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γ-Decay towards the shape-isomeric ground state of 239U
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A 589 (1995) 435-444
y-Decay towards the shape-isomeric ground state of 239U S. Oberstedt a, F. Gunsing b a Studiecentrum voor Kernenergie SCK-CEN, Boeretang 200, B-2400 Mol, Belgium b Institute for Reference Materials and Measurements IRMM, Retieseweg, B-2440 Geel, Belgium
Received 25 November 1994; revised 3 March 1995
Abstract In a high-energy-resolution measurement of y-rays from neutron capture in 238U five hitherto unknown y-transitions could be found exclusively in the y-ray spectra of s-wave neutron subthreshold fission resonances around 720 eV. Since it is generally believed that the strongest fission resonance in this cluster at a neutron energy En = 721.6 eV is a nearly pure class-II state, these transitions might be the first experimental observation of y-decay populating the shape-isomeric ground state in 239U. This assumption is supported by a quantitative analysis of the relative peak areas of each individual y-transition amongst these fission resonances, which show a strong correlation with the respective class-II admixture into the observed neutron resonance. Keywords: NUCLEAR REACTIONS 238U(n,y), E = 600-800 eV; measured E~,,Ir. 239Udeduced shape
isomeric ground-state feeding.
1. Introduction In fission research a still unresolved problem is the unambiguous determination o f the shape o f the double humped fission barrier in actinide nuclei with Z < 94, which in the parabola approximation reduces to the determination o f the heights (relative to the normal ground state) and curvatures of the inner and outer barriers. Although the interpretation o f the so-called intermediate-structure phenomenon as the coupling between a so-called class-II state located above the second potential minimum and class-I states located in the first potential well leads to a better understanding of the fission process at subthreshold neutron energies [ 1,2], this does imply more transparent barriers in the compound nuclei 239U and 238Np than what is expected from the analysis o f fast-fission data. An attempt to bring shape-isomeric (delayed) fission with a halt' 0375-9474/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDI 0375-9474(95) 0008 I-X
436
s. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
life of the order of a few tens of ns into play to solve this discrepancy [3] could not be confirmed experimentally [4]. There it was shown, that a possibly existing shape isomer in 239U should have a half life larger than 0.25 /xs decaying to a considerably large extent via y-decay back to the normal ground state. Indeed, the half life of the shape isomer for fission or y-decay back to the normal ground state provides a good measure for the penetrability of the outer or inner barrier, respectively. The population of such a shape isomer in slow neutron-capture reactions proceeds through y-decay of an excited quasi-class-II state, which is mainly located in the intermediate well. As a consequence additional y-transitions should appear in the y-ray spectrum of the corresponding resonances. The best known, essentially pure class-II state in 239U is populated by s-wave neutron capture in 238U at a neutron energy En = 721.6 eV. This resonance appears strongest in the intermediate structure around En = 720 eV in the subthreshold fission cross section of 238U. Its high class-II admixture into the observed resonance ( ~ 80%) and its small fission width makes 239Uthe most suitable compound system to investigate y-decay towards the shape-isomeric ground state.
2. Experimental conditions and data analysis In the course of a neutron-capture experiment, which has been performed at the time-of-flight spectrometer GELINA of the Institute for Reference Materials and Measurements in order to investigate p-wave neutron resonances in 239Ubelow an incident neutron energy En = 400 eV [5], y-ray spectra with high energy resolution were obtained for s-wave resonances between 600 and 800 eV. For the measurement a disc of highly enriched 23Su (9 ppm 235U) of 11.1 cm diameter and 694 g total weight was placed in the neutron beam with its normal at 30 ° with respect to the beam axis. The sample was viewed by a coaxial intrinsic germanium detector of 70% efficiency, placed at an angle of 120 ° with respect to the neutron beam direction. The y-energy resolution of the detector at about 1.5 MeV was measured to be 5.5 keV at full-width half-maximum (FWHM) by means of well-known y-ray transitions from s-wave resonances. In order to protect the detector against neutrons scattered from the sample a shielding consisting of a mixture of Li2CO3 and wax lined with 6Li metal was placed around sample and detector. The whole assembly was shielded against outside radiation with walls made out of lead and a mixture of boric acid and wax with a thickness of 10 cm each. For each event the amplitude information measured with a fast 8k ADC for the yray energy range between 0.3 and 4.81 MeV, which is the neutron separation energy of the compound nucleus 239U, and the time-of-flight information, measured with a 25 bit multiple-shot time digitizer, were recorded in list mode. In Fig. 1 the time-offlight spectrum corresponding to the neutron energy between 600 and 800 eV is shown. Since the experiment has been designed for the investigation of weak epithermal p-wave neutron resonances [6,7], a short flight path of 12.85 m was chosen, and resonances appear with a poor resolution at higher energies in the time-of-flight spectrum. Since we
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
2.2xl 0 s
,,,~
....
I ....
i ....
I ....
i ....
I ....
i ....
I ....
i ....
I , , , , l , , , l l
....
i ....
