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Measurement of the efficiency of solid state detectors using atomic-field bremsstrahlung
1028
Nuclear Instruments and Methods in Physics Research B24/25 (1987) 1028-1030 North-tlolland, Amsterdam
MEASUREMENT OF THE EFFICIENCY OF SOLID STATE DETECTORS USING ATOMIC-FIELD BREMSSTRAHLUNG J.C. A L T M A N *, R a j k u m a r A M B R O S E **, C.A. Q U A R L E S a n d Gregory L. W E S T B R O O K Department of Physics, Texas Christian Universit). Fort Worth, Texas 76129, USA
We report on the current status of our efforts to use the atomic-field bremsstrahlung spectrum from the bombardment of thin-film targets by an electron beam to measure the photon energy dependence of the efficiency of HPGe and Si(Li) detectors. Results are presented for an HPGe detector from 15 to 100 keV and for an Si(Li) detector from 2 to 40 keV. Points on the efficiency curve are also obtained using calibrated radioactive sources for comparison and for absolute normalization. Problems associated with the background in bremsstrahlung measurements are also discussed.
1. Introduction The precise measurement of the efficiency of solid stale detectors is important in many applications. Efforts to improve, systematize, and simplify the measurement of efficiency have been the subject of numerous studies over the past ten years. A recent paper by Campbell and McGhee [1] summarizes the current state-of-the-art with regard to Si(Li) detectors, and provides an extensive bibliography for the research on efficiency measurement. The approach adopted in ref. [1] is the traditional one of using carefully prepared, calibrated radioactive sources with X-rays and gamma rays spanning the photon energy region of interest. The u p d a t i n g of the relative intensities of the lines used is an important part of this work, as new data on the nuclear and atomic physics processes such as internal conversion coefficients, fluorescence yields, and relative X-ray intensities become available. Atomic-field bremsstrahlung (AFB) can provide an alternative to the conventional method employing radioactive sources. AFB was first used by Palinkas and Schlenck [2] who bombarded a thin carbon target with 10 keV electrons to obtain a bremsstrahlung spectrum. At that time, accurate calculations of the angular distribution corrections for the bremsstrahlung spectrum were not available, so their published results require a small systematic correction, especially at the lower photon energies. Nevertheless, the results they obtained were in good agreement with the radioactive source and X-ray fluorescence results within the experimental error. Ae-
* Current address: Lockheed Missiles and Space Co., Palo Alto, CA 94304, USA. * * Current address: Physics Department, Monmouth College, Monmouth, IL 61462, USA. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
curate calculations of the bremsstrahlung spectrum have now been tabulated [3]. The problem associated with the distortion of the bremsstrahhing spectrum due to plural scattering in the target when using low energy electrons on thin-film targets has been recognized [4]; and use of a gas target is one way to overcome this problem. Another way is to use high energy electrons for which the effects of plural scattering in a target film are negligible. One of the potential advantages of using AFB in efficiency measurement is that the theory is independent of the atomic and nuclear physics processes which form the theoretical basis of the line intensities of radioactive or X-ray fluorescence sources. Thus the AFB process provides an independent photon source with the potential of absolute calibration to the accurate theory. It is our belief that with reasonable attention to minimizing the background inherent in any bremsstrahlung measurement, the use of the AFB source can provide a simple, accurate, reproducible, broad-band efficiency measurement which will facilitate the modelling of the photon energy dependence of the detector efficiency. In some cases, the AFB method can provide better measurements in situ, since the AFB source may be a better approximation to the acutal photon source from irradiation of the target than a radioactive source prepared with the usual deposition techniques.
