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Variation in fwhm of gamma photopeak resolution with sample composition during neutron activation analysis using a low-energy photon spectrometer
Nuclear Instruments and Methods in Physics Research 219 (1984) 601-602 North-Holland, Amsterdam
601
L e t t e r to the Editor
V A R I A T I O N I N F W H M OF GAMMA P I - I O T O P E A K R E S O L U T I O N W I T H S A M P L E COMPOSITION DURING NEUTRON ACTIVATION ANALYSIS USING A LOW-ENERGY PHOTON SPECTROMETER P.J. POTTS, Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, England
T. T W O M E Y EG&G Instruments, Doncastle House, Doncastle Road, Bracknell, Berks. RG12 4PG, England
Received 16 May 1983 and in revised form 16 September 1983
Data are presented to show that the fwhm resolution of a low energy photon spectrometer in the 0-400 keV range can be affected by the proportion of overload pulses derived from higher energy gamma emissions from an irradiated sample.
Low energy photon spectrometers (LEPS detectors) are extensively used to measure the gamma spectra of samples irradiated during neutron activation analysis of geological material [1-3]. In this application, it is unusual to accumulate gamma emissions above about 300 keV due to the fall-off in LEPS detector efficiences at these higher energies. It is then most convenient to adjust system gain to 100 eV per channel so that gamma energies in the range 0 to 400 keV may be recorded in a 4096 channel multichannel analyser. Electronic pulses from gamma ray events derived from this energy region are then amplified in the range 0 to 10 V. However, most amplifiers used for gamma spectroscopy saturate at about 12 V with higher energy gamma rays producing overload or 'clipped' output pulses. We report here that the proportion of these clipped output pulses in a spectrum can affect the resolution of photopeaks in the recorded spectrum de: pending on the time constants and overload recovery characteristics of the main amplifier. This in turn can reduce the effectiveness with which a peak fitting program is able to analyse photopeak areas especially in the case of algorithms which require pre-calibration of fwhm as a function of energy (for example SAMPO [4]). It is suggested that this effect is caused by the breakup of the amplifier baseline during the recovery from saturation as the baseline restorer and overload protection circuitry attempt to restore the baseline. The effect is similar to the familar phenomenon of pulse pile-up, which occurs when count rates approach the reciprocal of the pulse-width of a single non-overloaded amplifier output pulse. The use of the so called pile-up rejector circuit 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
detects and vetos events which are distorted by the arrival of subsequent events before the amplifier has recovered. The recovery time of an amplifier lengthens substantially as the degree of overload is increased. The presence of overload pulses has the effect of lowering the count-rate threshold at which 'pile-up' is seen to occur, and at which spectral peaks begin to distort. The overall effect is that the entire energy resolution of the spectrum may be degraded by an amount which depends on the proportion of overload pulses in the spectrum. When related ~o the analysis of geological materials by INAA this effect will be correlated with the sample composition 6f principally 46Sc (889, 1120 keV), 59Fe (1099, 1292 keV) and 6°Co (1172, 1332 keV). In order to demonstrate this effect, measurements of standard gamma ray sources were made using a LEPS hyperpure germanium detector (of nominal resolution 540 eV at 122 keV) coupled to a 7032 Data Acquisition and Analysis System (all E G & G Ortec). Measurements were made with two spectroscopy amplifiers: (a) A standard linear Gaussian amplifier ( E G & G Ortec 472A). (b) A more modem Gaussian amplifier with improved baseline restoration and pulse pile-up circuitry (EG &G Ortec 572). In order to demonstrate the change in peak resolution with overload events, the following spectra were recorded using time constant settings of 1 and 2/~s. (1) 57Co source alone (all peaks within range - 122, 136 keV). (2) S7Co+6°Co sources (6°Co gammas at 1172 and 1332 keV saturate amplifier).