661.1
2.0
73ol ev
437
Iiiii
eV
/
e~ 1.61 ~ ~ ¢,,
1,4~ . 1.2-
8
1.0~ 0.8 ~ 0.61 0
20
40
60
80
1O0
120
140
channel (32 n s / e h ) Fig. 1. Time-of-ttight spectrum at neutron energies between 600 and 800 eV. The investigated s-wave resonances are indicated. From the hatched area a background 7-ray spectrum was obtained.
were only searching for additional decay lines in the y-ray spectra and not interested in their absolute intensities, our investigations were not affected by that. During the analysis y-ray spectra were generated from the experimental data for swave fission resonances at neutron energies En = 708.3, 721.6, 730.1 and 765.1 eV, respectively, as indicated in Fig. 1. These four resonances carry almost 100% of the fission intensity of this intermediate structure. For comparison y-ray spectra from 12 other s-wave fission as well as pure capture resonances between En = 20 and 700 eV [ 8 ], from which is assumed that they are pure class-I resonances, were also investigated. From the hatched area in Fig. 1 a y-ray spectrum was obtained for background correction. In a first step a background subtraction was applied to all y-ray spectra. The contents of each channel in the corrected spectrum Yi,res is: Yi,res -- Y/,tot - ( N r e s / N b g ) Y/,bg,
(i)
where Y/,tot and Y/,bg denote the channel contents in the uncorrected spectrum of the resonance and in the background spectrum, respectively. Nres and Nbg denote the number of time-of-flight channels from which the spectra were constructed in the analysis. After that, the channels of the corrected spectra were converted to the corresponding y-ray energy. The respective conversion factor is 0.6 keV per ADC channel. Then, all spectra were investigated for y-transitions, which exclusively appear in spectra of those fission resonances carrying a class-II fraction, and therefore might be attributed to y-decay within the second potential well. In the y-ray spectrum of the 721.6 eV resonance several statistically significant ytransitions could be found, which have not been mentioned yet in literature [9-13]. Two very clear peaks appear at y-ray energies of 1340 and 1480 keV and a smaller
438
S. Oberstedt, F. Gunsing/Nuclear Physics A 589 (1995) 435-444
1100: 1000] 900 t 800]
700]
"~ 400C Annn~ 0
1300
1400
1500
E~(keY) Fig. 2. High-resolution T-my spectra from s-wave neutron resonances in 239U at neutron energies of (a) 721.6 eV and (b) 661.1 eV, respectively. The thick full lines show the result of a least-square fit to the experimental data. In this representation one ADC channel corresponds to a v-energy interval of 0.6 keV. The energies of the transitions, which are ascribed to v-decay in the second potential well, are indicated.
peak at Er ~ 1300 keV. They are also visible to a much weaker extent in the other fission resonances around En = 720 eV but in no spectrum of any other investigated neutron resonance. Since about 15 s-wave resonances have been investigated, their missing in any other )'-ray spectrum cannot be attributed to Porter-Thomas fluctuations of the respective partial T-decay widths from resonance to resonance [ 14]. In Fig. 2 the background-corrected T-ray spectra for the relevant )'-energy region is shown. The upper part shows the ),-ray spectrum at En = 721.6 eV and the lower part the corresponding )'-ray spectrum of the neutron-capture resonance at En = 661.1 eV for comparison. Although the accumulated counts for higher )'-energies, Er ~ 3 MeV, are rather low, some indications exist for further weak transitions at Er = 2933 and 3110 keV, which do not appear in )'-ray spectra of the investigated pure capture resonances. These transitions are shown in the upper part of Fig. 3 together with its counterpart at En = 661.1 eV in the lower part. However, due to their weakness these transitions will not be considered in the further quantitative analysis. In the following the investigations will be restricted to the )'-transitions found between 1240 and 1580 keV, which are shown in Fig. 2. First, the peak resolution was determined as a function of the )'-ray energy, which turned out to be linear. During the peak shape
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
439
¢.. ~nn
o o
-
2900
3000
3100
3200
E~(keV) Fig. 3. High-energy y-ray spectra from s-wave neutron resonances in 239U at neutron energies of (a) 721.6 eV and (b) 661.1 eV, respectively. In this representation one ADC channel corresponds to a y-energy interval of 0.6 keV. The energies of the y-transitions, which may be ascribed to decay within the second potential well, are indicated. However, due to their weakness we restrict ourselves to a purely qualitative assignment without fitting the experimental data.
analysis the FWHM of the gaussian function fitted to the experimental data was then chosen according to the calibration function and kept fixed during the least-square minimization: n
Y( X) = p( X) + ~ & exp [ - In 16(X
-
Er,j)2 / F W H M 2 ( E.r,j) ] ,
(2)
j=l
where n denotes the number of peaks and E~,j the respective peak position. For the baseline p(X) a quadratic polynomial function was assumed. From this analysis it turned out, that the peaks around Er = 1340 and 1480 keV have a much larger FWHM than expected from the calibration function and therefore may be interpreted as doublets with energies at (1339.2, 1344.6) and (1475.8, 1.479.6) keV, respectively. The weak y-decay line in the spectrum of the 661.1 eV resonance (see Fig. 2b) at E r = 1346.2 keV may not be neglected during the peak analysis of the 1344.6 keV y-line of the 721.6 eV resonance. However, since both peak positions differ less than one FWHM the respective peak areas cannot be determined uniquely. The result of a least-square fit to the experimental data in Figs. 2a and 2b is shown by the thick full line. The most left peak indicated in Fig. 2a corresponds to a y-energy E~, = 1299.2 keV.
440
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
Table 1 Resonance parameters used to theoreticallyestimate the relative peak area lrt~n°r (from Refs. [4,18] ) E,
r,
rf
rr
r
(eV)
(meV)
(meV)
(meV)
(meV)
708.3 721.6 730.1 765.1
22 4- 2 1.64-0.2 1.04-0.1 7.04- 1.0
0.034 4- 0.001 0.80 4-0.15 0.1664-0.010 0.0084-0.001
23.1 4- 1.2 12.04-2.9 20.84-3.4 17.04-2.0
45.1 4- 2.3 12.84-2.9 21.64-3.4 24.04-2.4
cH
cn l~n/ r
( 10-3) 0.03 4- 0.00 0.794-0.17 0.174-0.02 0.01 4-0.00
15 4- 2 994-33 84-2 34- 1
Once the respective peak positions were obtained with good statistics from the )'-ray spectrum of the 721.6 eV resonance, they were also kept fixed during the analysis of the )'-ray spectra from the other resonances. The uncertainty in energy calibration is about 1 keV.
