will be lost. After the sample is dry, it is repeatedly
None None
evaporated with conc. HNO3 in a covered beaker until the liquid is colorless or almost colorless.
6. Standardization and errors Two independent standard 5¢Mn sources, one obtained from NBS and one from New England Nuclear, were used to determine the counter's efficiency. The standards were first cross checked on a 40 cm 3 Ge(Li) detector. The errors quoted for the measurement are 1 a for the total count rate plus the background. An additional 5% error for standardization and 5% error for the chemical yield determination were added quadratically. Thus if S is the total signal, N is the net signal, B is the background so that S = N + B , C is the error of the chemical y i e l d = 5 % and T is the standardization error= 5% then
EN(% ) : 5. Recovery of the sample
None None 53Mn t+ = 3.7My 5aMn t~. = 3.7 My
IO0
(S2+B2+C2+T2)½ : N
= 100 (S2 -~"B2 + 50)½ N and the error quoted = (E)(N)/IO0.
7. Samples The samples OP-1 through OP-6 for which the results below are quoted are samples of increasing depth below the surface taken from lunar rock 12002 collected during the Apollo 12 mission. For details of the sampling procedure and chemical processing prior to the counting of the 54Mn, the reader is referred to Finkel et al.3).
353
LIQUID SCINTILLATION-NaI(T1) COINCIDENCE DETECTOR TABLE 2 Details of X-y counting of two lunar 54Mn samples in the liquid scintillation-NaI(Tl)coincidence detector. Sample
Gross counts
Time
Background
Net count rate
622 382
16049' 8461"
0.0255 0.0254
0.0132 0.0197
OP 2 OP-6
TABLE 3 Summary of results of a4Mn (t½ = 303 d) determinations in lunar rock 12002. Total 5 4 M n (dpm/kg)
Sample
OP-I OP-2 OP-3 OP-4 OP-5 OP-6
Calculated galactic contribution (dpm/kg)
984- 17 775:12 534- 11 504-9 395:10 31 5:5
6.3% 6.3%
775:19 555:14 31 5:19 285:13 165:13 35:6
, '
,
'
1
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,
i
I
i
i
i
0.00550 0.0218
774- 12 31 5:5
0.496 0.456
This counter is in principle applicable to all nuclides which emit X-rays and g a m m a - r a y s in coincidence. The 5lCr used as an internal standard during the 54Mn counts is an example of a n o t h e r such nuclide. A requirement in the case of samples which c a n n o t be measured carrier free is that a colorless (or at least faintly colored) salt of the element of which the nuclide is an isotope be available. A n example of a nuclide for which this problem was encountered was 56Co. All c o m m o n salts of Co are rather highly colored in aqueous solution, so that it was f o u n d that when 10 mg of Co as Co(NO3) 2 was solubilized in the liquid scintillator, the efficiency was reduced to 88% of the weightless efficiency. The counter can, of course, also be use] to c o u n t nuclides which emit fi particles in coincidence with 7 rays. For this application, however, a coincidence
The spectrum o b t a i n e d from the c o u n t i n g of the OP-1 M n sample is shown in fig. 5. A s t a n d a r d 5 4 M n + 5 1 C r solution run in the c o u n t e r is shown in fig. 6 for comparison. Details of the actual measurernents of S4Mn in L u n a r Rock 12002 samples OP-2 i
Final dpm/kg
9. Applicability to nuclides other than S4Mn
8. Results
,
kg sample counted
and OP-6 made using this c o u n t e r are shown in table 2. The final results for all six samples are summarized in table 3.
Net solar produced 54Mn (dpm/kg)
21 4- 6 225:6 225:6 225:6 235:6 285:6
Efficiency Decay (measured factor by internal 51Cr)
I
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~
-7
.......
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,
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' 7
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F
i
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200
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0%
I
250
CHANNEL
Fig. 5. Spectrum of OP-1 Mn sample in liquid scintillationNaI(TI) coincidence counter,
0
I
I
i l 5O
I
I
I
] I00
I
~'N. J I 150 CHANNEL
I
r
I
J 200
i
i
250
Fig. 6. 54Mn gamma spectrum in liquid scintillation-Nal(Tl) coincidence counter.
