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Energy levels of 49V from 49Ti(p, n)49V
[I.E.I:3.A] |
1
Nuclear Physics A195 (1972) 596--608; (~) 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 f r o m the publisher
ENERGY LEVELS OF
49V F R O M
49Ti(p,
n)49V
J. G. MALAN, E. BARNARD, J. A. M. DE VILLIERS, J. W. TEPEL and P. VAN DER MERWE Atomic Energy Board, Pelindaba, Transvaal, Republic of South Africa
Received 17 April 1972 (Revised 12 June 1972) A bstraet: Energy levels of 49V were populated by the 49Ti(p, n) reaction at proton energies between 2.03 and 3.25 MeV. Spins were determined within the framework of the statistical theory from the angular distribution of de-excitation v-rays measured at five incident proton energies. Excitation strengths and decay branching ratios of the various levels were taken into account. A complete level and decay scheme for 49V up to an excitation of 1.7 MeV is presented. [ E
NUCLEAR REACTIONS 49Ti(p, n), (p,n~'),E = 2.0-3.3 MeV; measuredo'(E; E,L o'(E; E~,, 07.), E~,, 1~.. 49V deduced levels, J, =, )'-branching. Enriched targets.
]
1. Introduction The level structure of 49V has been studied through a variety of nuclear reactions. The results of these investigations have been summarized and evaluated t). Recently two more high-resolution studies of 49V appeared, viz. a 46Ti(~, pT)49V experiment by Mo et al. 2) and, during the course of the present experiment, a 49Ti(p, nT)49V experiment by Blasi et al. a). Together with the earlier work, this results in a fairly comprehensive and accurate level and decay scheme for 49V up to an excitation of 2.5 MeV. The spin values of a number of the observed levels are however uncertain or completely unknown. Information on spins has been furnished mainly by the (p, ~,), (3He, d) and (t, ~) experiments 4 - t l ) . In the (p, ~) experiments, the decays of resonances with probable spins of ½ or ~ were studied, effectively limiting information on low-lying levels to those with spin < ~. Since stripping and pick-up experiments were performed on spin-zero targets, similar limitations apply to levels studied by these methods. In addition the latter reaction mechanisms are somewhat selective. Highspin states in f~ shell nuclei are often particularly suited to investigation by means of (p, n) and (p, nT) reactions ~2). The non-selective nature of these compound-nuclear processes ensures an excitation probability for any particular level solely dependent on its spin/parity value and independent of any complicated structural considerations. Previous investigations of the 49Ti(p, n) and (p, ny) reactions 3, 13-16) were concerned mainly with level energies and the decay scheme of 49V. The present investigation was therefore initiated in order to obtain more complete data on the spins o f 596
49Ti(p, n)*gV
597
low-lying49V levels, primarily by way of a comparison of y/-ray angular distributions at various proton energies with the predictions of the statistical theory. The information simultaneously obtained on the relative excitation probabilities of the different levels and their decay characteristics, as well as data from other experiments, were taken into account in making spin assignments. It was considered prudent to verify the generally assumed spin values of the ground and first two excited states of 49V, to which most of the higher levels decay, before proceeding with the analysis. This was done by measuring relative neutron intensities to these levels over an adequate energy range, and comparing the results with the predictions of the statistical theory.
2. Experimental procedure and data analysis The experimental arrangement was similar to that described previously 12). A pulsed ( ~ 1 ns pulse length) proton beam of < 12 #A was provided by the AEB 3 MV Van de Graaff accelerator at Pelindaba. The maximum proton energy was 3.25 MeV, limiting the excitation of 49V via the (p, n) reaction (Q = - 1.383 MeV) to 1.80 MeV. The targets consisted of up to 320/~g/cm z of TiO2, enriched to 66 ~ in 49Ti, evaporated onto 0.25 m m thick tantalum discs. The water- and air-cooled target was exposed to the proton beam at an angle of 45 °, giving an effective target thickness of ~ 40 keV in the energy range used. The neutron intensity measurements were performed with a standard time-of-flight system employing a plastic scintillator. The relative efficiency of the detector was calibrated against that of a standard long counter. For the y-ray angular-distribution measurements a 40 cm 3 Ge(Li) detector was fixed at 90 ° as a monitor, while another 40 cm 3 detector was moved between 0 ° and 90 ° in 15 ° steps. Monitoring in this way on the same 7-ray observed in the angular distribution, or on the total decay of the level from which the y-ray originates, ensured reliable results. Monitoring by means of the total neutron or y/-ray flux (or by means of a known y-ray angular distribution) proved to be unsatisfactory. This was probably due to slight drifts in proton beam energy giving rise to fluctuations of a few percent in the relative intensities of y-rays from different levels. When the latter methods of monitoring were used, every angle was usually repeated at least twice, and a few of the distributions were completely remeasured as a check on the results. The resolution of both Ge(Li) detectors was ~ 2.8 keV at E~ = 1.33 MeV. The front of each detector was shielded by 2.4 m m of lead which suppressed the high flux of low-energy 7-rays, but also effectively precluded the observation of weak ~-rays of energy < 200 keV from the target. Efficiency and energy calibrations were performed with standard 7-ray sources. Time-of-flight gating methods were used in order to eliminate background due to neutron effects in the Ge(Li) detector, which were especially troublesome in the vicinity of the 595 keV y/-ray transition in 49V. As most of the other y/-ray peaks of interest were undisturbed by the neutron-induced back-
598
J.G. MALAN
et al.
