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Nuclear Physics A123 (1969) 215--224; ~) North-HollandPublishing Co., Amsterdam N o t to be r e p r o d u c e d b y p h o t o p r i n t or microfilm without written permission from the publisher

T H E R M A L N E U T R O N C A P T U R E IN 19F R. HARDELL and A. HASSELGREN Chalmers University of Technology, G6tebory, Sweden Received 27 May 1968

Abstract: Gamma rays from thermal neutron capture in fluorine have been measured with Ge(Li) spectrometers. About 50 of the 80 lines observed have been tentatively fitted into the 2°F decay scheme. New energy values of several 2°F levels have been determined, and the spin assignments of some low-lying levels are discussed.

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NUCLEAR REACTION agF(n,~), E = thermal; measured Er, I s, deduced Q. ~°F deduced levels, ;~-branehings. Natural target, Ge(Li) detector.

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1. Introduction Several experimental investigations have aimed at the determination of spins and parities of low-lying levels in the doubly odd nucleus 2°F, but still only very few assignments seem to be well established. Recent theoretical studies assuming the 160 core to be inert and describing the level structure of 2°F in terms of interactions between nucleons in the ds shell t,2) have failed to explain the existence of four excited states below 1.1 MeV. Part of the level scheme of 2°F as deduced from (d, p) and (3He, p) reaction experiments 3-5) is shown in fig. 1. The de-excitation of higher excited states has been studied using the neutron capture reaction. Previous investigations with thermal neutrons are summarized in the work of Nadjakov 6). The gamma radiation following resonance neutron capture in 19F was recently studied by Bergqvist et al. 7) using a Na[ detector. A number of questions in the interpretation of previous (n, 7) spectra may be resolved by measurements with a Ge(Li) spectrometer.

2. Experimental arrangements A teflon (C2 F4), target was encapsulated in the centre of a graphite holder and placed in a tube traversing the reactor vessel of the 1 M W heavy water reactor R1 in Stockholm. The fluorine neutron capture cross section is only 9 mb, and the a / A value is consequently among the lowest to be found for natural elements. The effective target quantity is about 7 g of fluorine, and the thermal neutron flux in the target position is 3 • I012 n cm -2 • s - 1. The capture gamma rays pass a 6 mm diam. by 1.5 m long collimating system before reaching the detector situated about 4.5 m from the target. This 215

216

R. HARDELL AND A. HASSELGREN

arrangement prevents the detector from seeing the aluminium walls of the reactor tube. A further lowering of the background was obtained by evacuating the tube, thus avoiding neutron capture in nitrogen. A staff-shaped Ge(Li) detector, length 2.0 cm, volume 1.5 cm 3, followed by a cooled FET preamplifier, both manufactured by AB Atomenergi, Studsvik, Sweden, have been used for the detection of the gamma-ray spectra. The preamplifier is followed by a Tennelec TC 200 main amplifier and an Intertechnique 4096-channel pulse-height analyser.

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To simplify the response function of the detector above the pair production threshold, a pair spectrometer arrangement has been used. The two side crystals form together an annulus of NaI, 15 cm in diam. and 10 cm long with a through hole, 4 cm in diam. The annulus is bisected, and the two segments are optically isolated. For the measurement of low-energy gammas, the same N a I side crystals are used as part of an anticoincidence mantle. Furthermore, a N a I crystal of dimensions 12.5 cm diam. by 10 cm long with a through hole, diam. 1.8 cm, is placed in front of the N a I annulus to detect backscattered radiation. The pulses from the N a I crystals are summed,

19F(n, )')

217

amplified and fed via a discriminator and a pulse former to the anticoincidence gate of the multi-channel analyser. A detailed description of the whole arrangement is given in ref. s). To obtain energy calibration in the high-energy region, nitrogen gas was passed into the reactor channel, and a composite spectrum was taken. Well-determined energies of g a m m a rays from neutron capture in nitrogen and carbon are given in refs. 9-11). In the low-energy region, radioactive sources were used for the energy calibration. The resolution of the Ge(Li) detector was about 4 keV for the 6°Co peaks and about 8 keV at 6 MeV. The efficiency of the Ge(Li) pair spectrometer has been calculated using a natural nickel target. The relative nickel intensities have been taken from the measurements of Groshev et al. 12). In the low-energy region, radioactive sources have been used for the determination of the relative photopeak-efficiency curve.