3. Results and discussion At the present stage of the analysis it is already indicated to ascribe the T-transitions exclusively found in the )'-ray spectra of s-wave resonances in the intermediate structure around En = 720 eV to )'-decay within the second potential well towards the shapeisomeric ground state. Since we did not observe these transitions in any other )'-ray spectrum and taking into account the number of investigated s-wave resonances, the possibility of having observed )'-decay within the first potential well may be excluded. The alternative interpretation as hitherto unobserved deexcitation )'-quanta from fission fragments seems also to be very unlikely, because these )'-transitions, which appear to be relatively strong at least in the spectrum of the 721.6 eV resonance, are not observed in any )'-ray spectrum of the fission resonances of class-I type at neutron energies below 700 eV. Moreover, these )'-transitions have not been reported earlier from investigations of the deexcitation of fission fragments [ 11-13,15,16]. Although these results were obtained by investigating neighbouring isotopes, e.g. 235'236'238U, fission-fragment distributions do not differ that much from one fissioning isotope to its neighbour. At first sight it is interesting that, if the low-energy component of one doublet is simply combined with the high-energy component of the other to a 2)'-cascade, both combinations sum up to an energy of 2.82 MeV considering the accuracy of the underlying energy calibration. This might imply a shape-isomeric ground-state energy Eli = Bn - 2.82 MeV ~ 2.0 MeV which is compatible with limits given in Ref. [4]. However, there is absolutely no indication for a )'-transition at 2.82 MeV. On the other hand, the possibly existing weak transition at about 3.1 MeV might imply EII ~ 1.7 MeV in pretty good agreement with estimations based on experimentally determined class-II level spacings in 239U [8,17]. Anyhow, one strong support for the assumption of indeed having observed )'-decay in the second potential well would be, if the relative peak areas I v of these )'-decay lines
S. Oberstedt, F. Gunsing/Nuclear Physics A 589 (1995) 435-444
441
are correlated with the class-II admixture cll into the observed resonances j, which may roughly be estimated from the fission widths Ffj:
4
(3)
Cll'i = Ffi/ Z /-'fj' j=l
where the summation extends over the four fission resonances mentioned above. The area of a neutron-capture resonance i:
A~i) ~ Fni/Fi [(1 - Cll,i)Fyl -1- Cll,iF~,2] ,
(4)
where Fni is the neutron width, and F~,l and F~,2 are the total ,y-decay widths in the first and second potential well, respectively. Then, the area of a single "y-transition from A(i) may be calculated by the cascade towards the shape-isomeric ground state ,lrn,a A(i) 71i,A ,-w Cll,iFy2,,~Fni/ Fi '
(5)
where Fr2,a is the partial decay width for the transition ,L Finally, the expected relative peak areas of class-II T-transitions may be estimated by
lthe°r(i) 1 ) ~,,II = cn,iFni/ ( Fi~ _ ~Cil,kFnk/Fk
.
(6)
All resonance parameters used to estimate the relative peak areas l ~ i °r are listed in Table 1. The fission width Ff of the 721.6 eV resonance was chosen as the average from the values given in Ref. [4], and the other Ff values were taken from Ref. [ 18]. If not explicitly known, a total ,y-decay width in the first potential well Frl = 23.2 meV [ 18] was used. From the estimated shape-isomeric ground-state energy En < 2.0 MeV and estimates given in Ref. [4] Fr2 = 9 ± 1 meV has been chosen. In order to compare the measured peak areas with the estimated /theor "r,II , they have to be normalized to their respective total peak area summed over the four major fission resonances of the intermediate structure according to:
4
1~,,./= gjFWHM ( Er,j) / Z giFWHM (gy,i),
( 7)
i=1 where the symbols have the same meaning as in Eq. (2). At the same time relative peak areas of well-known "y-transitions of the "y-cascade within the first potential well should show an anti-correlation with 1theor "y,ll due to the first term on the right-hand side of Eq. (4) and because the subsequent y-deexcitation from the shape-isomeric ground state may also proceed through other decay branches. In Table 2 the relative peak area 1~ of all investigated y-transitions are listed for each of the four fission resonances. In the last row of Table 2 the estimated relative peak areas for a transition within the second potential well l~h,~l°r are given. In the last column of Table 2 the correlation coefficient r of the relative peak areas I v with their estimated values is given. As expected the relative
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
442
Table 2 Relative peak area I r of all investigated 7-decay lines. The * denotes 7-decay lines ascribed to decay within the second potential well. The other transitions are from the decay within the first potential well. In the last column the coefficient r for the correlation of the relative peak areas with the theoretical values I 7,11 the°r is given Er (keV)
1299.2 1339.2 1344.6 1475.8 1479.6 629.7 638.5 787.2 794.1 ltheor 7,II
* * * * *
/r
r
En (eV) =708.3
721.6
730.1
765.1
0.024-0.08 0.09 4- 0.05 0.18 4- 0.23 0.12 4- 0.04 0.02-4-0.07 0.29 4- 0.02 0.344-0.02 0.35 4- 0.04 0.27 4- 0.03
0.424-0.10 0.59 4- 0.07 0.52 4- 0.52 0.67 -t- 0.09 0.694-0.18 0.15 4- 0.01 0.114-0.01 0.06 4- 0.02 0.13 4- 0.02
0.354-0.11 0.29 4- 0.06 0.23 4- 0.24 0.15 -t- 0.04 0.294-0.08 0.31 -t- 0.02 0.344-0.02 0.27 4- 0.03 0.30 4- 0.03
0.214-0.09 0.03 4- 0.04 0.07 + 0.19 0.07 4- 0.04 0.004-0.05 0.25 4- 0.01 0.214-0.01 0.33 4- 0.03 0.29 4- 0.03
0.12 4- 0.04
0.79 4- 0.34
0.07 4- 0.03
0.02 4- 0.01
0.589 0.903 0.958 0.995 0.909 -0.905 -0.777 -0.954 -0.993 -
peak areas of all presumable class-II transitions, in Table 2 indicated by an asterisk, show a high correlation with the estimated values (correlation coefficient r close to unity), whereas the normal class-I transitions show a strong anti-correlation with rvalues less than -0.77. In Fig. 4 the relative peak area averaged over all presumable class-II transitions Ir,n is shown as a function of the neutron resonance energy En ([~)
1.0
o.8~
q
0.6
0.4
:iiiiii .......................... 0.2
0.0 . 700
.