2. ThcoD'
The theory of AFB has been calculated by Pratt and his co-workers [3,5]. The calculation is first order in quantum electrodynamics and treats the process as a single electron transition in a relativistically self-consistent potential. When electrons with kinetic energy T
J.C. A Itmatz et aL / Effieien(v of solid state detectors bombard a target of atomic number Z, the radiation produced is a continuum with energy k ranging from zero to T, the so-called kinematic endpoint of the bremsstrahhing spectrum. A detector placed at angle 0 with respect to the incident beam, subtending a solid angle 812 will detect N ( k ) photons within an energy width of 8k given by :
1.0[ I
'
1029
I
'
I
,
I
'
I
,
I
'--
I
,
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'
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,
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0.6
N ( k ) = Not(do~dO d k )S126kc( k ),
(1)
where No is the incident electron beam intensity, t is the target thickness, c(k) is the photon energy dependent efficiency, and d o / d E dk is the theoretical AFB cross section which is a function of Z, T, k, and 0. Absolute efficiency measurements require the precise determination of the target thickness and No, and this is a limiting factor in the accuracy achievable with this technique. Relative efficiency c(k) can be determined from the ratio N ( k ) / ( d o / d I 2 dk) with much more accuracy since it does not depend on the target thickness. If good relative measurements are available over the desired range of energies, the efficiency can be placed on an absolute scale by the measurement of one line from a calibrated source.
3. Experimental details The bremsstrahlung spectrum used in this work is produced by the bombardment of thin-film targets with a 100 keV electron beam from a 300 keV Cockcroft-Walton accelerator. An incident energy of 100 keV was selected since it was high enough to span the photon energy region of interest in both detectors, and be largely unaffected by plural ormultiple interactions in the targets used; yet low enough to allow minimization of background with reasonable shielding. The targets, which were purchased commercially, were thin films of pure elements and compounds and were either self-supporting or carbon backed. The target thicknesses were typically in the 50 # g / c m 2 range and were known to about 20%, but, of course, this does not figure in the relative efficiency measurement. A variety of targets were used to demonstrate the consistency and independence of the results on target element. The radioactive sources used were 133Ba, l°9Cd, and 57Co. They were calibrated and traceable to NBS standards, The line intensities were taken from the most recent tabulation [1].
4. HPGe detector The HPGe detector was a 100 mm 2 planar detector with a thickness of 10 mm. The detector had a I mil Be window and was coupled to the vacuum with a 5 mil
0.4
0.2
o.o
,
10
30
50
ENERGY
70
90
110
(keY)
Fig. 1. The absolute efficiency for an HPGe detector versus photon energy in keV. The data is the smoothed average of AFB runs from targets of carbon, copper, silver, gold, and uranium with the characteristic X-rays omitted. The source points are from 57C0 and 133Ba.
mylar window. The solid angle was established by lead collimators, and was calculated geometrically. The efficiency of the detector is shown in fig. 1. The curve is the result of a smoothed averaging of AFB runs from targets of carbon, copper, silver, gold, and uranium, omitting any region of characteristic target X-rays. The source points are also shown. The relative efficiency determined from the AFB measurement has--been normalized to the source points in a one parameter least square fit. The absolute efficiency calculated from eq. (1) for each target was also found to be within the typical uncertainty in the target thickness of about 20%.
5. Si(Li) detector The Si(Li) detector was a 100 mm 2, 3 mm thick detector with a 0.3 nail Be window. It was coupled directly to the vacuum chamber. A small permanent magnet was placed between the detector and the target to deflect electrons scattered from the target. This is important to reduce the background from scattered electrons with sufficient energy to penetrate the Be window. The solid angle was again determined by lead collimators and calculated geometrically. The efficiency is shown in fig. 2. The curve, which has not been smoothed in this case, is the average of the AFB results obtained from targets of carbon, aluminum, copper, and silver, omitting the characteristic X-rays XI. DETECTORS
1030
J.C. A hman et al. 1.2
'
I
'
/ F.fficien¢y
of solid state detectors
cannot be subtracted out with a standard target-in target-out procedure, can be estimated from the known elastic scattering cross section and the thick-target bremsstrahlung cross section [7]. We have estimated the background to be less than 10% at the lowest photon energy. This is supported by the agreement between the source and the AFB measurements; but further experimental work is needed to establish this background with more certainty.
I
1.0
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0.4
7. Conclusions 0.2
,
0.0 0
I
,
iO
I 20
ENERGY
,
I 30
,
40
[keY)
Fig. 2. The absolute efficiency of a Si(Li) detector. The broken cuiwe is the result of averaging without smoothing the data from AFB runs from targets of carbon, aluminum, copper and silver, omitting the characteristic X-rays. The source points are from SVCo,1°9Cd, and 133Ba.The smooth line is the theoretical photoabsorption curve for 3 mm thick silicon. from the target. The AFB results have been normalized to the source points in a one-parameter fit. Also shown is the scaled theoretical efficiency curve calculated from the photoabsorption data of Storm and Israel [6] for a 3 mm thick Si detector. The absolute efficienty calculated for each target from eq. (1) is found to agree with the normalization to the radioactive sources within the uncertainty in the target thickness of about 20%.