354
J.S. FRUCHTER
counter gated by a gas proportional or geiger counter is superior if high sensitivity is desired, because such a counter achieves comparable efficiencies with a much lower background.
for help with the electronics. The work was supported by NASA Contract NAS 9-7891 and NASA Grant NGL-05-009-148. References
I would like to express my gratitude to Prof. J. R. Arnold for extending me this opportunity to undertake this work and also for providing valuable guidance. J. Evans also contributed several valuable suggestions. ! thank N. Fong, J. Hollon and N. Powers for assistance in construction of the counter and D. Sullivan
1) N. G. Bhandari, Nucl. Instr. and Meth. 67 (1969) 251. 2) E. Schram, Organic scintillation detectors (Elsevier Publ. Co., New York, 1963). 3) R.C. Finkel, J . R . Arnold, M. lmamura, R.C. Reedy, J.S. Fruchter, H. H. Loosli, J. C. Evans, A. C. Delany and and J. P. Shedlovsky, Proc. 2nd Lunar science Conf. (Houston, 1971) in press.
INSTRUMENTS
AND
METHODS
99
(I972)
349-354;
©
NORTH-HOLLAND
PUBLISHING
CO.
D E V E L O P M E N T OF A HIGH SENSITIVITY LIQUID SCINTILLATION-NaI(TI) COINCIDENCE DETECTOR FOR X-? E M I T T E R S I N L U N A R R O C K S J.S. FRUCHTER*
University of California, San Diego, La Jolla, California 92037, U.S.A. Received 8 N o v e m b e r 1971 A new type o f high sensitivity counter e m p l o y i n g a liquid scintillation cell as a coincidence gate for a 3" NaI(T1) well crystal has been developed for the detection o f small a m o u n t s of X - 7 emitting nuclides. This c o u n t e r has approximately 10 times the efficiency o f a c o m p a r a b l e detector e m p l o y i n g a conventional gas proportional c o u n t e r as the coincidence gate. T h e b a c k g r o u n d
o f the existing system u n d e r the full width at half m a x i m u m o f the 54Mn p h o t o p e a k is 0 . 0 2 c p m . M e a s u r e m e n t s to the 15% precision level of as little as 0.25 d p m o f 54Mn in l u n a r rock samples are reported. T h e applicability o f the counter to X - t, emitters other t h a n 54Mn is discussed.
1. Introduction
2. Counter design and operation
X - ? coincidence counters have been used successfully by a number of investigators in order to detect very small amounts of those radionuclides which decay by electron capture to an excited state [cf., Bhandaril)]. An example of such a nuclide is the 303 d 54Mn which emits an 834 keV g a m m a in coincidence with 5.41 keV C.r K X-ray. Measurements of this nuclide in lunar rocks were desired as a monitor for solar proton emission during time scales of the order of 1-2 y, corresponding to the mean life of 5+Mn. Calculations showed that ordinary X-7 counters using a conventional gas X-ray counter as the coincidence gate would not be suitable for this application, because although they achieve very low backgrounds, they do so only by sacrificing a great deal of efficiency. This efficiency loss is, due to the low efficiency of small gas counters for X-rays coupled with the loss of Auger electrons in solid samples. These combined factors generally yield in the order of a 3-5% efficiency for the emitted X-rays. Coupled with the 11% efficiency of a 3" Na[(TI) well detector for the 834 keV photon, the instrument has a coincidence efficiency of only 0.6%. Because of the rather small absolute activity present in our lunar samples ( < 0.5 dpm) at the time of counting, a detector with such low efficiency could not produce a measurement of acceptable statistical precision in a reasonable length of time. To alleviate this problem, a new type of counter, described below, was developed.