.
. . 720
.
.
.
. . 740
.
~ 760
. 780
E n (eV) Fig. 4. Relative peak area 17 as a function of the neutron resonance energy compared to the estimated values l~,.l~r based on the neutron resonance parameters (o): The symbols (I-7) and ( ~ ) give the relative peak areas averaged over all presumable class-ll transitions, l~,,n, and normal class-I transitions, It. 1, respectively. The peak areas are normalized to the total peak areas summed over the four major resonances of the intermediate structure at En = 720 eV.
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
443
together with the estimated values l ~ l °r ( o ) . Still the characteristic correlation holds and the corresponding correlation coefficient rn = 0.931. For comparison the averaged relative peak areas lr,i o f the class-I transitions are also included ( / ~ ) , and the respective correlation coefficient rl = --0.955. The indicated error margins extend over one standard deviation from the mean values. Both the high correlation of lr,lt as well as the anti-correlation o f Ir,i with the class-II admixture cn into the observed resonances strongly support the assumption that indeed T-transitions within the second potential well are observed.
4. Conclusion From a neutron-capture experiment T-ray spectra with a high energy resolution were obtained for 239U in the neutron energy region between 600 and 800 eV. Five hitherto unknown T-transitions with T-energies at 1298.8, 1339.2, 1344.6, 1475.8 and 1479.6 keV could be found exclusively in the s-wave resonances within the intermediate structure around En = 720 eV. Their relative peak areas 1~, in the T-ray spectra of each resonance show a high correlation with the corresponding class-II fraction estimated from the experimentally observed fission widths. At the same time well-known T-transitions towards the normal ground state are attenuated. Therefore, it is concluded, that for the first time y-decay within the second potential well is observed feeding the shape-isomeric ground state in 239U. Rather weak transitions around Er = 2.9 and 3.1 MeV in the ?/-ray spectrum o f the strongest fission resonance at E, = 721.6 eV indicate a shape-isomeric ground-state energy of about 1.7 MeV in good agreement with results obtained from subthreshold fission measurements. Nevertheless, due to the small number of accumulated counts in this energy region transitions with higher T-energies within the second potential well may not yet be excluded definitely.
References I II H. Weigmann, Z. Phys. 214 (1968) 7. 121 J.E. Lynn, Proc. 2nd IAEA Symp. on Physics and chemistry of fission, Vienna, 28 July-1 August 1969 (IAEA, Vienna, 1969) p. 249. 131 J.E. Lynn, Proc. Int. Conf. 50 Years with nuclear fission, vol. 1, Gaithersburg, 25-28 April 1989 (1989) p. 418. [4] S. Oberstedt, J.P. Theobald, H. Weigmann, J.A. Wartena and C. Btirkholz, Nucl. Phys. A 573 (1994) 467. 151 F. Gunsing, Ph.D. thesis, Delft University (1995). 161 F. Corvi, F. Gunsing, H. Postma and K. Athanassopulos, in Time reversal invariance and parity violation in neutron reactions, eds. C. Gould, J.D. Bowman and Yu.P. Popov (Word Scientific, Singapore, 1994) p. 79. 171 E Gunsing, E Corvi, K. Athanassopulos, H. Postma, Yu.P. Popov and E.I. Shaparov, in Capture gammaray spectroscopy and related topics, ed. J. Kern (World Scientific, Singapore, 1994) p. 797. 181 EC. Difillipo, R.B. Perez, G. de Saussure, D.K. Olsen and R.W. Ingle, Phys. Rev. C 21 (1980) 1400. 191 Nucl. Data Sheets 66 (1992) 4.
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[10] L.V. East and G.R. Keepin, in Proc. 2rid IAEA Symp. on Physics and chemistry of fission, Vienna, 28 July-1 August 1969 (IAEA, Vienna, 1969) p. 647. [ 11 ] R.G. Graves, R.E. Chrien, D.I. Garber, G.W. Cole and O.A. Wasson, Phys. Rev. C 8 (1973) 781. [ 12] W.J. Schindler and C.M. Fleck, Nucl. Phys. A 206 (1973) 374. [ 13] T.A. Khan, D. Hofmann and E Horsch, Nucl. Phys. A 205 (1973) 488. [ 14] J.E. Lynn, The theory of neutron resonance reactions (Clarendon, Oxford, 1968). [ 15] A.A. Bogdzel et al., JINR Internal Report P3-87-862, Dubna (1987). [ 16] K. Perseyn, S. Pomm6, E. Jacobs, D. de Frenne, K. Govaert and M.-L. Yoneama, Proc. Int. Workshop on High resolution spectroscopy of fission fragments, neutrons and y-rays, Dresden (1993) p. 57. [17] G.E Auchampaugh, G. de Saussure, D.K. Olsen, R.W. Ingle, R.B. Perez and R.L. Macklin, Phys. Rev. C 33 (1986) 125. [18] S.E Mughabghab, Neutron cross sections, vol. 1, part B (Academic Press, New York, 1984).
