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Semiconductor telescope spectrometer for beta-ray spectra
Nuclear Instruments and Methods 212 (1983) 221 225 North-Holland Publishing Company
SEMICONDUCTOR W. TRZASKA
221
TELESCOPE SPECTROMETER
*, J. A Y S T ( ~ A N D
J. K A N T E L E
FOR BETA-RAY SPECTRA
**
Department of Physics, University of Jyviiskylii, Jyviiskylii, Finland Received 15 October 1982
A telescope beta-ray spectrometer consisting of a 3 mm thick Si(Li) A E plus a 5 mm thick Ge(HP) E detector was built and tested. Simplicity and insensitivity to ),-rays (when operated in a coincidence mode) make the device well suited to fl+ and /3 end-point energy measurements, and it can also be used in on-line studies of very short-lived fl emitters. Due to a number of effects associated with the interactions of the fl-particles with the spectrometer, the calibration curves for/3 + and/3 end-point energies were found to be different and were determined using several well-known/3 activities. As an application of the method, the 4934gRufl- end-point energy was measured. The result, 5315 (85) keV, is in good agreement with the prediction from the mass-systematics of the 1977 Mass tables.
1. Introduction The d e t e r m i n a t i o n of nuclear masses is one of the basic tasks in nuclear physics. Most of the mass values of the isotopes close to the beta-stability region are already k n o w n with high accuracy. However, in the case of the nuclides far from stability, the available data are in general scarce a n d often unreliable. In such cases, direct mass d e t e r m i n a t i o n s are difficult and the beta e n d - p o i n t energy m e a s u r e m e n t s c o m b i n e d with the necessary spectroscopic information remain practically as the only way to determine the masses. There are several well-known techniques for that purpose. However, none of them is ideal. Magnetic spectrometers provide, for instance, good energy resolution b u t at the expense of the efficiency. Plastic scintillator spectrometers offer the opposite. Recently, a great deal of interest has been focussed on beta spectrometers based on semiconductor detectors [1-5]. They combine a good energy resolution with a sufficient efficiency a n d may be calibrated easily with gamma-rays. However, the sensitivity to gammarays often requires some additional means for removing the background, which destroys the simplicity of a single detector set-up and hence restricts the possible applications. Our idea was to develop a technique that could overcome these problems, without seriously affecting the advantages of a single detector spectrometer. Since the line-shape, especially at higher energies, is a prob* Permanent address: Institute of Nuclear Research, Warsaw, Poland. ** Temporary address: Los Alamos National Laboratory MS K480, New Mexico 87545, USA. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
lem, it was i m p o r t a n t to consider methods which would yield an improved line-shape, too. Our choice, a semic o n d u c t o r detector telescope described in this paper, seems to well fulfil these expectations.
2. The detector set-up description The semiconductor telescope spectrometer shown in fig. 1 consists of a 3 m m thick Si(Li) A E detector with a sensitive area of 2 cm 2 a n d a hyperpure g e r m a n i u m E detector, 5 m m thick a n d l0 m m in diameter. Silicon, having a relatively low Z-value, exhibits a low backscattering probability for electrons a n d low probabilities for the production of b r e m s s t r a h l u n g and annihilation of positrons in flight. Therefore, silicon is a detector material well suited to electron spectroscopy. O n the o t h e r hand, since the m a x i m u m available thicknesses a n d stopping efficiencies of silicon detectors are rather limited, the use of g e r m a n i u m E detector was found necessary. T h e thickness of the A E detector (3 mm) indicates a lower limit of a b o u t 2 MeV for the meausurable fl end-point energies. The upper limit, somewhat above 7 MeV, is determined by the total stopping efficiency of the telescope. The overall energy resolution of the telescope was a b o u t 25 keV. This figure reflects mainly the effect of a 39 m g / c m 2 dead layer at one side of the Si(Li) detector, estimated from the measured energy loss of electrons from a 2°7Bi source. To achieve a solid-angle m a t c h between the two detectors, the sensitive area of the Si(Li) crystal was limited by means of a lead collimator. T h e close face-to-face geometry was necessary go give a n o p t i m u m line-shape, to which the main c o n t r i b u t i o n
222 BEAM
>ET
W. Trzaska et aZ / Semiconductor telescope spectrometer
F e r m i - K u r i e plots. The end-point energies were then determined with the aid of straight lines fitted to the linear parts of the plots (cf. fig. 3).