The counter employed a liquid scintillation cell coupled to a 2" photomultiplier tube as a coincidence gate for a 3" NaI(TI) well clystal. The 8 ml quartz liquid scintillation vial was placed in the well and the entire assembly was coupled together in a light-tight container (see fig. 1). The counter was then placed inside an iron shield surrounded by a Geiger ring. In this configuration, the counter had a weightless efficiency of 6.1% (fwhm) and a working background of 0.025 epm. The sample was dissolved as MnCI2 in a commercially available xylene-napthalene based solvent*. As much as 50 mg of Mn could be solubilized in the 8 ml solution before an appreciable loss of efficiency occurred. (At 50 mg the efficiency was still 87% of the weightless
* Present address: Center for Volcanology, Oregon, Eugene, O r e g o n 97403, U.S.A.
University
t Marketed u n d e r the trade n a m e A q u a s o l by New E n g l a n d Nuclear Corp.
~
C O N E T R A I
of Fig. 1. Liquid scintillation-NaI(Tl) coincidence counter.
349
350
J.S. FRUCHTER IAquid Scintillation Counter PM
Ceil
NaI [ Ill
]
Preamp
]
RC-Shaping Ampiifier ]
Single Channel Analyzer
Spectroscopy [ Arnplifie r [ and Logic Pulse Shaper ! I Logic 1 Pulse
Linear Pulse Li~ ea r
F Fast Coincidence > | L e a d i n g Edge Overlap el. 5 ~.sec Resolving Tithe r Geiger ~ Guard Ring I
Slo\~ Anticoincidence I 200 j s e c Blanking Time Coincidence Pulse Multic hannel Analyze r
<
Fig. 2. Electronics and logic for liquid scintillation-Nal(TI) coincidence counter.
efficiency. When 60 mg of Mn was solubilized the efficiency fell to 61% of the weightless efficiency.) The Photomultiplier tube used was an E M ! 9635QB. It is necessary to use a low noise tube of this nature in order to keep chance coincidences from contributing appreciably to the counter background. Since many of the X-ray events produce ultimately only one electron at the photocathode, it is necessary either to set the electronic discriminators quite low or the gain quite high in order to avoid a serious loss of efficiency. The problem is compounded by the fact that it is desirable to operate the counter at room temperature. In the system described, the chance coincidence rate contributed around 10% of the total background. The 9635QB also has a high efficiency photocathode (25% quantum efficiency at 3800 A), which is desirable since the light pulses that the X-rays produce in the scintillator are small in amplitude. As mentioned previously, the optimum overall efficiency obtained for this particular system was 6.1% (weightless fwhm for the 835 keV 54Mn peak). Since the efficiency of the NaI(TI) crystal for this geometry was found to be almost exactly 11%, the liquid scintillation counter had a 55% efficiency for the Cr X - r a y s - Auger electrons. This efficiency is
10-20 times greater than that of small X-ray counters, and since there is no self-absorption correction, the efficiency increase can be even larger for large samples. This efficiency was stable over a period of eight months during which the counter was operated. The counter's electronic system is shown diagrammatically in fig. 2. The NaI(Tl) crystal was operated at 1050 V. A preamplifier was used with the particular crystal since it seemed to improve its resolution. The full width at half maximum of the 54Mn photopeak was 9.0%. The liquid scintillation counter in this particular system was operated at about 1200 V. The voltage is of course dependent on the individual tube and the amplifier gain setting. The single channel analyzer was run with a window found experimentally to cover the energy range of 0.5-14MeV. This window seems rather wide, but is to be expected in part due to the poor resolution of the liquid scintillation system at such low energies. If the upper-level discriminator is lowered the efficiency falls off rather slowly, whereas it falls off quickly as the lower-level discriminator is raised, showing that the bulk of the pulses come at low photoelectron numbers. Fig. 3 shows the response spectrum produced by the liquid scintillation counter for 54Mn when operated in coincidence with the NaI(T1) crystal. Leaving the SCA with as wide a window as possible was found to give the best efficiency to background ratio. Using the SCA with the lower-level discriminator only, however, caused an enormous increase in background (factor of 10) with only a small increase in efficiency (factor of 1.2). The coincidence module used was a Canberra Model 1446 which is of the leading edge overlap type with a i
~
J
t
I
J
i
i
J
I
J
r
,
i
I
J
J
J
i
I
I
r
i
i
3O0
2OO
o
~00
lO0
200
300
CHANNEL, FULL SCALE :
400 [0
500
VOLTS
Fig. 3. CrK X-ray response spectrum produced in liquid scintillation counter by ~4Mn.