602
P.J. Potts, [~ Twornev / Variation in fwhm of gamma photopeak
Table 1 Resolution of "STCo 122 keV photopeak on a LEPS detector using various amplifier combinations Radioactive sources counted
57Co only 57Co + 6oCo 57Co + 60Co + 137Cs
Fwhm resolution of 122 keV ~7Co photopeak "~ time constant = 1 /ts
time constant = 2 ~ts
472A amplifier
572 amplifier
472A amplifier
572 amplifier
615.4 612.2 620.0
611.0 611.6 614.9
551.0 570.9 591.1
570.6 574.3 574.8
~) Accuracy of an individual resolution measurement is estimated to be better than _+5 eV.
(3)
57Co-+-6°Co-+-137Cs sources (662 keV g a m m a of 137Cs also saturates amplifier). F o r all measurements, c o u n t i n g dead times were m a i n t a i n e d below 15%, (4000 input cps maximum), eliminating the possibility of significant distortion due to n o n - o v e r l o a d pile-up. T h e results of resolution m e a s u r e m e n t s on the 122 keV 57Co peak are shown in table 1 a n d indicate a large variation in resolution using the 472A amplifier set at 2 /~s. This variation is largely eliminated when the time c o n s t a n t setting is reduced to 1 # s a n d is n o t detected at all o n the more a d v a n c e d 572 amplifier at either 1 or 2 p.s. It is concluded that the resolution effects described here m a y cause unsuspected errors in peak fitting algorithms used in the routine analysis of geological samples
b y I N A A using a LEPS detector unless the appropriate optimisation of m a i n amplifier time c o n s t a n t has been carried out. Moreover, in any radiation spectroscopy system in which overload pulses are occurring, the effects of pulse pile-up should be checked at m u c h lower count-rates t h a n might otherwise be considered.
References [1] J. Hertogen and R. Gijbels, Anal. Chim. Acta 56 (1971) 61. [21 P. Henderson and C.T. Williams, J. Radional. Chem. 67 (1981) 445. [3] P.A. Baedecker, J.J. Rowe and E. Steinnes, J. Radioanal. Chem. 40 (1977) 115. [4] J.T. Rontti and S.G. Prussin, Nucl. Instr. and Meth. 72 (1969) 125.
601
L e t t e r to the Editor
V A R I A T I O N I N F W H M OF GAMMA P I - I O T O P E A K R E S O L U T I O N W I T H S A M P L E COMPOSITION DURING NEUTRON ACTIVATION ANALYSIS USING A LOW-ENERGY PHOTON SPECTROMETER P.J. POTTS, Department of Earth Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, England
T. T W O M E Y EG&G Instruments, Doncastle House, Doncastle Road, Bracknell, Berks. RG12 4PG, England
Received 16 May 1983 and in revised form 16 September 1983
Data are presented to show that the fwhm resolution of a low energy photon spectrometer in the 0-400 keV range can be affected by the proportion of overload pulses derived from higher energy gamma emissions from an irradiated sample.
