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Direct determination of the nuclear charge distribution of mass separated fission products from 235U(nth,ƒ)
Volume 53B, number 1
PHYSICSLETTERS
11 November 1974
DIRECT DETERMINATION OF THE NUCLEAR CHARGE DISTRIBUTION O F MASS S E P A R A T E D FISSION P R O D U C T S F R O M 235U(nth, f) G. SIEGERT, H. WOLLNIK* 1, j. GREIF, G. FIEDLER* 1, M. ASGHAR, G. BAILLEUL, J.P. BOCQUET .2 , J.P. GAUTHERON, H. SCHRADER, H. EWALD.1 , P. ARMBRUSTER .3 Institut Laue-Langevin, Grenoble,France Received 1 October 1974 The nuclear charge distribution of mass separated monoenergeticfissionproducts of the isobaric chains90 and 91 wererecorded with a AE Si-surfacebarrier detector. The achievednuclear chargeresolvingpower (fwhm)Z/AZ was 43. After the completion of the 'LOHENGRIN' fission product separator [1] at the high flux reactor of the Institut Laue-Langevin in Grenoble, mass and energy separated f i ~ o n products from the fission of 235U with thermal neutrons are available. The current density is about 1000 particles/sec-cm2 which is two orders of magnitude more than an earlier fission product separator in Munich [2] could supply. If such mass and energy separated fission products pass through a thin AE Si-surface barrier detector, the energy deposited therein depends on the nuclear charge. The nuclear charge resolving power of such a detector is determined by two main contributions: firstly the inherent pulse height variation due to imperfect charge collection or to the energy straggling of heavy ions passing through matter and secondly the thickness variation of the detector. For a very thin detector the unavoidable thickness variation becomes dominant. For a thicker detector, however, the thickness variation causes a ~aal]er spread in the energy deposit AE = (aE/ax) Ax since the specific energy loss aE/a x decreases with increasing detector depth x. This implies that the detector should be almost as thick as the particle range R o. Moreover, the pulse height difference for fission products of adjacent nuclear charges increases monotonously up to about Re~2 and decreases thereafter, thus favoring a detector thickness of Re/2. Since usually the first of these effects dominates, a detector thickness closer to R o is *1 UniversityGiessen,Ge~meny .2 D.R.F., C.F~N.Grenoble, France. *3 GSI Darmstadt, Germany;
preferable. The pulse height difference for fission products of neighboufing nuclear charges, however, in any case should stay larger than the pulse height variation due to the energy straggling and variation in charge collection efficiency mentioned above, thus giving an upper limit to the detector thickness. For fission products of the light group, we have chosen a 8/an thick detector (ORTEC No. 25-15-8-122A). After limiting the detector area of 25 mm 2 to 1 mm 2 and 2 mm 2, the thickness variation was small enough to obtain a sufficient nuclear charge resolving power. To demonstrate the feasibility of the method, fig. 1 shows spectra for the mass chains 90 and 91. Fitting several equally spaced Gauss-functions to the data we found a nuclear charge resolving power (fwhm) Z/AZ = 43 for particles of 100 MeV kinetic energy. The areas of the fitted Gauss.functions give directly the fractional independent yields (FLY). For each mass several pulse height spectra were recorded. The independent yields computed from all these spectra agreed within their errors, indicating the good reproducibility of the method (fig. 2). The weighted average values are presented in table 1 together with the radiochemical F l y values from ref. [3], which contain experimental results together with interpolated values according to the odd-even systematics. Both data are in good agreement; our errors, however, are smaller by about a factor of two. In comparing the FIY values of ref. [3] with our experimental results one should keep in mind that our data refer to particles of a selected kinetic energy and ionic charge, while the radiochemical results contain particles of all occurring kinetic energies and ionic 45
Volume 53B, number 1
PHYSICS LETTERS
,
40°
+,
NKP
11 November 1974
,
°0Kr I I
0.8
/~
mlee oheln DO
aO
m a a c chain
T0 . 6
m o.4
J _e ~Brl ÷÷
OIKr ~
0
memm oheln g l
O.6
14o
°lKr_~
m a e a chain 0 1
b
elRb_~
~-- O.4 B
I 0> 0
_" 0 . 6
m
L _
I
I I ohennel number
_
91Br
I
.
O
Fig. 1. The pulse height spectra of energy selected fission fragments of the masses 90 and 91 passing through a 8 ~m Si-surface barrier detector (recording time: one hour).
charges. Since the sum of the kinetic energy and the excitation energy is constant in a specific fission event, particles of different kinetic energies have different excitation energies. This can influence the nuclear charge distribution [4]. In the experiment reported here such effects should be rather small because the correlation between the kinetic energy and the excitation energy is considerably washed out by the energy absorption in a 400 # g / c a 2 thick uranium target (0 to I0 MeV) that we used. We have chosen particles with initial kinetic energies close to their well known average values; this should give a representative selection of fission products of all kinetic energies. The fission product separator 'LOHENGRIN' selects particles of one kinetic energy over charge ratio and one mass over charge ratio. This ionic charge depends on the velocity and nuclear charge of the selected fission product. Thus the recorded fractional 46
! ,I
g'lar . 5
tm •
a's
"
d8
"~'?
nuclear charge
Z
• FI "
3'.