LIQUID SCINTILLATION-Na[(T1)
variable resolving time from 0.5-3.0 #sec. A resolving time of this order is necessary, again to avoid the chance-coincidence problem. The analyzer was an N D 1100, and the single-channel analyzer and amplifiers were also Nuclear Data modules.
3. Background The best background during development of the counter was 0.025 cpm (fwhm for the 835 keV 54Mn peak). The background was also quite stable during the eight months of actual operation. As pointed out previously, approximately 10% of this background was due to chance coincidences due to P.M. tube noise. This percentage was determined by measuring the P.M. tube noise rate and then applying this count rate to the circuit with a pulser. The P.M. tube itself could not be used because of a more significant effect, namely the sensitivity of the P.M. tubes themselves to cosmic rays. This effect has been reported previously and is described by Schram2). It is difficult to eliminate using an anticoincidence counter in the usual ring configuration due to the geometry of the two P.M. tubes facing one another. As a result of this sensitivity, the background of the counter without the liquid scintillation cell in the system was found to be 0.013 cpm or about 60% of the total background with the liquid scintillation cell included. The remainder of the background is presumably due to events in the liquid scintillator itself. As many of the standard practices of low level counting as possible, considering limitations imposed by cost and existing facilities, were utilized in order to minimize the background. The counter was tested in both a very clean lead shield (4" St. Joe Lead) and in a clean iron shield (6" Fe). The counter was found to have a slightly lower background in the 54Mn photopeak region in the Fe shield, 0.033 instead of 0.039. This reduction is presumably due to a reduction in the amount of cosmic-ray produced bremsstrahlung in the lowm Z shield material. Addition of a Geiger guard ring ultimately reduced the background to 0.025 cpm under the 54Mn peak. A NaI(TI) or plastic scintillation anticoincidence mantle would probably have been superior to the Geiger ring, but a suitable scintillation device was not available during development of the counter. It is hoped that one may be acquired in the near future for testing. The NaI(T1) of the crystal contained less than 1 p p m K, also in order to reduce the background, and the liquid scintillation vial was made of quartz for the same reason. The body of the counter was constructed of PVC (poly vinyl chloride) which had been previously checked for g a m m a contamination and the reflector was built of shim steel which
COINCIDENCE
DETECTOR
351
had been checked for fl contamination. AI foil was also tried as a reflector, but since it gave, in this case, no noticeable increase in sensitivity, it was rejected because of a slight increase in the background as well as its inferior mechanical qualities. The background of the NaI(T1) crystal in the iron shield with the Geiger guard ring but without the liquid scintillation coincidence was 1.98 cpm under the 54Mn peak so that it can be seen that a background reduction of around 100 is achieved with an efficiency reduction of less than a factor of 2. Small peaks of the radium daughters 214pb and z 14Bi are present in the background. Since these peaks lie below the 54Mn energy and cause no difficulty, their source was not isolated. A typical coincidence background is shown in fig. 4. The 51Cr internal standard was always included in background measurements to make sure that any contaminants it might contain did not contribute to the 54Mn peak. The background is fairly flat and featureless above 650 keV. Table 1 shows a comparison of characteristics of this counter with other methods for counting 54Mn samples.
4. Quenching One of the most troublesome problems in a liquid scintillation system is the scintillation quenching effect exerted by dissolved 02 as well as other dissolved impurities. In order to eliminate atmospheric 02, a stream of pure dry N2 was bubbled through the dissolved sample for 10 rain prior to counting. In the case of 54Mn a check for further quenching effects was possible by dissolving a known amount of 51Cr along with the sample or standard. 51Cr w a s chosen because the 4.9 keV X-ray is close to the 54Mn X-ray energy, it ] i i i i l i , l l l , t l r
Cr 5t 300
200
~÷
I00
Bi 214
Mn54
K
ooJ O0
I
50
~
I
i
J
I
J
J
i
/00
t
I
~50
t
I
I
l
I
r
t
i
200
CHANNEL
Fig. 4. Background of liquid scintillation-Nal(T1) counter.
i
?~0
352
J.S. F R U C H T E R TABLE 1
C o m p a r i s o n o f liquid scintillation NaI(TI) coincidence detector with other m e t h o d s o f counting 54Mn in a 15 g lunar sample (30 m g Mn).