Nuclear Physics A 589 (1995) 435-444
y-Decay towards the shape-isomeric ground state of 239U S. Oberstedt a, F. Gunsing b a Studiecentrum voor Kernenergie SCK-CEN, Boeretang 200, B-2400 Mol, Belgium b Institute for Reference Materials and Measurements IRMM, Retieseweg, B-2440 Geel, Belgium
Received 25 November 1994; revised 3 March 1995
Abstract In a high-energy-resolution measurement of y-rays from neutron capture in 238U five hitherto unknown y-transitions could be found exclusively in the y-ray spectra of s-wave neutron subthreshold fission resonances around 720 eV. Since it is generally believed that the strongest fission resonance in this cluster at a neutron energy En = 721.6 eV is a nearly pure class-II state, these transitions might be the first experimental observation of y-decay populating the shape-isomeric ground state in 239U. This assumption is supported by a quantitative analysis of the relative peak areas of each individual y-transition amongst these fission resonances, which show a strong correlation with the respective class-II admixture into the observed neutron resonance. Keywords: NUCLEAR REACTIONS 238U(n,y), E = 600-800 eV; measured E~,,Ir. 239Udeduced shape
isomeric ground-state feeding.
1. Introduction In fission research a still unresolved problem is the unambiguous determination o f the shape o f the double humped fission barrier in actinide nuclei with Z < 94, which in the parabola approximation reduces to the determination o f the heights (relative to the normal ground state) and curvatures of the inner and outer barriers. Although the interpretation o f the so-called intermediate-structure phenomenon as the coupling between a so-called class-II state located above the second potential minimum and class-I states located in the first potential well leads to a better understanding of the fission process at subthreshold neutron energies [ 1,2], this does imply more transparent barriers in the compound nuclei 239U and 238Np than what is expected from the analysis o f fast-fission data. An attempt to bring shape-isomeric (delayed) fission with a halt' 0375-9474/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDI 0375-9474(95) 0008 I-X
436
s. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
life of the order of a few tens of ns into play to solve this discrepancy [3] could not be confirmed experimentally [4]. There it was shown, that a possibly existing shape isomer in 239U should have a half life larger than 0.25 /xs decaying to a considerably large extent via y-decay back to the normal ground state. Indeed, the half life of the shape isomer for fission or y-decay back to the normal ground state provides a good measure for the penetrability of the outer or inner barrier, respectively. The population of such a shape isomer in slow neutron-capture reactions proceeds through y-decay of an excited quasi-class-II state, which is mainly located in the intermediate well. As a consequence additional y-transitions should appear in the y-ray spectrum of the corresponding resonances. The best known, essentially pure class-II state in 239U is populated by s-wave neutron capture in 238U at a neutron energy En = 721.6 eV. This resonance appears strongest in the intermediate structure around En = 720 eV in the subthreshold fission cross section of 238U. Its high class-II admixture into the observed resonance ( ~ 80%) and its small fission width makes 239Uthe most suitable compound system to investigate y-decay towards the shape-isomeric ground state.
2. Experimental conditions and data analysis In the course of a neutron-capture experiment, which has been performed at the time-of-flight spectrometer GELINA of the Institute for Reference Materials and Measurements in order to investigate p-wave neutron resonances in 239Ubelow an incident neutron energy En = 400 eV [5], y-ray spectra with high energy resolution were obtained for s-wave resonances between 600 and 800 eV. For the measurement a disc of highly enriched 23Su (9 ppm 235U) of 11.1 cm diameter and 694 g total weight was placed in the neutron beam with its normal at 30 ° with respect to the beam axis. The sample was viewed by a coaxial intrinsic germanium detector of 70% efficiency, placed at an angle of 120 ° with respect to the neutron beam direction. The y-energy resolution of the detector at about 1.5 MeV was measured to be 5.5 keV at full-width half-maximum (FWHM) by means of well-known y-ray transitions from s-wave resonances. In order to protect the detector against neutrons scattered from the sample a shielding consisting of a mixture of Li2CO3 and wax lined with 6Li metal was placed around sample and detector. The whole assembly was shielded against outside radiation with walls made out of lead and a mixture of boric acid and wax with a thickness of 10 cm each. For each event the amplitude information measured with a fast 8k ADC for the yray energy range between 0.3 and 4.81 MeV, which is the neutron separation energy of the compound nucleus 239U, and the time-of-flight information, measured with a 25 bit multiple-shot time digitizer, were recorded in list mode. In Fig. 1 the time-offlight spectrum corresponding to the neutron energy between 600 and 800 eV is shown. Since the experiment has been designed for the investigation of weak epithermal p-wave neutron resonances [6,7], a short flight path of 12.85 m was chosen, and resonances appear with a poor resolution at higher energies in the time-of-flight spectrum. Since we
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
2.2xl 0 s
,,,~
....
I ....
i ....
I ....
i ....
I ....
i ....
I ....
i ....
I , , , , l , , , l l
....
i ....
