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Calibration source effects in Si(Li) detector efficiency determinations
NUCLEAR
INSTRUMENTS
AND
METHODS
136 (I976) 347-348;
©
NORTH-HOLLAND
PUBLISHING
CO.
C A L I B R A T I O N S O U R C E E F F E C T S I N Si(Li) D E T E C T O R EFFICIENCY DETERMINATIONS* S A M J. C I P O L L A a n d M A R Y J. H E W I T T
Department of Physics, Creighton University, Omaha, Nebraska 68178, U.S.A. Received 2 F e b r u a r y 1976 a n d in revised f o r m 12 M a y 1976 Efficiency determinations o f Si(Li) detectors for low-energy characteristic X-rays (3-10 keV) using a physical five-parameter model have pointed to possible self-absorption effects in the usual drop-evaporated calibration sources. To further test this hypothesis, u n i f o r m a n d thin calibration sources prepared by a surface ion-implantation technique were used to re-determine the efficiency response o f a Si(Li) detector.
It has been found that the usual evaporated drop source t) for use in low-energy efficiency calibrations of Si(Li) detectors can cause errors in the results due to source self-absorption, which is difficult, if not impossible, to correctZ). U n i f o r m and thin source deposits are therefore needed in this kind o f work. There are several ways to produce such sources, including v a c u u m evaporation, electron sputtering in vacuum, electroplating, and direct surface ionimplantation, which was the method chosen here. Ref. 2 describes in detail the experimental procedures used and the analytical method employed to determine the detection efficiency o f a Si(Li) detector. The model equation can be written as,
collimated Si(Li) detector was performed with a set o f " t h i c k " drop-evaporated sources, and then repeated with a set o f " t h i n " ion-implanted sources. The sources w e r e 2 4 1 A m , 51Cr, 57Co, 137Cs, and 6 5 Z n . In the " t h i n " - s o u r c e set, the Z41Am source was .._ " . . . . . . . . . . . . . =. . . . . . . . . . _d=, .~-~._- . .~_~&-,~,,,, ....
b
Thin
×
e = f2 exp(~E ~) [1
-
-
exp(yE~)],
±
(1)
with the five parameters (~,/~, 7, 5, I2) to be specified from the fit of the measured efficiency e vs the p h o t o n energy E. A subsequent modification of the fitting procedure is the inclusion o f various restricted-fit options (setting/~ = constant, or setting f2 = constant). In ref. 2, the high-energy response turned out as expected, but the low-energy response seemed anomalous. Of the several hypotheses put forth to explain this behavior, the most plausible one seemed to be source self-absorption in the non-uniform and relatively thick drop sources. A n additional problem was the lack o f a sufficient n u m b e r of low-energy data points to adequately describe the functional dependence in the low-energy region. This is a critical situation since X-ray spectroscopy o f low-Z elements often requires a knowledge of the Si(Li) efficiency response if the superior energy resolution of this detector is to be exploited. This note concentrates on the problem o f source self-absorption. A n efficiency calibration of a well* W o r k supported in part by the Research Corporation, U.S.A.
ill
Thick
Th n_e,i (,~=-286)
OI 1.5
2
2 5 3
4
5
6 7 8 9 I0
15
20
25 5 0
40
50 6070
Energy (KEY)
Fig. 1. Efficiency response o f the Si(Li) detector using " t h i c k " a n d " t h i n " calibration sources o f 241Am, 137Cs, 51Cr, S7Co, and 6SZn. Corrections were m a d e for Mylar covers on the " t h i c k " sources, and for source misalignment, as in ref. 2. T h e results o f the model fit are s h o w n in the curves d r a w n t h r o u g h the data, a n d in table 1. The dotted curve is the fl-restricted fit to the " t h i n " - s o u r c e data, with fl = - 2 . 8 6 . Note the attenuation o f the low-energy points in the " t h i c k " - s o u r c e curve. Error bars are omitted for s o m e low-energy points for clarity, and A denotes the " t h i c k " source data, while (3 denotes the " t h i n " source data.
348
S. J. C I P O L L A
AND
M. J. H E W I T T
Table 1 P a r a m e t e r s o f the fits to the efficiency data.