3. Test measurements 3.1. Well-known t9 emitters
OPPER COLD FINGER 0
V//A m
rLASTIC INDIUM LEAD 5cm
Fig. 1. The geometry of the telescope ,8-ray spectrometer.
comes from the reabsorption of the electrons backscattered from the surface of the Ge(HP) detector. A proper cooling of the system was assured by indium rings and screws which with spring wires connected the detectors to the cold-finger. A plastic cup with a thin mylar window served as an additional protection against possible detector damage by oil-vapor condensation, when the detector was cooled down to the liquid-nitrogen temperature. For the use with a helium-jet system, which was a part of the several application tests for the spectrometer, the telescope vacuum was separated by a thin nickel window from the vacuum of the tape transport system. In all other experiments, the telescope was set to face the target at right angles with respect to the beam, as shown in fig. 1. Special attention was paid to minimize possible pileup effects by using amplifiers with pile-up rejection and by limiting the counting rates of both detectors to less than 1500 cps. Pulsed cyclotron beams were used throughout our experiments and the data were collected exclusively during the beam-off periods. Both the coincidence and the singles mode events were stored. The singles spectra were helpful for monitoring and calibration purposes, since the detectors were internally calibrated by gamma-rays, including quanta produced during the bombardments. For convenience, the gains of the E and A E detectors were equalized. The data analysis was carried out subsequently in a computer. The spectra were read out from magnetic tapes, the E and A E addresses were added, the energy losses in the window are in the dead layer of the Si(Li) detector were corrected for, and the results were finally plotted as
A series of experiments with several well-known 19+ and f l - emitters was carried out in studies of the behaviour of the telescope. All neutron-deficient beta emitters investigated were produced via the (p, n) reaction with proton beams of about 10 MeV energy from the MC-20 cyclotron of the University of Jyv~iskyl~. The targets were usually enriched, self-supporting, about 1 m g / c m 2 thick. A typical positron spectrum, resulting from the super-allowed decay of 54Co, and observed with the zl E detector (in coincidence with the E detector events) is shown in fig. 2a. It contains a broad transmission peak at about 1.1 MeV with a tail extending to higher energies and with a sharp edge in the low-energy region corresponding to a minimum energy loss. The lowest-energy counts below the transmission peak are mainly associated with the positron annihilation in the E detector. During the final analysis these events as well as those corresponding to a very high energy loss in the Si(Li) detector were rejected by a gating procedure. The spectrum of the E detector, as seen in fig. 2b, has a continuous shape, except at lower energies where the
I
l
800
lla)
SILL) AE detector
z
S
&00
0 800-
I,
z
:~ 400 o(o
0
(b)
-
- - ,
Oe (HP) E detector
T
2.5
~
.
I
5.0 ENERGY (MeV]
Fig. 2. Typica! coincidence spectra obtained with the telescope from 54Co ,8 + decay: (a) spectrum of AE detector, (b) spectrum of E detector.
W. Trzaska et al. / Semiconduclor telescope spectrometer
All the investigated beta decays, except the case of 90y, were of allowed type. Our results together with literature values taken from the 1977 Mass Tables [6] and, in the case of 1°6In, from the work of Davids et al. [7] are displayed in table 1. A significant disagreement between the values obtained from our F e r m i - C u r i e plots and the corresponding well-known literature values is obvious, as seen in table 1 and fig. 4. This discrepancy will be discussed in more detail in section 3.2. The main contribution to the errors indicated in table 1 results from the statistical effects. The contribution from other factors like the uncertainties in the energy calibration and in the thickness of the dead layer of the Si(Li) is of the order of a few keV. In general, higher end-point energies require longer runs for collecting sufficient statistics, but the situation may be reversed as a result of the disturbing influence of a fl-background. For example, the error bars in the case of 54Co are small because of the absence of the competing j~-branches in this nuclide, resulting in a long fitting region extending down to 3 MeV below the end-point energy.