Low energy photon spectrometers (LEPS detectors) are extensively used to measure the gamma spectra of samples irradiated during neutron activation analysis of geological material [1-3]. In this application, it is unusual to accumulate gamma emissions above about 300 keV due to the fall-off in LEPS detector efficiences at these higher energies. It is then most convenient to adjust system gain to 100 eV per channel so that gamma energies in the range 0 to 400 keV may be recorded in a 4096 channel multichannel analyser. Electronic pulses from gamma ray events derived from this energy region are then amplified in the range 0 to 10 V. However, most amplifiers used for gamma spectroscopy saturate at about 12 V with higher energy gamma rays producing overload or 'clipped' output pulses. We report here that the proportion of these clipped output pulses in a spectrum can affect the resolution of photopeaks in the recorded spectrum de: pending on the time constants and overload recovery characteristics of the main amplifier. This in turn can reduce the effectiveness with which a peak fitting program is able to analyse photopeak areas especially in the case of algorithms which require pre-calibration of fwhm as a function of energy (for example SAMPO [4]). It is suggested that this effect is caused by the breakup of the amplifier baseline during the recovery from saturation as the baseline restorer and overload protection circuitry attempt to restore the baseline. The effect is similar to the familar phenomenon of pulse pile-up, which occurs when count rates approach the reciprocal of the pulse-width of a single non-overloaded amplifier output pulse. The use of the so called pile-up rejector circuit 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
detects and vetos events which are distorted by the arrival of subsequent events before the amplifier has recovered. The recovery time of an amplifier lengthens substantially as the degree of overload is increased. The presence of overload pulses has the effect of lowering the count-rate threshold at which 'pile-up' is seen to occur, and at which spectral peaks begin to distort. The overall effect is that the entire energy resolution of the spectrum may be degraded by an amount which depends on the proportion of overload pulses in the spectrum. When related ~o the analysis of geological materials by INAA this effect will be correlated with the sample composition 6f principally 46Sc (889, 1120 keV), 59Fe (1099, 1292 keV) and 6°Co (1172, 1332 keV). In order to demonstrate this effect, measurements of standard gamma ray sources were made using a LEPS hyperpure germanium detector (of nominal resolution 540 eV at 122 keV) coupled to a 7032 Data Acquisition and Analysis System (all E G & G Ortec). Measurements were made with two spectroscopy amplifiers: (a) A standard linear Gaussian amplifier ( E G & G Ortec 472A). (b) A more modem Gaussian amplifier with improved baseline restoration and pulse pile-up circuitry (EG &G Ortec 572). In order to demonstrate the change in peak resolution with overload events, the following spectra were recorded using time constant settings of 1 and 2/~s. (1) 57Co source alone (all peaks within range - 122, 136 keV). (2) S7Co+6°Co sources (6°Co gammas at 1172 and 1332 keV saturate amplifier).
602
P.J. Potts, [~ Twornev / Variation in fwhm of gamma photopeak
Table 1 Resolution of "STCo 122 keV photopeak on a LEPS detector using various amplifier combinations Radioactive sources counted
57Co only 57Co + 6oCo 57Co + 60Co + 137Cs
Fwhm resolution of 122 keV ~7Co photopeak "~ time constant = 1 /ts
time constant = 2 ~ts
472A amplifier
572 amplifier
472A amplifier
572 amplifier
615.4 612.2 620.0
611.0 611.6 614.9
551.0 570.9 591.1
570.6 574.3 574.8
~) Accuracy of an individual resolution measurement is estimated to be better than _+5 eV.
(3)
57Co-+-6°Co-+-137Cs sources (662 keV g a m m a of 137Cs also saturates amplifier). F o r all measurements, c o u n t i n g dead times were m a i n t a i n e d below 15%, (4000 input cps maximum), eliminating the possibility of significant distortion due to n o n - o v e r l o a d pile-up. T h e results of resolution m e a s u r e m e n t s on the 122 keV 57Co peak are shown in table 1 a n d indicate a large variation in resolution using the 472A amplifier set at 2 /~s. This variation is largely eliminated when the time c o n s t a n t setting is reduced to 1 # s a n d is n o t detected at all o n the more a d v a n c e d 572 amplifier at either 1 or 2 p.s. It is concluded that the resolution effects described here m a y cause unsuspected errors in peak fitting algorithms used in the routine analysis of geological samples
b y I N A A using a LEPS detector unless the appropriate optimisation of m a i n amplifier time c o n s t a n t has been carried out. Moreover, in any radiation spectroscopy system in which overload pulses are occurring, the effects of pulse pile-up should be checked at m u c h lower count-rates t h a n might otherwise be considered.
References [1] J. Hertogen and R. Gijbels, Anal. Chim. Acta 56 (1971) 61. [21 P. Henderson and C.T. Williams, J. Radional. Chem. 67 (1981) 445. [3] P.A. Baedecker, J.J. Rowe and E. Steinnes, J. Radioanal. Chem. 40 (1977) 115. [4] J.T. Rontti and S.G. Prussin, Nucl. Instr. and Meth. 72 (1969) 125.