3. Experimental results The gamma-ray spectra recorded are shown in figs. 2 and 3. Several lines are due to reactions other than 19F(n, ~?)2°F. The 6131 keV line follows the beta decay of the ~6N ground state reached by the 19F(n, c@6N reaction. The 4946, 3684 and 1262 keV transitions are 13C capture lines, and the 4430 keV line is due to inelastic neutron scattering in l z c . A few weak lines can be ascribed to neutron interactions with aluminium and boron construction parts and the Ge(Li) detector itself. A previous investigation 6) of the purity of the teflon target has revealed small amounts ofBa, K and Ca. No lines, however, were found that could be ascribed to neutron capture in these elements. Several low-lying states of ~9F may be excited by inelastic neutron scattering. The 110 and 198 keV levels appeared to be very strongly excited, and a lead shield had to be introduced in front of the detector to reduce the very large number of low-energy photons. Even so, the low-energy distribution is considerable as seen in fig. 2 owing to neutron and g a m m a scattering in the target. Using the given total flux of fast neutrons in the reactor, the theoretical curve for the energy distribution of fission neutrons and the curves for the excitation functions for different gamma rays as determined by Freeman 13), a rough calculation of the intensities of a few 19F lines was performed. The results indicate that, compared to the 100 % line at 1632 keV following the beta decay of 2°F to 2°Ne, the intensities of the 1.24, 1.35, 1.46 and 2.58 MeV lines should have upper limits of 55, 125, 20 and 12 %, respectively. These lines appear in the spectra with intensities estimated to be 24, 90, 4 and 4 ~ , respectively. It should be observed that the strong 19F line observed at 1354 keV may correspond to a strong transition of this energy previously reported 6) gut ascribed to 2°F. The experimental results are summarized in table 1. The energy values are corrected for recoil, and the intensities are given in photons/100 neutron captures. The intensities were calculated from the experimental efficiency curves and with corrections applied for self-absorption in the target and for neutron and g a m m a shielding in the gamma-ray beam. The errors in the intensities are estimated to be about 15 ~ for

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R. FIARDELL AND A. HASSELGREN

s t r o n g lines a n d i n c r e a s e s u c c e s s i v e l y t o a b o u t 100 ~ f o r t h e w e a k e s t lines. A q u e s t i o n m a r k i n t h e t a b l e i n d i c a t e s d o u b t s a b o u t t h e e x i s t e n c e o f t h e line. T h e

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100 B (B = 6.602 M e V ) is f o u n d t o b e 0.81. U s i n g a c o m p u t e r f o r t h e c a l c u l a t i o n a n d o r d e r i n g o f all d i f f e r e n c e s b e t w e e n levels, it w a s p o s s i b l e t o f i n d all a l t e r n a t i v e p o s i TABLE 1 Gamma rays from neutron interactions with agF No.

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546.4~0.5 556.5zk0.5 583.6±0.5 603.0221.0 656.3220.5 665.0220.5 781 222 (?) 907 222 (9.) 983.5220.7 1056.9221.0 1253.2221.0 1276.4221.0 1282.3221.0 1309.1 220.7 1388.2221.0 1424.6221.0(?) 1437.0221.0(?) 1646.4221.5 1797.7221.5 1833.3221.5 1843.3221.0 1971 222 2043 222 2143 222 2178 ± 2 2193 ±2 2255 ± 2 2325 222 2344 ~ 2 (?) 2430 ±1

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a) C is the capturing state.

Assignment a)