Table 1 Uncorrected fl end-point energies measured with a semiconductor telescope spectrometer in comparison with well-known literature values [6,7]. Nuclide
End-point energy (keV)
62 Cu( fl + ) 27Si(/8 + ) ~6Ga(/~ + ) 1°6In(3 + state)(/~ +) 64Ga(/3 +) 54Co(/3+) 9° Y(,8 ) 144Pr(fl-) 2° F(/8 )
From Fermi-Curie plot
From literature mass values
3085(33) 3910(40 4294(24) 4980(90) 6080(30) 7084(10) 2290(15) 3003(34) 5286(44)
2927(5) 3787( 1.3) 4153(3) 4904(13) 6143(8) 7219.8(1.2) 2283.9(2.5) 2996(2.9) 5392.2(0.9)
223
effects of the 511 keV annihilation radiation are clearly seen.
The only B - emitter studied in an " i n - b e a m " experiment was 2°F. It was produced with 1 nA, 5.2 MeV deuterons on a 5 m g / c m 2 thick LiF target. The remaining two /3 end-points were acquired in experiments with 9°Sr and 144Ce sources.
3.2. Correct energy calibration In the case of an ideal detector, an allowed beta decay produces a linear relation between N ~ / ~ p F W and
\
30-
"~
~"
20-
90y
I~, pc
x3
x7
I~- E M I T T E R S
O-
10-
0
•
2
-
~
,~ BETA
g ENERGY
~ (MeV)
~
0
2
3 BETA
I
i
4
~
ENERGY
''
6
(MeV)
Fig. 3. Comparison of some positron and electron spectra measured with the semiconductor telescope spectrometer. The data are presented as Fermi-Curie plots. Straight lines fitted to linear parts of the plots were used to determine experimental end-point energies. Note a curving part of each fl+ spectrum excluded from fitting.
224
W. Trzaska et al. / Semiconductor telescope spectrometer
200 v
62Cu-~
°f'
100
l
27Si !]" ~
G6Go fIl O G ,c,
In
kl
{J Z W tY W h i.L
o --,--_+
t
64Ga
"IT ----_
~ 10 0
5 -200 l 2
I 3
I 4
I 5
l 6
I 7
END-POINT ENERGY (i',4eV)
Fig. 4. Differences between the end-point energies obtained from Fermi-Curie plots as shown in fig. 3 and the well-established literature values. Dashed lines, serving as correction curves for further experiments, indicate linear fits to the data.
energy• However, the complicated nature of the beta-ray detection causes noticeable disturbances to this ideal picture. To begin with, as one can see from fig. 3, where some of our F e r m i - C u r i e plots are displayed, all the positron spectra tend to curve up at the upper end. The plots are linear only up to some 150-300 keV below the extrapolated crossing point with the abscissa axis. Even then, neglecting the non-linear part of the plots, the results deviate significantly from the expected values• In fig. 4 our results are compared with the well-established values from the literature mass measurements: the deviations are mostly large and are certainly not accidental. This behaviour is understandable in view of the following phenomena: (1) summing of events due to positron annihilation, (2) escape of primary kinetic energies, (3) escape of bremsstrahlung quanta, (4) backscattering, and (5) annihilation of positrons in flight. Monte Carlo calculations reproduce very well both the shape and the shift toward higher energies in the positron spectra observed with semiconductor detectors [8]. The phenomena (1) and (5) are sufficient for explaining the differences in behaviour of the observed/3+ and /3- spectra. When one compares the experimental endpoint values of the low-energy electrons and positrons, the effect of the strong summing-up with partially absorbed annihilation quanta in the latter case is clearly seen (fig. 4); at the same time the electron spectra are practically undistorted. With increasing energy, the role of the bremsstrahlung and other escape-events give rise to undervaluation of the experimental results for electrons, and result in a compensation effect for positrons. At about 6 MeV, even the fl+ end-point energies are undervaluated, despite of summing-up with annihilation quanta• The probability for annihilation in flight increases with /3 + energy [9], resulting in a higher energy for the annihilation quanta. This decreases the probability for them to interact with the detector. The same is true when the annihilation takes place near the edges of
