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Lineshape factors and counting rate in annihilation radiation
Solid State Communications, Vol. 12, pp. 443—445, 1973. Pergamon Press.
Printed in Great Britain
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION Ch. Dauwe, M. Dorikens and L. Dorikens.Vanpraet* Rijksuniversiteit Gent, I.N.W. Proeftuinstraat. 86, B-9000, Gent, Belgium (Received 4 November 1972 by S. Amelinckx)
Annihilation lineshape factor measurements are found to be influenced by the total counting rate observed by the Ge(Li) detector, and by the output pulse width of the main (semi-Gauss) amplifier. Counting rates have to be kept very low, typically below 2kHz.
i ~ H ~•_i—i_~ I ~_‘k~ ~ f ~
POSITRON annihilation experiments in metals have shown an amazing dependence upon defects such as vacancies and dislocations. One of the suitable quantities to be measured for this pupose is the lineshape factor, also called S-factor, defined by MacKenzie et An alternative quantity is the absolute counting rate obtained in angular correlation measurements for zero angle2 (1800 correlation angle).
___ _____
___ _____ J
~.{CA2S _______
J
_____
Sc
S-factor measurements need many precautions in order to eliminate possible influences which affect the value. Schulte et aL3 have reported recently on some variations of the S-value which they attributed to surface effects, that could eventually be minimized by using positron sources with high end-point energy such as Ge’8, and by frequently etching the samples. We have performed some experiments in order to establish the influence of two other experimental factors, namely the total counting rate observed by the Ge(Li) detector, and the output pulse shape of the main amplifier. It is well known that both factors affect the resolution obtained with Ge(Li) detectors and hence theyare expected to be important for S-factor measurements.
ii
_____
Sc 2
FIG. 1. Block diagram of the set-up. Ge: Ge(Li) detectoT. P.A.: preamplifier. A: Amplifier, semi-Gauss shaping type. CA 25: ADC type Intertechnique CA 25. T: pulse train from ADC. STAB1: digital window stabilizer, modified type SES G201. STAB2: digital windows (modified SES G201 ,non stabilizing). Sd: scaler counting the summed contents of windows over the centroid of the 511keV peak (Ia). Sc2: scaler counting the summed contents of windows over the full peak (Jr). (modified SES G 201) and an ADC (Intertechnique CA 25) as shown in Fig. 1. The principle of operation has been briefly described before.4 A second semiGauss amplifier, identical to the first one, except for the shaping time constants, was also used; it had 2psec FWIIM of the output pulse.
The detector we used was a 19 cm3 Ge(Li) true coaxial by Ortec. The best resolution (l.SlkeV at 5 l4keV) was obtained with a semi Gauss amplifier of own design, with 4~secFWHM of the output pulse. This was coupled to a digital gain stabilizing system
The digital windows ofSTAB1 (see Fig. 1) are set symmetrically and adjacent over the centroid of the 511 keV annihilation peak, whilst STAB2 selects the full annihilation line. Note that only STAB 1 acts as gain stabilizer. The central counts J~and the full counts are directly counted by two scalers, without really needing the memory of the multichannel analyser.
_____________
*
___ _____
Research worker of the I.I.K.W., Brussels, Belgium. 443
444
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION Vol. 12, No.6
In this set-up the peak stabilizers, besides stabilizing against amplifier drift, act as digital single channel analysers and the S-factor is given by ~ This has to be corrected for the contribution to I,~,and I~,of the Compton continuum of the higher energy gamma radiation (i.e. the l2l4keVtransition if Na22 is used as a source). However our measurements are made with only one source-convertor combination, which means that this correction is not necessary as long as we compare relative values for S. The source was Na22 sandwiched between two pieces of annealed and electropolished OFHC copper. The gain of the amplifiers, DC offsets and conversion gain of the ADC and the windows of the peak stabilizers were chosen in order that I~,be taken over the central 2keV of the annihilation line. This corresponds to an S-factor between 0.5 and 0.6. The form of stabilization we use, is of course only one point stabilization, while Schulte2 used two point stabilization. Nevertheless, in our case each measurement takes only 10mm, so that one point stabilization is satisfactory, especially so since the stabilization point is identical with the measuring point,
The results are presented in Fig. 2. The abscissa of the plot represents the total counting rate as seen by the Ge(Li) detector. The measured S.value drops for both amplifier conditions, when the counting rate increases. This effect is most pronounced for the ‘long’ pulse shape. However, since better intrinsic Ge(Li) resolution is obtained for longer time constants, the whole ‘long pulse’ S curve, lies above the ‘short pulse’ one. It seems that just as in any high resolution experiment, long shaping constants for the main amplifier are imperative, but then the total counting rate has to be kept very low, typically below 2kHz. A second series of measurements was done with a baseline restorer (Ortec model 438) after the amplifier. As expected this had no influence at low counting rates. The counting rate at which the S-factor started to deviate from a constant was barely higher than in the series without baseline restoration, but the deviation was less important. It seems that baseline restoration offers no solution to the problem. Pile-up rejection in the chain is expected to improve the results, but really comes to the same thing as using a lower counting rate. In any case, if absolute S-factor measurements be attempted and eventually compared between various experimenters, it would be adequate to use
-
.5,
the zero intensity extrapolated values. If this is not possible, because it is too time consuming, at least very low counting rate values should be used. -
Acknowledgements The authors are indebted to Dr. J. Uyttenhove for the design of the excellent semiGausse amplifiers. They wish to thank Prof. Dr. G. Robbrecht for his interest in this work. This research is part of the programme of the I.I.K.W., Brussels; the authors are indebted for financial support. —
-
I
.