661.1
2.0
73ol ev
437
Iiiii
eV
/
e~ 1.61 ~ ~ ¢,,
1,4~ . 1.2-
8
1.0~ 0.8 ~ 0.61 0
20
40
60
80
1O0
120
140
channel (32 n s / e h ) Fig. 1. Time-of-ttight spectrum at neutron energies between 600 and 800 eV. The investigated s-wave resonances are indicated. From the hatched area a background 7-ray spectrum was obtained.
were only searching for additional decay lines in the y-ray spectra and not interested in their absolute intensities, our investigations were not affected by that. During the analysis y-ray spectra were generated from the experimental data for swave fission resonances at neutron energies En = 708.3, 721.6, 730.1 and 765.1 eV, respectively, as indicated in Fig. 1. These four resonances carry almost 100% of the fission intensity of this intermediate structure. For comparison y-ray spectra from 12 other s-wave fission as well as pure capture resonances between En = 20 and 700 eV [ 8 ], from which is assumed that they are pure class-I resonances, were also investigated. From the hatched area in Fig. 1 a y-ray spectrum was obtained for background correction. In a first step a background subtraction was applied to all y-ray spectra. The contents of each channel in the corrected spectrum Yi,res is: Yi,res -- Y/,tot - ( N r e s / N b g ) Y/,bg,
(i)
where Y/,tot and Y/,bg denote the channel contents in the uncorrected spectrum of the resonance and in the background spectrum, respectively. Nres and Nbg denote the number of time-of-flight channels from which the spectra were constructed in the analysis. After that, the channels of the corrected spectra were converted to the corresponding y-ray energy. The respective conversion factor is 0.6 keV per ADC channel. Then, all spectra were investigated for y-transitions, which exclusively appear in spectra of those fission resonances carrying a class-II fraction, and therefore might be attributed to y-decay within the second potential well. In the y-ray spectrum of the 721.6 eV resonance several statistically significant ytransitions could be found, which have not been mentioned yet in literature [9-13]. Two very clear peaks appear at y-ray energies of 1340 and 1480 keV and a smaller
438
S. Oberstedt, F. Gunsing/Nuclear Physics A 589 (1995) 435-444
1100: 1000] 900 t 800]
700]
"~ 400C Annn~ 0
1300
1400
1500
E~(keY) Fig. 2. High-resolution T-my spectra from s-wave neutron resonances in 239U at neutron energies of (a) 721.6 eV and (b) 661.1 eV, respectively. The thick full lines show the result of a least-square fit to the experimental data. In this representation one ADC channel corresponds to a v-energy interval of 0.6 keV. The energies of the transitions, which are ascribed to v-decay in the second potential well, are indicated.
peak at Er ~ 1300 keV. They are also visible to a much weaker extent in the other fission resonances around En = 720 eV but in no spectrum of any other investigated neutron resonance. Since about 15 s-wave resonances have been investigated, their missing in any other )'-ray spectrum cannot be attributed to Porter-Thomas fluctuations of the respective partial T-decay widths from resonance to resonance [ 14]. In Fig. 2 the background-corrected T-ray spectra for the relevant )'-energy region is shown. The upper part shows the ),-ray spectrum at En = 721.6 eV and the lower part the corresponding )'-ray spectrum of the neutron-capture resonance at En = 661.1 eV for comparison. Although the accumulated counts for higher )'-energies, Er ~ 3 MeV, are rather low, some indications exist for further weak transitions at Er = 2933 and 3110 keV, which do not appear in )'-ray spectra of the investigated pure capture resonances. These transitions are shown in the upper part of Fig. 3 together with its counterpart at En = 661.1 eV in the lower part. However, due to their weakness these transitions will not be considered in the further quantitative analysis. In the following the investigations will be restricted to the )'-transitions found between 1240 and 1580 keV, which are shown in Fig. 2. First, the peak resolution was determined as a function of the )'-ray energy, which turned out to be linear. During the peak shape
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
439
¢.. ~nn
o o
-
2900
3000
3100
3200
E~(keV) Fig. 3. High-energy y-ray spectra from s-wave neutron resonances in 239U at neutron energies of (a) 721.6 eV and (b) 661.1 eV, respectively. In this representation one ADC channel corresponds to a y-energy interval of 0.6 keV. The energies of the y-transitions, which may be ascribed to decay within the second potential well, are indicated. However, due to their weakness we restrict ourselves to a purely qualitative assignment without fitting the experimental data.
analysis the FWHM of the gaussian function fitted to the experimental data was then chosen according to the calibration function and kept fixed during the least-square minimization: n
Y( X) = p( X) + ~ & exp [ - In 16(X
-
Er,j)2 / F W H M 2 ( E.r,j) ] ,
(2)
j=l
where n denotes the number of peaks and E~,j the respective peak position. For the baseline p(X) a quadratic polynomial function was assumed. From this analysis it turned out, that the peaks around Er = 1340 and 1480 keV have a much larger FWHM than expected from the calibration function and therefore may be interpreted as doublets with energies at (1339.2, 1344.6) and (1475.8, 1.479.6) keV, respectively. The weak y-decay line in the spectrum of the 661.1 eV resonance (see Fig. 2b) at E r = 1346.2 keV may not be neglected during the peak analysis of the 1344.6 keV y-line of the 721.6 eV resonance. However, since both peak positions differ less than one FWHM the respective peak areas cannot be determined uniquely. The result of a least-square fit to the experimental data in Figs. 2a and 2b is shown by the thick full line. The most left peak indicated in Fig. 2a corresponds to a y-energy E~, = 1299.2 keV.
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S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
Table 1 Resonance parameters used to theoreticallyestimate the relative peak area lrt~n°r (from Refs. [4,18] ) E,
r,
rf
rr
r
(eV)
(meV)
(meV)
(meV)
(meV)
708.3 721.6 730.1 765.1
22 4- 2 1.64-0.2 1.04-0.1 7.04- 1.0
0.034 4- 0.001 0.80 4-0.15 0.1664-0.010 0.0084-0.001
23.1 4- 1.2 12.04-2.9 20.84-3.4 17.04-2.0
45.1 4- 2.3 12.84-2.9 21.64-3.4 24.04-2.4
cH
cn l~n/ r
( 10-3) 0.03 4- 0.00 0.794-0.17 0.174-0.02 0.01 4-0.00
15 4- 2 994-33 84-2 34- 1
Once the respective peak positions were obtained with good statistics from the )'-ray spectrum of the 721.6 eV resonance, they were also kept fixed during the analysis of the )'-ray spectra from the other resonances. The uncertainty in energy calibration is about 1 keV.