6. Discussion of background Background can be a problem in the use of AFB. In addition to the possible penetration of energetic electrons through the Be window, the scattering of incident electrons into the window can produce a thick-target bremsstrahlung background. The effect of this background is to enhance the apparent efficiency, especially at the lower photon energies. The background, which
We have demonstrated that the photon energy dependence of the efficiency of an HpGe and Si(Li) detector can be determined over a wide range of photon energy using an AFB source. With an electron bombarding energy of 100 keV, and targets in the 50 p g / c m 2 range, distortion from multiple interactions in the target is negligible. Care must be taken, however, to minimize the thick-target bremsstrahlung background from the detector or vacuum window. The relative efficiency can be placed on an absolute scale either by normalization to a calibrated radioactive source or to the absolute theoretical bremsstrahlung cross section if the target thickness is accurately known. This work was supported in part by the Robert A. Welch Foundation under Grant. No. P-968 and by the Texas Christian University Research Fund. One of us (R.A.) received a Grant-in-Aid of Research from the Society of Sigma Xi for partial support of this research.
References [1] J.L. Campbell and P.L. McGhee, Nucl. Instr. and Moth. A248 (1986) 393. [2] J. Palinkas and B. Schlenk, Nucl. Inst. and Meth. 169 (1980) 493. [3] L. Kissel, C.A. Quarles, and R.H. Pratt, At. Data and Nucl. Data Tables 28 (1983) 381. I4] L. F.step and C.A. Quarles, IEEE Trans. Nucl. Sci. NS-30 (1983) 1518. [5] H.K. Tseng and R.H. Pratt, Phys. Rev. A3 (1971) 100. [6] E. Storm and H.I. Istrael, Nucl. Data Tables 7 (1970) 565. [7] E. Storm, Phys. Rev. A5 (1972) 2328.
Nuclear Instruments and Methods in Physics Research B24/25 (1987) 1028-1030 North-tlolland, Amsterdam
MEASUREMENT OF THE EFFICIENCY OF SOLID STATE DETECTORS USING ATOMIC-FIELD BREMSSTRAHLUNG J.C. A L T M A N *, R a j k u m a r A M B R O S E **, C.A. Q U A R L E S a n d Gregory L. W E S T B R O O K Department of Physics, Texas Christian Universit). Fort Worth, Texas 76129, USA
We report on the current status of our efforts to use the atomic-field bremsstrahlung spectrum from the bombardment of thin-film targets by an electron beam to measure the photon energy dependence of the efficiency of HPGe and Si(Li) detectors. Results are presented for an HPGe detector from 15 to 100 keV and for an Si(Li) detector from 2 to 40 keV. Points on the efficiency curve are also obtained using calibrated radioactive sources for comparison and for absolute normalization. Problems associated with the background in bremsstrahlung measurements are also discussed.