4085-3528 C-6018 5870-5270 656-0 C-5937 3967-3184 984-0 1057-0 6520-5270

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tions for the g a m m a lines in the decay scheme. W h e r e possible the alternatives are given in o r d e r o f preference in the table. U s i n g the i n f o r m a t i o n s u m m a r i z e d in table l, it is possible to calculate the Q-value for the 19F(n, y ) 2 ° F reaction. I n table 2, the g a m m a - r a y cascades are given which are used for the d e t e r m i n a t i o n o f the weighted m e a n value Q = 6 6 0 2 . 0 + 0 . 6 keV. This value m a y be c o m p a r e d with the previous values 6 5 9 7 . 3 + 4 . 7 keV [ref. 14)] a n d 6 6 0 2 + 1 keV [ref. 15)]. The energy values o f several 2°F levels have been modified slightly, a n d the new values are given in table 3. W h e n constructing the decay scheme shown in fig. 4, it was f r o m intensity a r g u m e n t s f o u n d necessary to include a few lines that fit at different positions in the scheme a n d thus m a y consist o f two or three closeTABLE 2 Determination of the Q-value (given in keV) for the l~F(n, ~')2°Freaction 6602.5 : 6602.5 4-1.0 5618 + 983.5 =6601.5±2.1 5545 +1056.9 = 6601.94-2.2 5293 +1309.1 = 6602.14-2.1 4758 +1843.3 = 6601.34-2.2 4558 +2043 = 6601.04-2.8 3113 +3488 = 6601.04-2.8 3015 +3588 =6603.04-2.8 2636 +3967 = 6603.0zk2.8 2518 +4085 = 6603.04-3.6 665.0+5937 = 6602.04-2.1 583.6+6018 = 6601.6=I_1.1 weighted mean value Q = 6602.0 4- 0.6 TABLE 3 Excitation energies (in keV) of some ~°F levels populated in the neutron capture reaction 656:t_1 984 4-1 10574-1 1309 _:cl

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6018 ±1 6602.0 ±0.6

lying g a m m a rays. P a r t o f the m i s m a t c h in the intensity balance for some o f the levels in the decay scheme can be explained by this fact and by the omission o f a n u m b e r o f transitions in the scheme.

4. Discussion The ~ OF energy levels a b o v e 5 M e V have been d e t e r m i n e d with (d, p ) reaction experiments a n d are given with errors -t-20 keV [ref. 16)]. Consequently, the assignment o f transitions feeding or de-exciting other levels in this region than those at 6018 a n d 5937 keV is very tentative. In the low-energy region, the energy values are determined

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more accurately, and furthermore In values are given, thus making some conclusions possible on the characters of these states. The 2°F ground state has been found to be 2 + [ref. 17)], and a 1 ÷ assignment 18) of the 1057 keV state results from a study of the 2 oO beta decay. For several other low-lying levels, probable spin values are given in fig. 1. Theoretical spin values were obtained by Dazai 29) from a shell-model calculation of stripping reduced widths of even parity and show a reasonable agreement with experimental results. The conclusions drawn from the present experiment are summarized below. The 656 keV level. In the present study, no transition is observed from the 0 +, 1 + capturing state to the 656 keV state, and the upper limit for the intensity of such a transition is estimated to 0.2 photons/100 captures. Bissinger et al. 4) have found the most probable spin value for this level to be 3; this assignment is not in disagreement with our data. The exclusion of J(656) = 3 by Bergqvist et al. 7) is not unambiguous because of the poor resolution of the NaI detectors. The 828 keV level. From the gamma-ray spectra recorded, it can be estimated that the intensity of a possible transition from the capturing state to the 828 keV level must be lower than 0.1 photons/100 captures. Furthermore, a possible 828 keV ground state transition must be weaker than 0.8 photons/100 captures, whereas a 172 keV transition to the 656 keV level would be hidden in the strong low-energy background. In the work of Bissinger et al. 4), spins 3 and 1 are ruled out, whereas spin 2 is found to be probable. As there remain some doubts about the l n value from the (d, p) reaction, the J = 4 possibility is also discussed. The lack of an observable transition from the capturing state implies that this possibility should be seriously considered. The 984, 1057, 1309 and 1843 keV levels. Gamma-ray cascades are observed proceeding from the 0 +, 1 + capturing state via these levels to the 2 + ground state. Possible de-excitation lines from the 984, 1057 and 1309 keV states to other excited states in 2 oF are hidden in the low-energy background distribution or under other gamma lines. For the de-excitation of the 1843 keV state, the upper limits of possible transitions to the 656, 828, 984, 1057 and 1309 keV states are 0.4, 0.4, 0.6, 0.6 and 1.5 photons/100 captures, respectively. The present results do not contradict the assignments of Bissinger et al. 4) and give preference to the spin values 1 or 2 for all these levels. The 2044 keV level. This level has been found to be strongly populated in the decay of the 49 keV (1 - ) resonance 7), and the spin was concluded to be 1 + or 2 +. In the present investigation, this level was also found to be fed from the capturing state; the de-excitation mode was found to agree with earlier measurements as shown in fig. 1. The 2194 keV level. According to a previous investigation 3), this level mainly decays (80 ~ ) through a 1366-828 keV cascade. In view of the intensity of the 2194 keV ground state transition, the cascade would be expected to have an intensity of 4.4 photons/100 captures. The 1366 keV transition might very well be hidden in the