‘3.
I mti~.
~
FIG .2. Lineshape factor as a function of total counting rate for two different pulse shapes of the main amplifier output pulse. Dots (top curve): FWHM of output pulse = 4~sec.Open circles: FWHM of output pulse = 2psec. REFERENCES 1.
MACKENZIE I.K., EADY J.A. and GINGERICH R.R.,Phys. Lett. 33A, 279 (1970).
2.
MCKEE B.T.A., TRIFTHAUSER W. and STEWART A.T., Phys. Rev. Lett. 28,358(1972).
3. 4.
SCHULTE C.W., LICHTENBERGER P.C., GINGERICH R.R. and CAMPBELL J.L., Phys. Lett. 41A, 305 (1972). DORIKENS M., DEMUYNCK J. and DORIKENS-VANPRAET L., Z. Phys. 244,321(1971).
Vol. 12, No.6
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION La mesure du facteur S, décrivant la forme de Ia raie d’annihilation est fortement influencée par le taux de comptage dans le détecteur Ge(Li), et par la forme de l’impulsion de sortie de l’amplificateur semi-Gauss. Un faible taux de comptage (<2kHz) semble obligatoire.
445
Printed in Great Britain
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION Ch. Dauwe, M. Dorikens and L. Dorikens.Vanpraet* Rijksuniversiteit Gent, I.N.W. Proeftuinstraat. 86, B-9000, Gent, Belgium (Received 4 November 1972 by S. Amelinckx)
Annihilation lineshape factor measurements are found to be influenced by the total counting rate observed by the Ge(Li) detector, and by the output pulse width of the main (semi-Gauss) amplifier. Counting rates have to be kept very low, typically below 2kHz.
i ~ H ~•_i—i_~ I ~_‘k~ ~ f ~
POSITRON annihilation experiments in metals have shown an amazing dependence upon defects such as vacancies and dislocations. One of the suitable quantities to be measured for this pupose is the lineshape factor, also called S-factor, defined by MacKenzie et An alternative quantity is the absolute counting rate obtained in angular correlation measurements for zero angle2 (1800 correlation angle).
___ _____
___ _____ J
~.{CA2S _______
J
_____
Sc
S-factor measurements need many precautions in order to eliminate possible influences which affect the value. Schulte et aL3 have reported recently on some variations of the S-value which they attributed to surface effects, that could eventually be minimized by using positron sources with high end-point energy such as Ge’8, and by frequently etching the samples. We have performed some experiments in order to establish the influence of two other experimental factors, namely the total counting rate observed by the Ge(Li) detector, and the output pulse shape of the main amplifier. It is well known that both factors affect the resolution obtained with Ge(Li) detectors and hence theyare expected to be important for S-factor measurements.
ii
_____
Sc 2
FIG. 1. Block diagram of the set-up. Ge: Ge(Li) detectoT. P.A.: preamplifier. A: Amplifier, semi-Gauss shaping type. CA 25: ADC type Intertechnique CA 25. T: pulse train from ADC. STAB1: digital window stabilizer, modified type SES G201. STAB2: digital windows (modified SES G201 ,non stabilizing). Sd: scaler counting the summed contents of windows over the centroid of the 511keV peak (Ia). Sc2: scaler counting the summed contents of windows over the full peak (Jr). (modified SES G 201) and an ADC (Intertechnique CA 25) as shown in Fig. 1. The principle of operation has been briefly described before.4 A second semiGauss amplifier, identical to the first one, except for the shaping time constants, was also used; it had 2psec FWIIM of the output pulse.