the detector: the probability of interaction is smaller and so is the summing-up effect. From a number of measured "apparent" end-point energies in a wide energy range (table 1), we have estimated the necessary corrections that would give correct end-point values obtained via the F e r m i - C u r i e analysis of the spectra. Since no theoretical predictions were available, a linear fit to the data was a natural solution. The dashed lines in fig. 4 indicate our adopted energy-dependent corrections. 3.3• Decay energy of 93Ru The 93Ru end-point energy measurement is one of the typical cases to which we have applied our telescope. An enriched, 5 m g / c m 2 92Mo target was bombarded with 23.9 MeV, 0.2 nA 3He beam in 2 min irradiation cycles followed by 2 min data-acquisition cycles. The telescope was placed at right angles with respect to the beam direction and at 2 cm distance away from the target (fig. 1). Two independent runs were carried out to assure the reliability of the set-up. 93Ru has two fl-decaying states with half-lives of 10.8 s and 59.7 s and well-established energy difference of 734.4 keV [10]. The expected end-point energy of the shorter-lived component is higher by 241.8 keV. In order to remove most of the fl activity associated with the short-lived isomer, we skipped the first 60 s from each data-acquisition period. Fig. 5 represents a sample spectrum after a 14 hour data acquisition. The F e r m i - C u r i e analyses of spectra obtained in two sep-
3000
. . . . • "
11/2~) 0734 10 8s (9/2*) 00 " ~ 5 9 , 7 s
•
13°
-pwF
89
c~
FK PLOT
]000
o
4'
5~ BETA ENERGY (i'4eV)
6'
Fig. 5. 93Ru positron decay spectrum shown in a linear scale and as a Fermi-Curie plot. Straight line, fitted to points with indicated statistical error bars, served to determine the value of the end-point energy•
W. Trzaska et al. / Semiconductor telescope spectrometer
arate runs result in a combined value of 5239 (72) keV for the crossing point with the energy axis, which with the correction described in section 3.2 ( - 14 + 44 keV), gives 5315 + 85 keV as the final result for the beta end-point energy of the 93g RH decay. The value obtained for the total beta-decay energy (6337 + 85 keV) is in a good agreement with the predicted value of 6300 keV given in the 1977 Mass Tables [6].
225
This work has been supported in part by the National Research Council for Sciences. We wish to thank Miss L. Parviainen for her valuable help during the measurements and the data analysis, and Prof. J. Zylicz for his inspiring suggestions in the early stages of this work.
References 4. Conclusions A semiconductor telescope spectrometer is useful, easy to operate and broadly applicable in fl end-point energy determinations. The sensitivity to "/-rays is sufficiently controlled by coincidence and gating requirements. The use of a thick Si(Li) A E detector allows an accurate energy calibration by y-rays, and provides good time and energy resolution. Also, the line shape of this type of a telescope is good due to a greatly improved peak-to-total ratio [11]. This telescope may be operated without any additional magnetic electron transport systems. In the studies of fl÷ activities, disturbances caused by annihilation radiations are a limiting factor, when using semiconductor-type spectrometers. Therefore, it is usually difficult to study cases where a second/3 branch is separated by just a few hundred keV. Due to certain summing and escape probabilities, all measured endpoint energies (both fl+ and fl-) have to be adjusted using computer-calculated, or, as in the present case, experimentally determined correction curves.
[1] M.J. Berger, S.M. Seltzer, S.E. Chappell, J.C. Humphreys and J.W. Motz, Nucl. Instr. and Meth. 69 (1969) 181. [2] C.N. Davids, C.A. Gagliardi, M.J. Murphy and E.B. Norman, Phys. Rev. C19 (1979) 1463. [3] B.J. Varley, J.E. Kitching, W. Leo, J. Miskin, R.B. Moore, K.D. Wunsch, R. Decker, H. Wollnik and G. Siegert, Nucl. Instr. and Meth. 190 (1981) 543. [4] D.M. Moltz, K.S. Toth, F.T. Avignone III, H. Noma, B.G. Ritchie and B.D. Kern, Phys. Lett. 113B (1982) 16. [5] R. Decker, K.D. WOnsch, H. Wollnik, G. Jung, E. Koglin and G. Siegert, Nucl. Instr. and Meth. 192 (1982) 261. [6] A.H. Wapstra and K. Bos, At. Data Nucl. Data Tables 19 (March 1977). [7] l°6Cd(p, n)-threshold measurement by C.N. Davids et al., private communication. [8] F.T. Avignone III, H. Noma, D.M. Moltz and K.S. Toth, Nucl. Instr. and Meth. 189 (1981) 453. [9] J. Kantele and M. Valkonen, Nucl. Instr. and Meth. 112 (1973) 501. [10] J.C. de Lange, J. Bron, A. van Poelgeest and H. Verheul, Z. Physik A279 (1976) 79. [11] J. Kantele and A. Passoja, Nucl. Instr. and Meth. 92 (1971) 247.