Fig. 2. Fractional independent yields for fission products of the mass chains 90 and 91 which have primary kinetic energies U and ionic charges q. The horizontal lines indicate the weighted average yields. The error bars represent the standard deviations of the individual yield measurements. For each nucleus of the mass chain 90 the first three values are characterized by U = 100 MeV with two o f them corresponding to q = 21 and one to q = 22: The last three values are characterized by U = 95 MeV with two of them corresponding to q = 20 and one to q = 21. For each nucleus of the mass chain 91 the first four values were taken for U= 95 MeV and q = 21. The next seven were taken for U = 100 MeV with six corresponding to q = 21 and one to q = 22. The final value is characterized by U = 104 MeV and q=22.
independent yields should vary slightly with the ionic charge selected by the spectrometer. To take this effect into account corrections of about 1% to 5% are necessary. If one stays close to the average kinetic energy and the average ionic charge, the effect is small as may be seen in fig. 2 where the uncorrected experimental FIY values are plotted for three different kinetic energies and ionic charges.
Volume 53B, number 1
PHYSICS LETTERS
11 November 1974
Table 1 Fractional independent yields and the average nuclear charge values (Zp) of fission products of the mass chains 90 and 91. Values in brackets are from ref. [3]. Mass
ssBr
36Kr
90
0.106 ± 0.01 (0.109 ± 0.025) 0.040 ± 0.005 (0.044 ± 0.018)
0.753 (0.761 0.542 (0.546
91
37Rb ± 0.02 ± 0.032) ± 0.01 ± 0.015)
We shall apply this m e t h o d o f determining fractional independent yields to all the isobaric chains for which the ~ E detector can resolve the nuclear charges reasonably w e l l
0.142 (0.130 0.373 (0.380
± 0.01 ± 0.02 ± 0.01 ± 0.03)
ss Sr
Zp
0.045 ± 0.05 (0.030 ± 0.030)
36.04 (36.02 36.42 (36.37)
References [1] p. Armbruster et al., Axkiv L Fysik 36 (1967) 305; E. Moll et al., Proc- 8th int. EMIS conf., Sk6vde, Sweden (1973) 249. [2] H. Gunther, G. Siegert, R.L. Ferguson, H. Ewald and E. Kunecny, Nucl. Phys. A196 (1972) 401. [3] S. Amiel and H. Feldstein, IAEA/SMoI74/25 (1973). [4] E. Konecny, H. Gunther, G. Siegert and I~ Winter, Nucl. Phys. A100 (1967) 465.
47
PHYSICSLETTERS
11 November 1974
DIRECT DETERMINATION OF THE NUCLEAR CHARGE DISTRIBUTION O F MASS S E P A R A T E D FISSION P R O D U C T S F R O M 235U(nth, f) G. SIEGERT, H. WOLLNIK* 1, j. GREIF, G. FIEDLER* 1, M. ASGHAR, G. BAILLEUL, J.P. BOCQUET .2 , J.P. GAUTHERON, H. SCHRADER, H. EWALD.1 , P. ARMBRUSTER .3 Institut Laue-Langevin, Grenoble,France Received 1 October 1974 The nuclear charge distribution of mass separated monoenergeticfissionproducts of the isobaric chains90 and 91 wererecorded with a AE Si-surfacebarrier detector. The achievednuclear chargeresolvingpower (fwhm)Z/AZ was 43. After the completion of the 'LOHENGRIN' fission product separator [1] at the high flux reactor of the Institut Laue-Langevin in Grenoble, mass and energy separated f i ~ o n products from the fission of 235U with thermal neutrons are available. The current density is about 1000 particles/sec-cm2 which is two orders of magnitude more than an earlier fission product separator in Munich [2] could supply. If such mass and energy separated fission products pass through a thin AE Si-surface barrier detector, the energy deposited therein depends on the nuclear charge. The nuclear charge resolving power of such a detector is determined by two main contributions: firstly the inherent pulse height variation due to imperfect charge collection or to the energy straggling of heavy ions passing through matter and secondly the thickness variation of the detector. For a very thin detector the unavoidable thickness variation becomes dominant. For a thicker detector, however, the thickness variation causes a ~aal]er spread in the energy deposit AE = (aE/ax) Ax since the specific energy loss aE/a x decreases with increasing detector depth x. This implies that the detector should be almost as thick as the particle range R o. Moreover, the pulse height difference for fission products of adjacent nuclear charges increases monotonously up to about Re~2 and decreases thereafter, thus favoring a detector thickness of Re/2. Since usually the first of these effects dominates, a detector thickness closer to R o is *1 UniversityGiessen,Ge~meny .2 D.R.F., C.F~N.Grenoble, France. *3 GSI Darmstadt, Germany;