Detector
Radiation detected (mode)
Efficiency
Self-absorption correction
Efficiency corrected for selfabsorption
Background (cpm)
S2/B (relative)
Signal/ background
Specificity
Interferences
S/B (relative)
LS-NaI(3") Nal(3") only Gas-proportional (small) Gas-proportional Gas-proportionalN a l (3") Ge(Li) 40 cm '3
X ?~ 7e X
6.3% 12% 4.3%
None None 0.60
6.3% 12.0% 2.6%
0.025 2.0 0.010
1.00 0.045 0.42
1.00 0.024 1.05
Good Good Fair
X
4.5%
0.90
4.1%
0.041
0.26
0.40
Fair
X- T ~
0.6% 0.4%
0.60 None
0.4% 0.4%
0.002 0.020
0.050 0.0050
0.80 0.08
Good Excellent
is a pure K-capture isotope and the 320 keV gamma falls below the 54Mn gamma. It is somewhat inconvenient because of its short half-life (28 d) and because the gamma is only a 9% branch. Of course, if one wished to count a low energy gamma such as 57Co, it would be necessary to use a calibration source external to the liquid scintillation vial so that it could be removed during the actual counting of the sample. 57Co was itself not chosen as the internal standard because of the presence of a 14 keV gamma ray in addition to the X-ray in coincidence with the main gamma ray. It was found in the case of two samples that quenching did occur, at least in the case of 51Cr" If quenching occurs which cannot be remedied by further degassing with N2, then it is necessary to recover the sample as described below and start over again.
The basic procedure for recovering an inorganic salt from the organic scintillation liquid is wet washing with HNO3. It is advisable at least in the case of the Aquasol mixture, to evaporate the liquid first. This evaporation must be done carefully in a large container because of the fact that when the xylene naphthalenewater emulsion is heated, separation into aqueous and organic phases occurs. Since the liquid phase with the lower boiling point (aqueous) is on the bottom, there will inevitably be some splattering as evaporation occurs. If, however, the process is carried out in a sufficiently large container, then none of the sample will be lost. After the sample is dry, it is repeatedly
None None
evaporated with conc. HNO3 in a covered beaker until the liquid is colorless or almost colorless.
6. Standardization and errors Two independent standard 5¢Mn sources, one obtained from NBS and one from New England Nuclear, were used to determine the counter's efficiency. The standards were first cross checked on a 40 cm 3 Ge(Li) detector. The errors quoted for the measurement are 1 a for the total count rate plus the background. An additional 5% error for standardization and 5% error for the chemical yield determination were added quadratically. Thus if S is the total signal, N is the net signal, B is the background so that S = N + B , C is the error of the chemical y i e l d = 5 % and T is the standardization error= 5% then
EN(% ) : 5. Recovery of the sample
None None 53Mn t+ = 3.7My 5aMn t~. = 3.7 My
IO0
(S2+B2+C2+T2)½ : N
= 100 (S2 -~"B2 + 50)½ N and the error quoted = (E)(N)/IO0.
7. Samples The samples OP-1 through OP-6 for which the results below are quoted are samples of increasing depth below the surface taken from lunar rock 12002 collected during the Apollo 12 mission. For details of the sampling procedure and chemical processing prior to the counting of the 54Mn, the reader is referred to Finkel et al.3).