SEMICONDUCTOR W. TRZASKA
221
TELESCOPE SPECTROMETER
*, J. A Y S T ( ~ A N D
J. K A N T E L E
FOR BETA-RAY SPECTRA
**
Department of Physics, University of Jyviiskylii, Jyviiskylii, Finland Received 15 October 1982
A telescope beta-ray spectrometer consisting of a 3 mm thick Si(Li) A E plus a 5 mm thick Ge(HP) E detector was built and tested. Simplicity and insensitivity to ),-rays (when operated in a coincidence mode) make the device well suited to fl+ and /3 end-point energy measurements, and it can also be used in on-line studies of very short-lived fl emitters. Due to a number of effects associated with the interactions of the fl-particles with the spectrometer, the calibration curves for/3 + and/3 end-point energies were found to be different and were determined using several well-known/3 activities. As an application of the method, the 4934gRufl- end-point energy was measured. The result, 5315 (85) keV, is in good agreement with the prediction from the mass-systematics of the 1977 Mass tables.
1. Introduction The d e t e r m i n a t i o n of nuclear masses is one of the basic tasks in nuclear physics. Most of the mass values of the isotopes close to the beta-stability region are already k n o w n with high accuracy. However, in the case of the nuclides far from stability, the available data are in general scarce a n d often unreliable. In such cases, direct mass d e t e r m i n a t i o n s are difficult and the beta e n d - p o i n t energy m e a s u r e m e n t s c o m b i n e d with the necessary spectroscopic information remain practically as the only way to determine the masses. There are several well-known techniques for that purpose. However, none of them is ideal. Magnetic spectrometers provide, for instance, good energy resolution b u t at the expense of the efficiency. Plastic scintillator spectrometers offer the opposite. Recently, a great deal of interest has been focussed on beta spectrometers based on semiconductor detectors [1-5]. They combine a good energy resolution with a sufficient efficiency a n d may be calibrated easily with gamma-rays. However, the sensitivity to gammarays often requires some additional means for removing the background, which destroys the simplicity of a single detector set-up and hence restricts the possible applications. Our idea was to develop a technique that could overcome these problems, without seriously affecting the advantages of a single detector spectrometer. Since the line-shape, especially at higher energies, is a prob* Permanent address: Institute of Nuclear Research, Warsaw, Poland. ** Temporary address: Los Alamos National Laboratory MS K480, New Mexico 87545, USA. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
lem, it was i m p o r t a n t to consider methods which would yield an improved line-shape, too. Our choice, a semic o n d u c t o r detector telescope described in this paper, seems to well fulfil these expectations.
2. The detector set-up description The semiconductor telescope spectrometer shown in fig. 1 consists of a 3 m m thick Si(Li) A E detector with a sensitive area of 2 cm 2 a n d a hyperpure g e r m a n i u m E detector, 5 m m thick a n d l0 m m in diameter. Silicon, having a relatively low Z-value, exhibits a low backscattering probability for electrons a n d low probabilities for the production of b r e m s s t r a h l u n g and annihilation of positrons in flight. Therefore, silicon is a detector material well suited to electron spectroscopy. O n the o t h e r hand, since the m a x i m u m available thicknesses a n d stopping efficiencies of silicon detectors are rather limited, the use of g e r m a n i u m E detector was found necessary. T h e thickness of the A E detector (3 mm) indicates a lower limit of a b o u t 2 MeV for the meausurable fl end-point energies. The upper limit, somewhat above 7 MeV, is determined by the total stopping efficiency of the telescope. The overall energy resolution of the telescope was a b o u t 25 keV. This figure reflects mainly the effect of a 39 m g / c m 2 dead layer at one side of the Si(Li) detector, estimated from the measured energy loss of electrons from a 2°7Bi source. To achieve a solid-angle m a t c h between the two detectors, the sensitive area of the Si(Li) crystal was limited by means of a lead collimator. T h e close face-to-face geometry was necessary go give a n o p t i m u m line-shape, to which the main c o n t r i b u t i o n
222 BEAM
>ET
W. Trzaska et aZ / Semiconductor telescope spectrometer
F e r m i - K u r i e plots. The end-point energies were then determined with the aid of straight lines fitted to the linear parts of the plots (cf. fig. 3).