1. Introduction The precise measurement of the efficiency of solid stale detectors is important in many applications. Efforts to improve, systematize, and simplify the measurement of efficiency have been the subject of numerous studies over the past ten years. A recent paper by Campbell and McGhee [1] summarizes the current state-of-the-art with regard to Si(Li) detectors, and provides an extensive bibliography for the research on efficiency measurement. The approach adopted in ref. [1] is the traditional one of using carefully prepared, calibrated radioactive sources with X-rays and gamma rays spanning the photon energy region of interest. The u p d a t i n g of the relative intensities of the lines used is an important part of this work, as new data on the nuclear and atomic physics processes such as internal conversion coefficients, fluorescence yields, and relative X-ray intensities become available. Atomic-field bremsstrahlung (AFB) can provide an alternative to the conventional method employing radioactive sources. AFB was first used by Palinkas and Schlenck [2] who bombarded a thin carbon target with 10 keV electrons to obtain a bremsstrahlung spectrum. At that time, accurate calculations of the angular distribution corrections for the bremsstrahlung spectrum were not available, so their published results require a small systematic correction, especially at the lower photon energies. Nevertheless, the results they obtained were in good agreement with the radioactive source and X-ray fluorescence results within the experimental error. Ae-
* Current address: Lockheed Missiles and Space Co., Palo Alto, CA 94304, USA. * * Current address: Physics Department, Monmouth College, Monmouth, IL 61462, USA. 0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
curate calculations of the bremsstrahlung spectrum have now been tabulated [3]. The problem associated with the distortion of the bremsstrahhing spectrum due to plural scattering in the target when using low energy electrons on thin-film targets has been recognized [4]; and use of a gas target is one way to overcome this problem. Another way is to use high energy electrons for which the effects of plural scattering in a target film are negligible. One of the potential advantages of using AFB in efficiency measurement is that the theory is independent of the atomic and nuclear physics processes which form the theoretical basis of the line intensities of radioactive or X-ray fluorescence sources. Thus the AFB process provides an independent photon source with the potential of absolute calibration to the accurate theory. It is our belief that with reasonable attention to minimizing the background inherent in any bremsstrahlung measurement, the use of the AFB source can provide a simple, accurate, reproducible, broad-band efficiency measurement which will facilitate the modelling of the photon energy dependence of the detector efficiency. In some cases, the AFB method can provide better measurements in situ, since the AFB source may be a better approximation to the acutal photon source from irradiation of the target than a radioactive source prepared with the usual deposition techniques.
2. ThcoD'
The theory of AFB has been calculated by Pratt and his co-workers [3,5]. The calculation is first order in quantum electrodynamics and treats the process as a single electron transition in a relativistically self-consistent potential. When electrons with kinetic energy T
J.C. A Itmatz et aL / Effieien(v of solid state detectors bombard a target of atomic number Z, the radiation produced is a continuum with energy k ranging from zero to T, the so-called kinematic endpoint of the bremsstrahhing spectrum. A detector placed at angle 0 with respect to the incident beam, subtending a solid angle 812 will detect N ( k ) photons within an energy width of 8k given by :
1.0[ I
'
1029
I
'
I
,
I
'
I
,
I
'--
I
,
I
'
I
,
I
0.6
N ( k ) = Not(do~dO d k )S126kc( k ),
(1)
where No is the incident electron beam intensity, t is the target thickness, c(k) is the photon energy dependent efficiency, and d o / d E dk is the theoretical AFB cross section which is a function of Z, T, k, and 0. Absolute efficiency measurements require the precise determination of the target thickness and No, and this is a limiting factor in the accuracy achievable with this technique. Relative efficiency c(k) can be determined from the ratio N ( k ) / ( d o / d I 2 dk) with much more accuracy since it does not depend on the target thickness. If good relative measurements are available over the desired range of energies, the efficiency can be placed on an absolute scale by the measurement of one line from a calibrated source.
3. Experimental details The bremsstrahlung spectrum used in this work is produced by the bombardment of thin-film targets with a 100 keV electron beam from a 300 keV Cockcroft-Walton accelerator. An incident energy of 100 keV was selected since it was high enough to span the photon energy region of interest in both detectors, and be largely unaffected by plural ormultiple interactions in the targets used; yet low enough to allow minimization of background with reasonable shielding. The targets, which were purchased commercially, were thin films of pure elements and compounds and were either self-supporting or carbon backed. The target thicknesses were typically in the 50 # g / c m 2 range and were known to about 20%, but, of course, this does not figure in the relative efficiency measurement. A variety of targets were used to demonstrate the consistency and independence of the results on target element. The radioactive sources used were 133Ba, l°9Cd, and 57Co. They were calibrated and traceable to NBS standards, The line intensities were taken from the most recent tabulation [1].
4. HPGe detector The HPGe detector was a 100 mm 2 planar detector with a thickness of 10 mm. The detector had a I mil Be window and was coupled to the vacuum with a 5 mil
0.4
0.2
o.o
,
10
30
50
ENERGY
70
90
110
(keY)
Fig. 1. The absolute efficiency for an HPGe detector versus photon energy in keV. The data is the smoothed average of AFB runs from targets of carbon, copper, silver, gold, and uranium with the characteristic X-rays omitted. The source points are from 57C0 and 133Ba.
mylar window. The solid angle was established by lead collimators, and was calculated geometrically. The efficiency of the detector is shown in fig. 1. The curve is the result of a smoothed averaging of AFB runs from targets of carbon, copper, silver, gold, and uranium, omitting any region of characteristic target X-rays. The source points are also shown. The relative efficiency determined from the AFB measurement has--been normalized to the source points in a one parameter least square fit. The absolute efficiency calculated from eq. (1) for each target was also found to be within the typical uncertainty in the target thickness of about 20%.