Parameter
"Thick" sources
"Thin" sources Free fit
Q ,8
Z~
( 5 . 1 0 ± 0 . 8 9 ) × 10 - 4 - 4.584-2.32 - 1.214-0.50 - ( 3 . 0 2 4 - 2 . 7 5 ) x 104 - 3.084-0.22 0.33
electroplated onto platinum instead of being ionimplanted, as it was purchased from stock through Monsanto Research Corporation. The photon energies and emissions per decay from the sources were mostly those adopted by Campbell and McNelles3), with the calibration data for 51Cr taken from Gehrke and Lokken4), and the 137Cs L X-ray data taken from ref. 2. The activities of the sources ranged from less than a microCurie to about 3 #Ci, except for the "thick" /41Am source, which had an activity of about 13 #Ci and did pose some problems with interfering americium X-rays due to source self-fluorescence. Fig. 1 and table 1 show the results of the fits of eq. (1) to the "thick"- and "thin"-source data. In the " thick "-source fit, the ' 37Cs data had to be normalized to the rest of the "thick" data, apparently due to misalignment of the source during the measurements. (See ref. 2 for the normalization procedure.) The high-energy parameter of the "thick"-source fit, 6=-3.08_0.22, is in excellent agreement with expectations; but, as anticipated, the low-energy parameter,/3 = - 1.21 _+0.50, appears unrealistic. The fit of the "thin"-source data to eq. (1) required no normalization. Again, the high-energy fit is excellent ( 6 = - 3 . 1 3 _ + 0 . 1 4 ) , but the low-energy fit is disappointingly unrealistic, in that / ~ = - 0 . 8 7 _ + 0 . 5 0 is well off the expected value of /3 = - 2 . 8 6 for a stacked 0.5 rail. beryllium and 0.5 rail. Mylar window combination (ignoring the dead-silicon and gold layers on the front of the detector). As mentioned before, the paucity of data in the low-energy ( < 5 keV) region, even for nearly ideal sources, will make the low-energy efficiency determination inaccurate. To test this hypothesis further, a restricted fit was also made to the "thin"-source data, in which the low-energy parameter was fixed at /3 = - 2 . 8 6 . The results of this fit are also shown in fig. 1. Statistically, the restricted fit is quite acceptable as seen from the reduced chi-squared value of Z~ = 1.31. Thus, it is
(5.87 =k 1.18) × 10 - 4 - 2.014-0.86 - 0.874-0.50 - ( 3 . 3 3 4 - 1 . 7 8 ) x 104 - 3.13-4-0.14 0.54
,8-restricted fit
(4.544-0.09) × 10 - 4 - 21.14-2.9 - 2.86 - ( 5 . 6 0 4 - 3 . 1 7 ) x 104 - 3.204-0.15 1.31
highly probable that the " thin "-source data are consistent with the model expectations, whereas before this behavior was masked by self-absorption effects in the "thick" sources. It is readily seen in fig. 1 that the low-energy " thick "-source data are attenuated. (Incidentally, a /Lrestricted fit on the "thick"-source data, similar to the one performed on the "thin"source data, yielded unacceptable 7~r z values, as well as incorrect estimates of the solid angle parameter f2; it clearly did not represent the efficiency measurement using the drop evaporated sources.) Besides demonstrating the need for better source preparation procedures, this note also reiterates the need to develop more calibration sources to fill in the low-energy range for efficiency determinations of Si(Li) detectors. Only if more calibrated low-energy sources are developed can the model be fully tested to determine if other effects, such as incomplete charge collection in the detector, or absorption in the detector gold layer, will necessitate significant changes in the model. The authors are very grateful for the assistance of J. L. Lerner and L. Glendenon of Argonne National Laboratory for preparation and calibration of the ion-implanted sources, and to J. L. Duggan of North Texas State University and the staff of the Oak Ridge Associated Universities for preparation and calibration of the drop-evaporated sources. A. Bennie is thanked for her assistance in the data analysis. References ') W. C. P a r k e r , H. Slatis, F. S. G o u l d i n g a n d R. A. A l l e n , in Alpha-, beta- and gamma-ray spectroscopy (ed. K. S i e g b a h n ; N o r t h - H o l l a n d Publ. Co., A m s t e r d a m , 1965) ch. 7. 2) W . J . G a l l a g h e r a n d S. J. C i p o l l a , Nucl. Instr. a n d M e t h . 122 (1974) 4O5. 3) j . L. C a m p b e l l a n d L. A. M c N e l l e s , Nucl. Instr. a n d M e t h . 125 (1975) 205. 4) R . J . G e h r k e and R . A . L o k k e n , Nucl. Instr. a n d Meth. 97 (1971) 219.