3. Test measurements 3.1. Well-known t9 emitters
OPPER COLD FINGER 0
V//A m
rLASTIC INDIUM LEAD 5cm
Fig. 1. The geometry of the telescope ,8-ray spectrometer.
comes from the reabsorption of the electrons backscattered from the surface of the Ge(HP) detector. A proper cooling of the system was assured by indium rings and screws which with spring wires connected the detectors to the cold-finger. A plastic cup with a thin mylar window served as an additional protection against possible detector damage by oil-vapor condensation, when the detector was cooled down to the liquid-nitrogen temperature. For the use with a helium-jet system, which was a part of the several application tests for the spectrometer, the telescope vacuum was separated by a thin nickel window from the vacuum of the tape transport system. In all other experiments, the telescope was set to face the target at right angles with respect to the beam, as shown in fig. 1. Special attention was paid to minimize possible pileup effects by using amplifiers with pile-up rejection and by limiting the counting rates of both detectors to less than 1500 cps. Pulsed cyclotron beams were used throughout our experiments and the data were collected exclusively during the beam-off periods. Both the coincidence and the singles mode events were stored. The singles spectra were helpful for monitoring and calibration purposes, since the detectors were internally calibrated by gamma-rays, including quanta produced during the bombardments. For convenience, the gains of the E and A E detectors were equalized. The data analysis was carried out subsequently in a computer. The spectra were read out from magnetic tapes, the E and A E addresses were added, the energy losses in the window are in the dead layer of the Si(Li) detector were corrected for, and the results were finally plotted as
A series of experiments with several well-known 19+ and f l - emitters was carried out in studies of the behaviour of the telescope. All neutron-deficient beta emitters investigated were produced via the (p, n) reaction with proton beams of about 10 MeV energy from the MC-20 cyclotron of the University of Jyv~iskyl~. The targets were usually enriched, self-supporting, about 1 m g / c m 2 thick. A typical positron spectrum, resulting from the super-allowed decay of 54Co, and observed with the zl E detector (in coincidence with the E detector events) is shown in fig. 2a. It contains a broad transmission peak at about 1.1 MeV with a tail extending to higher energies and with a sharp edge in the low-energy region corresponding to a minimum energy loss. The lowest-energy counts below the transmission peak are mainly associated with the positron annihilation in the E detector. During the final analysis these events as well as those corresponding to a very high energy loss in the Si(Li) detector were rejected by a gating procedure. The spectrum of the E detector, as seen in fig. 2b, has a continuous shape, except at lower energies where the
I
l
800
lla)
SILL) AE detector
z
S
&00
0 800-
I,
z
:~ 400 o(o
0
(b)
-
- - ,
Oe (HP) E detector
T
2.5
~
.
I
5.0 ENERGY (MeV]
Fig. 2. Typica! coincidence spectra obtained with the telescope from 54Co ,8 + decay: (a) spectrum of AE detector, (b) spectrum of E detector.
W. Trzaska et al. / Semiconduclor telescope spectrometer
All the investigated beta decays, except the case of 90y, were of allowed type. Our results together with literature values taken from the 1977 Mass Tables [6] and, in the case of 1°6In, from the work of Davids et al. [7] are displayed in table 1. A significant disagreement between the values obtained from our F e r m i - C u r i e plots and the corresponding well-known literature values is obvious, as seen in table 1 and fig. 4. This discrepancy will be discussed in more detail in section 3.2. The main contribution to the errors indicated in table 1 results from the statistical effects. The contribution from other factors like the uncertainties in the energy calibration and in the thickness of the dead layer of the Si(Li) is of the order of a few keV. In general, higher end-point energies require longer runs for collecting sufficient statistics, but the situation may be reversed as a result of the disturbing influence of a fl-background. For example, the error bars in the case of 54Co are small because of the absence of the competing j~-branches in this nuclide, resulting in a long fitting region extending down to 3 MeV below the end-point energy.
Table 1 Uncorrected fl end-point energies measured with a semiconductor telescope spectrometer in comparison with well-known literature values [6,7]. Nuclide
End-point energy (keV)
62 Cu( fl + ) 27Si(/8 + ) ~6Ga(/~ + ) 1°6In(3 + state)(/~ +) 64Ga(/3 +) 54Co(/3+) 9° Y(,8 ) 144Pr(fl-) 2° F(/8 )
From Fermi-Curie plot
From literature mass values
3085(33) 3910(40 4294(24) 4980(90) 6080(30) 7084(10) 2290(15) 3003(34) 5286(44)
2927(5) 3787( 1.3) 4153(3) 4904(13) 6143(8) 7219.8(1.2) 2283.9(2.5) 2996(2.9) 5392.2(0.9)
223
effects of the 511 keV annihilation radiation are clearly seen.