3. Results and discussion At the present stage of the analysis it is already indicated to ascribe the T-transitions exclusively found in the )'-ray spectra of s-wave resonances in the intermediate structure around En = 720 eV to )'-decay within the second potential well towards the shapeisomeric ground state. Since we did not observe these transitions in any other )'-ray spectrum and taking into account the number of investigated s-wave resonances, the possibility of having observed )'-decay within the first potential well may be excluded. The alternative interpretation as hitherto unobserved deexcitation )'-quanta from fission fragments seems also to be very unlikely, because these )'-transitions, which appear to be relatively strong at least in the spectrum of the 721.6 eV resonance, are not observed in any )'-ray spectrum of the fission resonances of class-I type at neutron energies below 700 eV. Moreover, these )'-transitions have not been reported earlier from investigations of the deexcitation of fission fragments [ 11-13,15,16]. Although these results were obtained by investigating neighbouring isotopes, e.g. 235'236'238U, fission-fragment distributions do not differ that much from one fissioning isotope to its neighbour. At first sight it is interesting that, if the low-energy component of one doublet is simply combined with the high-energy component of the other to a 2)'-cascade, both combinations sum up to an energy of 2.82 MeV considering the accuracy of the underlying energy calibration. This might imply a shape-isomeric ground-state energy Eli = Bn - 2.82 MeV ~ 2.0 MeV which is compatible with limits given in Ref. [4]. However, there is absolutely no indication for a )'-transition at 2.82 MeV. On the other hand, the possibly existing weak transition at about 3.1 MeV might imply EII ~ 1.7 MeV in pretty good agreement with estimations based on experimentally determined class-II level spacings in 239U [8,17]. Anyhow, one strong support for the assumption of indeed having observed )'-decay in the second potential well would be, if the relative peak areas I v of these )'-decay lines
S. Oberstedt, F. Gunsing/Nuclear Physics A 589 (1995) 435-444
441
are correlated with the class-II admixture cll into the observed resonances j, which may roughly be estimated from the fission widths Ffj:
4
(3)
Cll'i = Ffi/ Z /-'fj' j=l
where the summation extends over the four fission resonances mentioned above. The area of a neutron-capture resonance i:
A~i) ~ Fni/Fi [(1 - Cll,i)Fyl -1- Cll,iF~,2] ,
(4)
where Fni is the neutron width, and F~,l and F~,2 are the total ,y-decay widths in the first and second potential well, respectively. Then, the area of a single "y-transition from A(i) may be calculated by the cascade towards the shape-isomeric ground state ,lrn,a A(i) 71i,A ,-w Cll,iFy2,,~Fni/ Fi '
(5)
where Fr2,a is the partial decay width for the transition ,L Finally, the expected relative peak areas of class-II T-transitions may be estimated by
lthe°r(i) 1 ) ~,,II = cn,iFni/ ( Fi~ _ ~Cil,kFnk/Fk
.
(6)
All resonance parameters used to estimate the relative peak areas l ~ i °r are listed in Table 1. The fission width Ff of the 721.6 eV resonance was chosen as the average from the values given in Ref. [4], and the other Ff values were taken from Ref. [ 18]. If not explicitly known, a total ,y-decay width in the first potential well Frl = 23.2 meV [ 18] was used. From the estimated shape-isomeric ground-state energy En < 2.0 MeV and estimates given in Ref. [4] Fr2 = 9 ± 1 meV has been chosen. In order to compare the measured peak areas with the estimated /theor "r,II , they have to be normalized to their respective total peak area summed over the four major fission resonances of the intermediate structure according to:
4
1~,,./= gjFWHM ( Er,j) / Z giFWHM (gy,i),
( 7)
i=1 where the symbols have the same meaning as in Eq. (2). At the same time relative peak areas of well-known "y-transitions of the "y-cascade within the first potential well should show an anti-correlation with 1theor "y,ll due to the first term on the right-hand side of Eq. (4) and because the subsequent y-deexcitation from the shape-isomeric ground state may also proceed through other decay branches. In Table 2 the relative peak area 1~ of all investigated y-transitions are listed for each of the four fission resonances. In the last row of Table 2 the estimated relative peak areas for a transition within the second potential well l~h,~l°r are given. In the last column of Table 2 the correlation coefficient r of the relative peak areas I v with their estimated values is given. As expected the relative
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
442
Table 2 Relative peak area I r of all investigated 7-decay lines. The * denotes 7-decay lines ascribed to decay within the second potential well. The other transitions are from the decay within the first potential well. In the last column the coefficient r for the correlation of the relative peak areas with the theoretical values I 7,11 the°r is given Er (keV)
1299.2 1339.2 1344.6 1475.8 1479.6 629.7 638.5 787.2 794.1 ltheor 7,II
* * * * *
/r
r
En (eV) =708.3
721.6
730.1
765.1
0.024-0.08 0.09 4- 0.05 0.18 4- 0.23 0.12 4- 0.04 0.02-4-0.07 0.29 4- 0.02 0.344-0.02 0.35 4- 0.04 0.27 4- 0.03
0.424-0.10 0.59 4- 0.07 0.52 4- 0.52 0.67 -t- 0.09 0.694-0.18 0.15 4- 0.01 0.114-0.01 0.06 4- 0.02 0.13 4- 0.02
0.354-0.11 0.29 4- 0.06 0.23 4- 0.24 0.15 -t- 0.04 0.294-0.08 0.31 -t- 0.02 0.344-0.02 0.27 4- 0.03 0.30 4- 0.03
0.214-0.09 0.03 4- 0.04 0.07 + 0.19 0.07 4- 0.04 0.004-0.05 0.25 4- 0.01 0.214-0.01 0.33 4- 0.03 0.29 4- 0.03
0.12 4- 0.04
0.79 4- 0.34
0.07 4- 0.03
0.02 4- 0.01
0.589 0.903 0.958 0.995 0.909 -0.905 -0.777 -0.954 -0.993 -
peak areas of all presumable class-II transitions, in Table 2 indicated by an asterisk, show a high correlation with the estimated values (correlation coefficient r close to unity), whereas the normal class-I transitions show a strong anti-correlation with rvalues less than -0.77. In Fig. 4 the relative peak area averaged over all presumable class-II transitions Ir,n is shown as a function of the neutron resonance energy En ([~)
1.0
o.8~
q
0.6
0.4
:iiiiii .......................... 0.2
0.0 . 700
.