353
LIQUID SCINTILLATION-NaI(T1) COINCIDENCE DETECTOR TABLE 2 Details of X-y counting of two lunar 54Mn samples in the liquid scintillation-NaI(Tl)coincidence detector. Sample
Gross counts
Time
Background
Net count rate
622 382
16049' 8461"
0.0255 0.0254
0.0132 0.0197
OP 2 OP-6
TABLE 3 Summary of results of a4Mn (t½ = 303 d) determinations in lunar rock 12002. Total 5 4 M n (dpm/kg)
Sample
OP-I OP-2 OP-3 OP-4 OP-5 OP-6
Calculated galactic contribution (dpm/kg)
984- 17 775:12 534- 11 504-9 395:10 31 5:5
6.3% 6.3%
775:19 555:14 31 5:19 285:13 165:13 35:6
, '
,
'
1
'
~
,
i
I
i
i
i
0.00550 0.0218
774- 12 31 5:5
0.496 0.456
This counter is in principle applicable to all nuclides which emit X-rays and g a m m a - r a y s in coincidence. The 5lCr used as an internal standard during the 54Mn counts is an example of a n o t h e r such nuclide. A requirement in the case of samples which c a n n o t be measured carrier free is that a colorless (or at least faintly colored) salt of the element of which the nuclide is an isotope be available. A n example of a nuclide for which this problem was encountered was 56Co. All c o m m o n salts of Co are rather highly colored in aqueous solution, so that it was f o u n d that when 10 mg of Co as Co(NO3) 2 was solubilized in the liquid scintillator, the efficiency was reduced to 88% of the weightless efficiency. The counter can, of course, also be use] to c o u n t nuclides which emit fi particles in coincidence with 7 rays. For this application, however, a coincidence
The spectrum o b t a i n e d from the c o u n t i n g of the OP-1 M n sample is shown in fig. 5. A s t a n d a r d 5 4 M n + 5 1 C r solution run in the c o u n t e r is shown in fig. 6 for comparison. Details of the actual measurernents of S4Mn in L u n a r Rock 12002 samples OP-2 i
Final dpm/kg
9. Applicability to nuclides other than S4Mn
8. Results
,
kg sample counted
and OP-6 made using this c o u n t e r are shown in table 2. The final results for all six samples are summarized in table 3.
Net solar produced 54Mn (dpm/kg)
21 4- 6 225:6 225:6 225:6 235:6 285:6
Efficiency Decay (measured factor by internal 51Cr)
I
,
~
-7
.......
•
~
7 " - -
,
~
i
i
i
Cr 51
' 7
i
i
~ n 54
i
Cr 51
~_
300
-
200
--
53O
== S
I
\
Bi 214 I00
ic0 I
c0! 0.0
I
I
1
50
I
i
t
L ~
I
i
I00
I
i
150
F
i
i
I
i
200
I _
~
I
0%
I
250
CHANNEL
Fig. 5. Spectrum of OP-1 Mn sample in liquid scintillationNaI(TI) coincidence counter,
0
I
I
i l 5O
I
I
I
] I00
I
~'N. J I 150 CHANNEL
I
r
I
J 200
i
i
250
Fig. 6. 54Mn gamma spectrum in liquid scintillation-Nal(Tl) coincidence counter.
354
J.S. FRUCHTER
counter gated by a gas proportional or geiger counter is superior if high sensitivity is desired, because such a counter achieves comparable efficiencies with a much lower background.
for help with the electronics. The work was supported by NASA Contract NAS 9-7891 and NASA Grant NGL-05-009-148. References
I would like to express my gratitude to Prof. J. R. Arnold for extending me this opportunity to undertake this work and also for providing valuable guidance. J. Evans also contributed several valuable suggestions. ! thank N. Fong, J. Hollon and N. Powers for assistance in construction of the counter and D. Sullivan
1) N. G. Bhandari, Nucl. Instr. and Meth. 67 (1969) 251. 2) E. Schram, Organic scintillation detectors (Elsevier Publ. Co., New York, 1963). 3) R.C. Finkel, J . R . Arnold, M. lmamura, R.C. Reedy, J.S. Fruchter, H. H. Loosli, J. C. Evans, A. C. Delany and and J. P. Shedlovsky, Proc. 2nd Lunar science Conf. (Houston, 1971) in press.