INSTRUMENTS
AND
METHODS
136 (I976) 347-348;
©
NORTH-HOLLAND
PUBLISHING
CO.
C A L I B R A T I O N S O U R C E E F F E C T S I N Si(Li) D E T E C T O R EFFICIENCY DETERMINATIONS* S A M J. C I P O L L A a n d M A R Y J. H E W I T T
Department of Physics, Creighton University, Omaha, Nebraska 68178, U.S.A. Received 2 F e b r u a r y 1976 a n d in revised f o r m 12 M a y 1976 Efficiency determinations o f Si(Li) detectors for low-energy characteristic X-rays (3-10 keV) using a physical five-parameter model have pointed to possible self-absorption effects in the usual drop-evaporated calibration sources. To further test this hypothesis, u n i f o r m a n d thin calibration sources prepared by a surface ion-implantation technique were used to re-determine the efficiency response o f a Si(Li) detector.
It has been found that the usual evaporated drop source t) for use in low-energy efficiency calibrations of Si(Li) detectors can cause errors in the results due to source self-absorption, which is difficult, if not impossible, to correctZ). U n i f o r m and thin source deposits are therefore needed in this kind o f work. There are several ways to produce such sources, including v a c u u m evaporation, electron sputtering in vacuum, electroplating, and direct surface ionimplantation, which was the method chosen here. Ref. 2 describes in detail the experimental procedures used and the analytical method employed to determine the detection efficiency o f a Si(Li) detector. The model equation can be written as,
collimated Si(Li) detector was performed with a set o f " t h i c k " drop-evaporated sources, and then repeated with a set o f " t h i n " ion-implanted sources. The sources w e r e 2 4 1 A m , 51Cr, 57Co, 137Cs, and 6 5 Z n . In the " t h i n " - s o u r c e set, the Z41Am source was .._ " . . . . . . . . . . . . . =. . . . . . . . . . _d=, .~-~._- . .~_~&-,~,,,, ....
b
Thin
×
e = f2 exp(~E ~) [1
-
-
exp(yE~)],
±
(1)
with the five parameters (~,/~, 7, 5, I2) to be specified from the fit of the measured efficiency e vs the p h o t o n energy E. A subsequent modification of the fitting procedure is the inclusion o f various restricted-fit options (setting/~ = constant, or setting f2 = constant). In ref. 2, the high-energy response turned out as expected, but the low-energy response seemed anomalous. Of the several hypotheses put forth to explain this behavior, the most plausible one seemed to be source self-absorption in the non-uniform and relatively thick drop sources. A n additional problem was the lack o f a sufficient n u m b e r of low-energy data points to adequately describe the functional dependence in the low-energy region. This is a critical situation since X-ray spectroscopy o f low-Z elements often requires a knowledge of the Si(Li) efficiency response if the superior energy resolution of this detector is to be exploited. This note concentrates on the problem o f source self-absorption. A n efficiency calibration of a well* W o r k supported in part by the Research Corporation, U.S.A.
ill
Thick
Th n_e,i (,~=-286)
OI 1.5
2
2 5 3
4
5
6 7 8 9 I0
15
20
25 5 0
40
50 6070
Energy (KEY)
Fig. 1. Efficiency response o f the Si(Li) detector using " t h i c k " a n d " t h i n " calibration sources o f 241Am, 137Cs, 51Cr, S7Co, and 6SZn. Corrections were m a d e for Mylar covers on the " t h i c k " sources, and for source misalignment, as in ref. 2. T h e results o f the model fit are s h o w n in the curves d r a w n t h r o u g h the data, a n d in table 1. The dotted curve is the fl-restricted fit to the " t h i n " - s o u r c e data, with fl = - 2 . 8 6 . Note the attenuation o f the low-energy points in the " t h i c k " - s o u r c e curve. Error bars are omitted for s o m e low-energy points for clarity, and A denotes the " t h i c k " source data, while (3 denotes the " t h i n " source data.
348
S. J. C I P O L L A
AND
M. J. H E W I T T
Table 1 P a r a m e t e r s o f the fits to the efficiency data.