5. Si(Li) detector The Si(Li) detector was a 100 mm 2, 3 mm thick detector with a 0.3 nail Be window. It was coupled directly to the vacuum chamber. A small permanent magnet was placed between the detector and the target to deflect electrons scattered from the target. This is important to reduce the background from scattered electrons with sufficient energy to penetrate the Be window. The solid angle was again determined by lead collimators and calculated geometrically. The efficiency is shown in fig. 2. The curve, which has not been smoothed in this case, is the average of the AFB results obtained from targets of carbon, aluminum, copper, and silver, omitting the characteristic X-rays XI. DETECTORS
1030
J.C. A hman et al. 1.2
'
I
'
/ F.fficien¢y
of solid state detectors
cannot be subtracted out with a standard target-in target-out procedure, can be estimated from the known elastic scattering cross section and the thick-target bremsstrahlung cross section [7]. We have estimated the background to be less than 10% at the lowest photon energy. This is supported by the agreement between the source and the AFB measurements; but further experimental work is needed to establish this background with more certainty.
I
1.0
O.B
O,G
0.4
7. Conclusions 0.2
,
0.0 0
I
,
iO
I 20
ENERGY
,
I 30
,
40
[keY)
Fig. 2. The absolute efficiency of a Si(Li) detector. The broken cuiwe is the result of averaging without smoothing the data from AFB runs from targets of carbon, aluminum, copper and silver, omitting the characteristic X-rays. The source points are from SVCo,1°9Cd, and 133Ba.The smooth line is the theoretical photoabsorption curve for 3 mm thick silicon. from the target. The AFB results have been normalized to the source points in a one-parameter fit. Also shown is the scaled theoretical efficiency curve calculated from the photoabsorption data of Storm and Israel [6] for a 3 mm thick Si detector. The absolute efficienty calculated for each target from eq. (1) is found to agree with the normalization to the radioactive sources within the uncertainty in the target thickness of about 20%.
6. Discussion of background Background can be a problem in the use of AFB. In addition to the possible penetration of energetic electrons through the Be window, the scattering of incident electrons into the window can produce a thick-target bremsstrahlung background. The effect of this background is to enhance the apparent efficiency, especially at the lower photon energies. The background, which
We have demonstrated that the photon energy dependence of the efficiency of an HpGe and Si(Li) detector can be determined over a wide range of photon energy using an AFB source. With an electron bombarding energy of 100 keV, and targets in the 50 p g / c m 2 range, distortion from multiple interactions in the target is negligible. Care must be taken, however, to minimize the thick-target bremsstrahlung background from the detector or vacuum window. The relative efficiency can be placed on an absolute scale either by normalization to a calibrated radioactive source or to the absolute theoretical bremsstrahlung cross section if the target thickness is accurately known. This work was supported in part by the Robert A. Welch Foundation under Grant. No. P-968 and by the Texas Christian University Research Fund. One of us (R.A.) received a Grant-in-Aid of Research from the Society of Sigma Xi for partial support of this research.
References [1] J.L. Campbell and P.L. McGhee, Nucl. Instr. and Moth. A248 (1986) 393. [2] J. Palinkas and B. Schlenk, Nucl. Inst. and Meth. 169 (1980) 493. [3] L. Kissel, C.A. Quarles, and R.H. Pratt, At. Data and Nucl. Data Tables 28 (1983) 381. I4] L. F.step and C.A. Quarles, IEEE Trans. Nucl. Sci. NS-30 (1983) 1518. [5] H.K. Tseng and R.H. Pratt, Phys. Rev. A3 (1971) 100. [6] E. Storm and H.I. Istrael, Nucl. Data Tables 7 (1970) 565. [7] E. Storm, Phys. Rev. A5 (1972) 2328.