The only B - emitter studied in an " i n - b e a m " experiment was 2°F. It was produced with 1 nA, 5.2 MeV deuterons on a 5 m g / c m 2 thick LiF target. The remaining two /3 end-points were acquired in experiments with 9°Sr and 144Ce sources.
3.2. Correct energy calibration In the case of an ideal detector, an allowed beta decay produces a linear relation between N ~ / ~ p F W and
\
30-
"~
~"
20-
90y
I~, pc
x3
x7
I~- E M I T T E R S
O-
10-
0
•
2
-
~
,~ BETA
g ENERGY
~ (MeV)
~
0
2
3 BETA
I
i
4
~
ENERGY
''
6
(MeV)
Fig. 3. Comparison of some positron and electron spectra measured with the semiconductor telescope spectrometer. The data are presented as Fermi-Curie plots. Straight lines fitted to linear parts of the plots were used to determine experimental end-point energies. Note a curving part of each fl+ spectrum excluded from fitting.
224
W. Trzaska et al. / Semiconductor telescope spectrometer
200 v
62Cu-~
°f'
100
l
27Si !]" ~
G6Go fIl O G ,c,
In
kl
{J Z W tY W h i.L
o --,--_+
t
64Ga
"IT ----_
~ 10 0
5 -200 l 2
I 3
I 4
I 5
l 6
I 7
END-POINT ENERGY (i',4eV)
Fig. 4. Differences between the end-point energies obtained from Fermi-Curie plots as shown in fig. 3 and the well-established literature values. Dashed lines, serving as correction curves for further experiments, indicate linear fits to the data.
energy• However, the complicated nature of the beta-ray detection causes noticeable disturbances to this ideal picture. To begin with, as one can see from fig. 3, where some of our F e r m i - C u r i e plots are displayed, all the positron spectra tend to curve up at the upper end. The plots are linear only up to some 150-300 keV below the extrapolated crossing point with the abscissa axis. Even then, neglecting the non-linear part of the plots, the results deviate significantly from the expected values• In fig. 4 our results are compared with the well-established values from the literature mass measurements: the deviations are mostly large and are certainly not accidental. This behaviour is understandable in view of the following phenomena: (1) summing of events due to positron annihilation, (2) escape of primary kinetic energies, (3) escape of bremsstrahlung quanta, (4) backscattering, and (5) annihilation of positrons in flight. Monte Carlo calculations reproduce very well both the shape and the shift toward higher energies in the positron spectra observed with semiconductor detectors [8]. The phenomena (1) and (5) are sufficient for explaining the differences in behaviour of the observed/3+ and /3- spectra. When one compares the experimental endpoint values of the low-energy electrons and positrons, the effect of the strong summing-up with partially absorbed annihilation quanta in the latter case is clearly seen (fig. 4); at the same time the electron spectra are practically undistorted. With increasing energy, the role of the bremsstrahlung and other escape-events give rise to undervaluation of the experimental results for electrons, and result in a compensation effect for positrons. At about 6 MeV, even the fl+ end-point energies are undervaluated, despite of summing-up with annihilation quanta• The probability for annihilation in flight increases with /3 + energy [9], resulting in a higher energy for the annihilation quanta. This decreases the probability for them to interact with the detector. The same is true when the annihilation takes place near the edges of
the detector: the probability of interaction is smaller and so is the summing-up effect. From a number of measured "apparent" end-point energies in a wide energy range (table 1), we have estimated the necessary corrections that would give correct end-point values obtained via the F e r m i - C u r i e analysis of the spectra. Since no theoretical predictions were available, a linear fit to the data was a natural solution. The dashed lines in fig. 4 indicate our adopted energy-dependent corrections. 