.
. . 720
.
.
.
. . 740
.
~ 760
. 780
E n (eV) Fig. 4. Relative peak area 17 as a function of the neutron resonance energy compared to the estimated values l~,.l~r based on the neutron resonance parameters (o): The symbols (I-7) and ( ~ ) give the relative peak areas averaged over all presumable class-ll transitions, l~,,n, and normal class-I transitions, It. 1, respectively. The peak areas are normalized to the total peak areas summed over the four major resonances of the intermediate structure at En = 720 eV.
S. Oberstedt, E Gunsing/Nuclear Physics A 589 (1995) 435-444
443
together with the estimated values l ~ l °r ( o ) . Still the characteristic correlation holds and the corresponding correlation coefficient rn = 0.931. For comparison the averaged relative peak areas lr,i o f the class-I transitions are also included ( / ~ ) , and the respective correlation coefficient rl = --0.955. The indicated error margins extend over one standard deviation from the mean values. Both the high correlation of lr,lt as well as the anti-correlation o f Ir,i with the class-II admixture cn into the observed resonances strongly support the assumption that indeed T-transitions within the second potential well are observed.
4. Conclusion From a neutron-capture experiment T-ray spectra with a high energy resolution were obtained for 239U in the neutron energy region between 600 and 800 eV. Five hitherto unknown T-transitions with T-energies at 1298.8, 1339.2, 1344.6, 1475.8 and 1479.6 keV could be found exclusively in the s-wave resonances within the intermediate structure around En = 720 eV. Their relative peak areas 1~, in the T-ray spectra of each resonance show a high correlation with the corresponding class-II fraction estimated from the experimentally observed fission widths. At the same time well-known T-transitions towards the normal ground state are attenuated. Therefore, it is concluded, that for the first time y-decay within the second potential well is observed feeding the shape-isomeric ground state in 239U. Rather weak transitions around Er = 2.9 and 3.1 MeV in the ?/-ray spectrum o f the strongest fission resonance at E, = 721.6 eV indicate a shape-isomeric ground-state energy of about 1.7 MeV in good agreement with results obtained from subthreshold fission measurements. Nevertheless, due to the small number of accumulated counts in this energy region transitions with higher T-energies within the second potential well may not yet be excluded definitely.
References I II H. Weigmann, Z. Phys. 214 (1968) 7. 121 J.E. Lynn, Proc. 2nd IAEA Symp. on Physics and chemistry of fission, Vienna, 28 July-1 August 1969 (IAEA, Vienna, 1969) p. 249. 131 J.E. Lynn, Proc. Int. Conf. 50 Years with nuclear fission, vol. 1, Gaithersburg, 25-28 April 1989 (1989) p. 418. [4] S. Oberstedt, J.P. Theobald, H. Weigmann, J.A. Wartena and C. Btirkholz, Nucl. Phys. A 573 (1994) 467. 151 F. Gunsing, Ph.D. thesis, Delft University (1995). 161 F. Corvi, F. Gunsing, H. Postma and K. Athanassopulos, in Time reversal invariance and parity violation in neutron reactions, eds. C. Gould, J.D. Bowman and Yu.P. Popov (Word Scientific, Singapore, 1994) p. 79. 171 E Gunsing, E Corvi, K. Athanassopulos, H. Postma, Yu.P. Popov and E.I. Shaparov, in Capture gammaray spectroscopy and related topics, ed. J. Kern (World Scientific, Singapore, 1994) p. 797. 181 EC. Difillipo, R.B. Perez, G. de Saussure, D.K. Olsen and R.W. Ingle, Phys. Rev. C 21 (1980) 1400. 191 Nucl. Data Sheets 66 (1992) 4.
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[10] L.V. East and G.R. Keepin, in Proc. 2rid IAEA Symp. on Physics and chemistry of fission, Vienna, 28 July-1 August 1969 (IAEA, Vienna, 1969) p. 647. [ 11 ] R.G. Graves, R.E. Chrien, D.I. Garber, G.W. Cole and O.A. Wasson, Phys. Rev. C 8 (1973) 781. [ 12] W.J. Schindler and C.M. Fleck, Nucl. Phys. A 206 (1973) 374. [ 13] T.A. Khan, D. Hofmann and E Horsch, Nucl. Phys. A 205 (1973) 488. [ 14] J.E. Lynn, The theory of neutron resonance reactions (Clarendon, Oxford, 1968). [ 15] A.A. Bogdzel et al., JINR Internal Report P3-87-862, Dubna (1987). [ 16] K. Perseyn, S. Pomm6, E. Jacobs, D. de Frenne, K. Govaert and M.-L. Yoneama, Proc. Int. Workshop on High resolution spectroscopy of fission fragments, neutrons and y-rays, Dresden (1993) p. 57. [17] G.E Auchampaugh, G. de Saussure, D.K. Olsen, R.W. Ingle, R.B. Perez and R.L. Macklin, Phys. Rev. C 33 (1986) 125. [18] S.E Mughabghab, Neutron cross sections, vol. 1, part B (Academic Press, New York, 1984).