The detector we used was a 19 cm3 Ge(Li) true coaxial by Ortec. The best resolution (l.SlkeV at 5 l4keV) was obtained with a semi Gauss amplifier of own design, with 4~secFWHM of the output pulse. This was coupled to a digital gain stabilizing system
The digital windows ofSTAB1 (see Fig. 1) are set symmetrically and adjacent over the centroid of the 511 keV annihilation peak, whilst STAB2 selects the full annihilation line. Note that only STAB 1 acts as gain stabilizer. The central counts J~and the full counts are directly counted by two scalers, without really needing the memory of the multichannel analyser.
_____________
*
___ _____
Research worker of the I.I.K.W., Brussels, Belgium. 443
444
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION Vol. 12, No.6
In this set-up the peak stabilizers, besides stabilizing against amplifier drift, act as digital single channel analysers and the S-factor is given by ~ This has to be corrected for the contribution to I,~,and I~,of the Compton continuum of the higher energy gamma radiation (i.e. the l2l4keVtransition if Na22 is used as a source). However our measurements are made with only one source-convertor combination, which means that this correction is not necessary as long as we compare relative values for S. The source was Na22 sandwiched between two pieces of annealed and electropolished OFHC copper. The gain of the amplifiers, DC offsets and conversion gain of the ADC and the windows of the peak stabilizers were chosen in order that I~,be taken over the central 2keV of the annihilation line. This corresponds to an S-factor between 0.5 and 0.6. The form of stabilization we use, is of course only one point stabilization, while Schulte2 used two point stabilization. Nevertheless, in our case each measurement takes only 10mm, so that one point stabilization is satisfactory, especially so since the stabilization point is identical with the measuring point,
The results are presented in Fig. 2. The abscissa of the plot represents the total counting rate as seen by the Ge(Li) detector. The measured S.value drops for both amplifier conditions, when the counting rate increases. This effect is most pronounced for the ‘long’ pulse shape. However, since better intrinsic Ge(Li) resolution is obtained for longer time constants, the whole ‘long pulse’ S curve, lies above the ‘short pulse’ one. It seems that just as in any high resolution experiment, long shaping constants for the main amplifier are imperative, but then the total counting rate has to be kept very low, typically below 2kHz. A second series of measurements was done with a baseline restorer (Ortec model 438) after the amplifier. As expected this had no influence at low counting rates. The counting rate at which the S-factor started to deviate from a constant was barely higher than in the series without baseline restoration, but the deviation was less important. It seems that baseline restoration offers no solution to the problem. Pile-up rejection in the chain is expected to improve the results, but really comes to the same thing as using a lower counting rate. In any case, if absolute S-factor measurements be attempted and eventually compared between various experimenters, it would be adequate to use
-
.5,
the zero intensity extrapolated values. If this is not possible, because it is too time consuming, at least very low counting rate values should be used. -
Acknowledgements The authors are indebted to Dr. J. Uyttenhove for the design of the excellent semiGausse amplifiers. They wish to thank Prof. Dr. G. Robbrecht for his interest in this work. This research is part of the programme of the I.I.K.W., Brussels; the authors are indebted for financial support. —
-
I
.
‘3.
I mti~.
~
FIG .2. Lineshape factor as a function of total counting rate for two different pulse shapes of the main amplifier output pulse. Dots (top curve): FWHM of output pulse = 4~sec.Open circles: FWHM of output pulse = 2psec. REFERENCES 1.
MACKENZIE I.K., EADY J.A. and GINGERICH R.R.,Phys. Lett. 33A, 279 (1970).
2.
MCKEE B.T.A., TRIFTHAUSER W. and STEWART A.T., Phys. Rev. Lett. 28,358(1972).
3. 4.
SCHULTE C.W., LICHTENBERGER P.C., GINGERICH R.R. and CAMPBELL J.L., Phys. Lett. 41A, 305 (1972). DORIKENS M., DEMUYNCK J. and DORIKENS-VANPRAET L., Z. Phys. 244,321(1971).
Vol. 12, No.6
LINESHAPE FACTORS AND COUNTING RATE IN ANNIHILATION RADIATION La mesure du facteur S, décrivant la forme de Ia raie d’annihilation est fortement influencée par le taux de comptage dans le détecteur Ge(Li), et par la forme de l’impulsion de sortie de l’amplificateur semi-Gauss. Un faible taux de comptage (<2kHz) semble obligatoire.
445