ground, some angular distributions were obtained without time-gating. The target to detector distance was about 35 cm for the time-gated, and somewhat less for the ungated runs. Spectra were recorded with a dispersion o f < 1 keV per channel in a 2048-channel analyser. The data were analysed for peak energies and areas on a C D C 1700 and an I B M 360/40 c o m p u t e r with the p r o g r a m m e s L I Z ~7) and P E A K 18) respectively. The latter p r o g r a m m e was slightly modified in order to facilitate the handling o f 7-ray spectra. TABLE l Optical potentials used in the Hauser-Feshbach calculations Parameter
Potentials neutron A
neutron B
proton
45.85 4.845 0.62
44.36 4.543 0.62
47.08 4.845 0.62
13.92 5.345 0.50
9.0 4.797 0.50
13.48 5.345 0.50
R e a l well
depth (MeV) radius (fm) diffuseness (fro) I m a g i n a r y Gaussian s u r f a c e
depth (MeV) radius (fm) diffuseness (fm) Spin-orbit
depth (MeV)
7.0
7.0
7.0
The energy dependence of the potential depths is given in ref. 20). Values shown are for E o = 3.0 MeV and E. = 0.5 MeV. Saxon-Woods and Thomas radial shapes are used for the real well and the spin-orbit term respectively.
Statistical-model calculations were performed with the c o m p u t e r code P E L I N S C A [ref. ~9)]. Transmission coefficients for p r o t o n and neutron channels are calculated in this p r o g r a m m e f r o m the energy-dependent optical-model parameters r e c o m m e n d ed by Engelbrecht and Fiedeldey 2o). This potential (A in table 1) gives a g o o d account o f the s-wave neutron strength function which peaks strongly for A ~ 50 but predicts a (p, n) transition strength too large by a factor o f 3 or more for the 748 keV level in 49V (spin ~+ or ~-+, see sect. 4). However, in a recent study 21) o f elastic and inelastic neutron scattering from T i a slightly modified set o f parameters (B in table 1 ) with the same energy dependence as in ref. 2o) gave a much better fit to the experimental data, although not reproducing the s-wave strength function satisfactorily. The reason for the discrepancy is not clear, but may in part be due 22) to the omission o f level width fluctuation corrections in the usual Hauser-Feshbach calculations performed by P E L I N S C A . In this work all calculations are based on potential B in table 1.