preferable. The pulse height difference for fission products of neighboufing nuclear charges, however, in any case should stay larger than the pulse height variation due to the energy straggling and variation in charge collection efficiency mentioned above, thus giving an upper limit to the detector thickness. For fission products of the light group, we have chosen a 8/an thick detector (ORTEC No. 25-15-8-122A). After limiting the detector area of 25 mm 2 to 1 mm 2 and 2 mm 2, the thickness variation was small enough to obtain a sufficient nuclear charge resolving power. To demonstrate the feasibility of the method, fig. 1 shows spectra for the mass chains 90 and 91. Fitting several equally spaced Gauss-functions to the data we found a nuclear charge resolving power (fwhm) Z/AZ = 43 for particles of 100 MeV kinetic energy. The areas of the fitted Gauss.functions give directly the fractional independent yields (FLY). For each mass several pulse height spectra were recorded. The independent yields computed from all these spectra agreed within their errors, indicating the good reproducibility of the method (fig. 2). The weighted average values are presented in table 1 together with the radiochemical F l y values from ref. [3], which contain experimental results together with interpolated values according to the odd-even systematics. Both data are in good agreement; our errors, however, are smaller by about a factor of two. In comparing the FIY values of ref. [3] with our experimental results one should keep in mind that our data refer to particles of a selected kinetic energy and ionic charge, while the radiochemical results contain particles of all occurring kinetic energies and ionic 45
Volume 53B, number 1
PHYSICS LETTERS
,
40°
+,
NKP
11 November 1974
,
°0Kr I I
0.8
/~
mlee oheln DO
aO
m a a c chain
T0 . 6
m o.4
J _e ~Brl ÷÷
OIKr ~
0
memm oheln g l
O.6
14o
°lKr_~
m a e a chain 0 1
b
elRb_~
~-- O.4 B
I 0> 0
_" 0 . 6
m
L _
I
I I ohennel number
_
91Br
I
.
O
Fig. 1. The pulse height spectra of energy selected fission fragments of the masses 90 and 91 passing through a 8 ~m Si-surface barrier detector (recording time: one hour).
charges. Since the sum of the kinetic energy and the excitation energy is constant in a specific fission event, particles of different kinetic energies have different excitation energies. This can influence the nuclear charge distribution [4]. In the experiment reported here such effects should be rather small because the correlation between the kinetic energy and the excitation energy is considerably washed out by the energy absorption in a 400 # g / c a 2 thick uranium target (0 to I0 MeV) that we used. We have chosen particles with initial kinetic energies close to their well known average values; this should give a representative selection of fission products of all kinetic energies. The fission product separator 'LOHENGRIN' selects particles of one kinetic energy over charge ratio and one mass over charge ratio. This ionic charge depends on the velocity and nuclear charge of the selected fission product. Thus the recorded fractional 46
! ,I
g'lar . 5
tm •
a's
"
d8
"~'?
nuclear charge
Z
• FI "
3'.
Fig. 2. Fractional independent yields for fission products of the mass chains 90 and 91 which have primary kinetic energies U and ionic charges q. The horizontal lines indicate the weighted average yields. The error bars represent the standard deviations of the individual yield measurements. For each nucleus of the mass chain 90 the first three values are characterized by U = 100 MeV with two o f them corresponding to q = 21 and one to q = 22: The last three values are characterized by U = 95 MeV with two of them corresponding to q = 20 and one to q = 21. For each nucleus of the mass chain 91 the first four values were taken for U= 95 MeV and q = 21. The next seven were taken for U = 100 MeV with six corresponding to q = 21 and one to q = 22. The final value is characterized by U = 104 MeV and q=22.
independent yields should vary slightly with the ionic charge selected by the spectrometer. To take this effect into account corrections of about 1% to 5% are necessary. If one stays close to the average kinetic energy and the average ionic charge, the effect is small as may be seen in fig. 2 where the uncorrected experimental FIY values are plotted for three different kinetic energies and ionic charges.
Volume 53B, number 1
PHYSICS LETTERS
11 November 1974
Table 1 Fractional independent yields and the average nuclear charge values (Zp) of fission products of the mass chains 90 and 91. Values in brackets are from ref. [3]. Mass
ssBr
36Kr
90
0.106 ± 0.01 (0.109 ± 0.025) 0.040 ± 0.005 (0.044 ± 0.018)
0.753 (0.761 0.542 (0.546
91
37Rb ± 0.02 ± 0.032) ± 0.01 ± 0.015)
We shall apply this m e t h o d o f determining fractional independent yields to all the isobaric chains for which the ~ E detector can resolve the nuclear charges reasonably w e l l
0.142 (0.130 0.373 (0.380
± 0.01 ± 0.02 ± 0.01 ± 0.03)
ss Sr
Zp
0.045 ± 0.05 (0.030 ± 0.030)
36.04 (36.02 36.42 (36.37)
References [1] p. Armbruster et al., Axkiv L Fysik 36 (1967) 305; E. Moll et al., Proc- 8th int. EMIS conf., Sk6vde, Sweden (1973) 249. [2] H. Gunther, G. Siegert, R.L. Ferguson, H. Ewald and E. Kunecny, Nucl. Phys. A196 (1972) 401. [3] S. Amiel and H. Feldstein, IAEA/SMoI74/25 (1973). [4] E. Konecny, H. Gunther, G. Siegert and I~ Winter, Nucl. Phys. A100 (1967) 465.
47