Parameter
"Thick" sources
"Thin" sources Free fit
Q ,8
Z~
( 5 . 1 0 ± 0 . 8 9 ) × 10 - 4 - 4.584-2.32 - 1.214-0.50 - ( 3 . 0 2 4 - 2 . 7 5 ) x 104 - 3.084-0.22 0.33
electroplated onto platinum instead of being ionimplanted, as it was purchased from stock through Monsanto Research Corporation. The photon energies and emissions per decay from the sources were mostly those adopted by Campbell and McNelles3), with the calibration data for 51Cr taken from Gehrke and Lokken4), and the 137Cs L X-ray data taken from ref. 2. The activities of the sources ranged from less than a microCurie to about 3 #Ci, except for the "thick" /41Am source, which had an activity of about 13 #Ci and did pose some problems with interfering americium X-rays due to source self-fluorescence. Fig. 1 and table 1 show the results of the fits of eq. (1) to the "thick"- and "thin"-source data. In the " thick "-source fit, the ' 37Cs data had to be normalized to the rest of the "thick" data, apparently due to misalignment of the source during the measurements. (See ref. 2 for the normalization procedure.) The high-energy parameter of the "thick"-source fit, 6=-3.08_0.22, is in excellent agreement with expectations; but, as anticipated, the low-energy parameter,/3 = - 1.21 _+0.50, appears unrealistic. The fit of the "thin"-source data to eq. (1) required no normalization. Again, the high-energy fit is excellent ( 6 = - 3 . 1 3 _ + 0 . 1 4 ) , but the low-energy fit is disappointingly unrealistic, in that / ~ = - 0 . 8 7 _ + 0 . 5 0 is well off the expected value of /3 = - 2 . 8 6 for a stacked 0.5 rail. beryllium and 0.5 rail. Mylar window combination (ignoring the dead-silicon and gold layers on the front of the detector). As mentioned before, the paucity of data in the low-energy ( < 5 keV) region, even for nearly ideal sources, will make the low-energy efficiency determination inaccurate. To test this hypothesis further, a restricted fit was also made to the "thin"-source data, in which the low-energy parameter was fixed at /3 = - 2 . 8 6 . The results of this fit are also shown in fig. 1. Statistically, the restricted fit is quite acceptable as seen from the reduced chi-squared value of Z~ = 1.31. Thus, it is
(5.87 =k 1.18) × 10 - 4 - 2.014-0.86 - 0.874-0.50 - ( 3 . 3 3 4 - 1 . 7 8 ) x 104 - 3.13-4-0.14 0.54
,8-restricted fit
(4.544-0.09) × 10 - 4 - 21.14-2.9 - 2.86 - ( 5 . 6 0 4 - 3 . 1 7 ) x 104 - 3.204-0.15 1.31
highly probable that the " thin "-source data are consistent with the model expectations, whereas before this behavior was masked by self-absorption effects in the "thick" sources. It is readily seen in fig. 1 that the low-energy " thick "-source data are attenuated. (Incidentally, a /Lrestricted fit on the "thick"-source data, similar to the one performed on the "thin"source data, yielded unacceptable 7~r z values, as well as incorrect estimates of the solid angle parameter f2; it clearly did not represent the efficiency measurement using the drop evaporated sources.) Besides demonstrating the need for better source preparation procedures, this note also reiterates the need to develop more calibration sources to fill in the low-energy range for efficiency determinations of Si(Li) detectors. Only if more calibrated low-energy sources are developed can the model be fully tested to determine if other effects, such as incomplete charge collection in the detector, or absorption in the detector gold layer, will necessitate significant changes in the model. The authors are very grateful for the assistance of J. L. Lerner and L. Glendenon of Argonne National Laboratory for preparation and calibration of the ion-implanted sources, and to J. L. Duggan of North Texas State University and the staff of the Oak Ridge Associated Universities for preparation and calibration of the drop-evaporated sources. A. Bennie is thanked for her assistance in the data analysis. References ') W. C. P a r k e r , H. Slatis, F. S. G o u l d i n g a n d R. A. A l l e n , in Alpha-, beta- and gamma-ray spectroscopy (ed. K. S i e g b a h n ; N o r t h - H o l l a n d Publ. Co., A m s t e r d a m , 1965) ch. 7. 2) W . J . G a l l a g h e r a n d S. J. C i p o l l a , Nucl. Instr. a n d M e t h . 122 (1974) 4O5. 3) j . L. C a m p b e l l a n d L. A. M c N e l l e s , Nucl. Instr. a n d M e t h . 125 (1975) 205. 4) R . J . G e h r k e and R . A . L o k k e n , Nucl. Instr. a n d Meth. 97 (1971) 219.