224

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b r o a d 1354 keV 19F line, whereas a c c o r d i n g to fig. 1 the 828 keV line should have an intensity o f a b o u t 1.6 a n d w o u l d be o b s e r v e d in the spectrum. T h e u p p e r limit for the intensity of a possible 828 keV line is e s t i m a t e d to 0.8 p h o t o n s / 1 0 0 captures. A s s u m i n g the decay m o d e o f the 828 keV level given in fig. 1 to be correct, this implies t h a t the b r a n c h i n g f r o m the 2194 keV level to the 828 keV level should be smaller t h a n 67 ~ .

The 3488 keV level. The intensity o f the g r o u n d state t r a n s i t i o n f r o m this level m a y be smaller t h a n indicated in the decay scheme owing to a possible 4313-828 k e V transition, b u t as no de-excitation line is observed f r o m the 828 keV level, the feeding o f the 828 keV level is e s t i m a t e d to be less t h a n 2 p h o t o n s / 1 0 0 captures. T h e intensity of the de-excitation line to the 1057 keV level is very difficult to estimate as two o t h e r alternatives r e m a i n for the fitting of the 2430 keV transition. I n any case, the deexcitation m o d e f o u n d in this w o r k does n o t agree with the results shown in fig. 1, where transitions to the 984 keV a n d 1843 keV states are missing. The 5937 and 6018 keV levels. A n In = 1 value has been r e p o r t e d for the 5937 keV level, whereas no value is available for the 6018 keV level. It seems, however, p r o b a b l e t h a t b o t h o f these levels have negative parity, as they are strongly fed f r o m the c a p t u r i n g state. We w o u l d like to t h a n k Dr. S. E. A r n e l l for valuable discussions a n d Professor N . R y d e for his interest in this work. W e are also i n d e b t e d to Ing. N. Berglund a n d the r e a c t o r staff at R I , S t o c k h o l m , for c o n t i n u o u s assistance d u r i n g the measurements.

References

1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)

M. C. Bouten, J. P. Elliott and J. A. Pullen, Nucl. Phys. A97 (1967) 113 A. Arima, S. Cohen, R. D. Lawson and M. H. Macfarlane, Nucl. Phys. A108 (1968) 94 P. R. Chagnon, Nucl. Phys. 59 (1964) 257 G. A. Bissinger, R. M. Mueller, P. A. Quin and P. R. Chagnon, Nucl. Phys. A90 (1967) 1 P. A. Quin, A. A. Rollefson, G. A. Bissinger, C. P. Browne and P. R. Chagnon, Phys. Rev. 157 (1967) 991 E. G. Nadjakov, Nucl. Phys. 48 (1963) 492 I. Bergqvist, J. A. Biggerstaff, J. H. Gibbons and W. M. Good, Phys. Rev. 158 (1967) 1049 S.E. Arnell, R. Hardell, A. Hasselgren, L. Jonsson and O. Skeppstedt, Nucl. Instr. 54 (1967) 165 R. C. Greenwood, private communication L. Jonsson and R. Hardell, to be published W. V. Prestwich, R. E. Cot6 and G. E. Thomas, Phys. Rev. 161 (1967) 1080 L. V. Groshev, A. M. Demidov and N. Shadier, I. V. Kurchatov Atomic Energy Institute, Moscow (1966) J. M. Freeman, Phil. Mag. 2 (1957) 628 J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 32 D. E. Blatchley and G. E. Thomas, Bull. Am. Phys. Soc. 12 (1967) 53 F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1 E. Freiberg and V. Soergel, Z. Phys. 162 (1961) 114 G. Scharff-Goldhaber, A. Goodman and M. G. Silbert, Phys. Rev. Lett. 4 (1960) 25 T. Dazai, Prog. Theor. Phys. 27 (1962) 433