3.3• Decay energy of 93Ru The 93Ru end-point energy measurement is one of the typical cases to which we have applied our telescope. An enriched, 5 m g / c m 2 92Mo target was bombarded with 23.9 MeV, 0.2 nA 3He beam in 2 min irradiation cycles followed by 2 min data-acquisition cycles. The telescope was placed at right angles with respect to the beam direction and at 2 cm distance away from the target (fig. 1). Two independent runs were carried out to assure the reliability of the set-up. 93Ru has two fl-decaying states with half-lives of 10.8 s and 59.7 s and well-established energy difference of 734.4 keV [10]. The expected end-point energy of the shorter-lived component is higher by 241.8 keV. In order to remove most of the fl activity associated with the short-lived isomer, we skipped the first 60 s from each data-acquisition period. Fig. 5 represents a sample spectrum after a 14 hour data acquisition. The F e r m i - C u r i e analyses of spectra obtained in two sep-
3000
. . . . • "
11/2~) 0734 10 8s (9/2*) 00 " ~ 5 9 , 7 s
•
13°
-pwF
89
c~
FK PLOT
]000
o
4'
5~ BETA ENERGY (i'4eV)
6'
Fig. 5. 93Ru positron decay spectrum shown in a linear scale and as a Fermi-Curie plot. Straight line, fitted to points with indicated statistical error bars, served to determine the value of the end-point energy•
W. Trzaska et al. / Semiconductor telescope spectrometer
arate runs result in a combined value of 5239 (72) keV for the crossing point with the energy axis, which with the correction described in section 3.2 ( - 14 + 44 keV), gives 5315 + 85 keV as the final result for the beta end-point energy of the 93g RH decay. The value obtained for the total beta-decay energy (6337 + 85 keV) is in a good agreement with the predicted value of 6300 keV given in the 1977 Mass Tables [6].
225
This work has been supported in part by the National Research Council for Sciences. We wish to thank Miss L. Parviainen for her valuable help during the measurements and the data analysis, and Prof. J. Zylicz for his inspiring suggestions in the early stages of this work.
References 4. Conclusions A semiconductor telescope spectrometer is useful, easy to operate and broadly applicable in fl end-point energy determinations. The sensitivity to "/-rays is sufficiently controlled by coincidence and gating requirements. The use of a thick Si(Li) A E detector allows an accurate energy calibration by y-rays, and provides good time and energy resolution. Also, the line shape of this type of a telescope is good due to a greatly improved peak-to-total ratio [11]. This telescope may be operated without any additional magnetic electron transport systems. In the studies of fl÷ activities, disturbances caused by annihilation radiations are a limiting factor, when using semiconductor-type spectrometers. Therefore, it is usually difficult to study cases where a second/3 branch is separated by just a few hundred keV. Due to certain summing and escape probabilities, all measured endpoint energies (both fl+ and fl-) have to be adjusted using computer-calculated, or, as in the present case, experimentally determined correction curves.
[1] M.J. Berger, S.M. Seltzer, S.E. Chappell, J.C. Humphreys and J.W. Motz, Nucl. Instr. and Meth. 69 (1969) 181. [2] C.N. Davids, C.A. Gagliardi, M.J. Murphy and E.B. Norman, Phys. Rev. C19 (1979) 1463. [3] B.J. Varley, J.E. Kitching, W. Leo, J. Miskin, R.B. Moore, K.D. Wunsch, R. Decker, H. Wollnik and G. Siegert, Nucl. Instr. and Meth. 190 (1981) 543. [4] D.M. Moltz, K.S. Toth, F.T. Avignone III, H. Noma, B.G. Ritchie and B.D. Kern, Phys. Lett. 113B (1982) 16. [5] R. Decker, K.D. WOnsch, H. Wollnik, G. Jung, E. Koglin and G. Siegert, Nucl. Instr. and Meth. 192 (1982) 261. [6] A.H. Wapstra and K. Bos, At. Data Nucl. Data Tables 19 (March 1977). [7] l°6Cd(p, n)-threshold measurement by C.N. Davids et al., private communication. [8] F.T. Avignone III, H. Noma, D.M. Moltz and K.S. Toth, Nucl. Instr. and Meth. 189 (1981) 453. [9] J. Kantele and M. Valkonen, Nucl. Instr. and Meth. 112 (1973) 501. [10] J.C. de Lange, J. Bron, A. van Poelgeest and H. Verheul, Z. Physik A279 (1976) 79. [11] J. Kantele and A. Passoja, Nucl. Instr. and Meth. 92 (1971) 247.