'*9Ti(p, n)4°V
599
3. Experimental results 3.1. LEVEL AND DECAY SCHEME T h o u g h the present experiment was primarily concerned with spin assignments, some useful data o n the decay of 49V were also obtained, as s u m m a r i z e d in table 2. TABLE2 Level energies and branching ratios in 49V Level energy a) (MeV)
E3' (MeV)
748.2 ±0.2
595.4i0.2 657.5 ±0.2
1021.7~0.2
1021.7,,0.2
100.0
1140.44-0.2
392.2520.2 987.5320.3 1049.74-0.2
5.9-4-0.9 18.44-0,5 26.8 4-1.1 48.9 ±0.6
5.24-0.6 16.64-1.5 26.4 320.9 51.8 4-1.3
1140.44-0.2
Branching ratios present work
ref. 3)
54.0 ±0.8 46.0 --0.8
55.4 z~0.6 44.6 ±0.6
1155.3 320.2
1064.6 --0.2 1155.3 4-0.2 (133) b)
22.8 320.3 77.2 4-0.3
22.4 -4-0.4 74.7 4-0.7 2.94-0.6
1514.6 4-0.2
1361.5 4-0.3 1423.94-0.3 1514.8 4-0.3
56.0-4-1.2 11.3 -4-0.6 32.7-- 1.5
56.3 321.0 11.3 4-0.4 32.4 4-0.7
1602.74-0.2
447.4-4-0.3 462.2-4-0.2 854.5±0.3 1512.04-0.3 1602.6.4.0.2
0.9320.4 7.5 4-0.9 8.64-1.8 55.64-1.1 27.4-4-1.1
6.9 -4-0.6 8.04-0.8 59.55_ 1.3 25.64-0.6
1643.34-0.4 1661.8320.2
1490.44-0.4 c) 1571.1--0.2 (1508) d)
64 ± 4 36 4-4
") The level energies of the first and second excited states were taken as 90.7 and 152.9 keV respectively l). b) Not observable in the present experiment (see text). ~) Very weak transition. Background structure in this area makes a reliable determination difficult. d) Not observable in the present work due to proximity of the very strong 1512 keV line. 111 c o m p a r i n g these results with the work of Bias± et al. 3, 16) and M o et al. 2) it must be b o r n e in m i n d that the present experimental layout precluded the study of the decay o f the first two excited states a n d of the proposed weak 133 keV t r a n s i t i o n between the 1155 a n d 1022 keV levels 3). Due to their low spin values a n d the l i m i t a t i o n in m a x i m u m p r o t o n energy, the 1643 a n d 1662 keV levels were rather weakly excited. A very weak 898.2 +_0.5 keV ?-ray of u n k n o w n origin is observed in the present work.
600
J . G . M A L A N et al.
This y-ray was tentatively allocated to a 1643-748 keV transition by Blasi et aL 3), but in view of energy balance considerations this assignment cannot be supported. The proposed 1508 keV y-ray in the decay of the 1662 keV level 3) was not observed in the presence of the very strong 1512 keV line. The rest of the decay scheme and branching ratios are in excellent agreement with the results of ref. 3) and agrees substantially with ref. 2). A previously unreported 447 keV y-ray is allocated to a transition between the 1603 and 1155 keV levels. This assignment is made on the basis o f energy sums and its observed threshold. Assuming that this line originates from the 1603 keV level, its branching ratio remains constant over the energy range studied. 1661,8~ 1643,3 1602,7 J 1514,6
27569 8 1 33 1156
1155,3 ~
5027186
11 ~ , 0 , 4 ~ "
II
I
I II I
1
i
4654
112-, 312" - 172 ÷ ~__ 712 +
I I I t
7723
I
1oo
1021,7
748,2
f
I
912",(71
2- )
"~- 512 1112~'~
3•2 +
o
r.
152,9 90,7 712"
0
Fig. 1. Level and decay scheme of 49V.
A complete decay scheme for 49V up to and including the 1662 keV level ispresented in fig. 1. Transitions not observed in the present work but reported by Blasi et al. 3) are included as dashed lines. 3.2. R E L A T I V E I N T E N S I T I E S
Neutron intensity measurements for the groups feeding the ground and first two excited states of 49 V were made at 30 keV intervals between 2.03 and 2.12 MeV. N o prominent resonance structure or intensity fluctuations were observed in this energy region, indicating adequate averaging over levels in the compound nucleus. F r o m isolated neutron-intensity measurements at higher energies, as well as from y-ray angular distributions supplemented by yield measurements at 55 °, relative excitation strengths for all observed levels were deduced. All known 7-ray branching ratios and cascades from this and previous work were taken into account. After averaging over sufficiently large energy intervals, the relative intensities are compared
595 657
1022
392 988 1050 1140
1065 1155
1362 1424 1515
462 855 1603
1490
1571
748
1022
1140
1155
1515
1603
1643
1662
~14
--0.01±3 0.04±3 0.03~2
(/2
0.03£:2 - 0.03i2
0.19±2
Ev ~ 2.56 MeV
Quoted errors pertain to the last decimal digit.
E~.i (keV)
Energy level (keV)
T A BL E 3
0.02£3 0.02±2 0.00±2 0.05£2 --0.01±2
--0.00±2 0.02±1
--0.00±2
--0.02i3 --0.02±3
a4
0.02£3 --0.04±2 -0.00±2
0.19±1
0.03±2 --0.01£2
a2
Ep = 2.89 MeV
0.0122 -0.01±2 0.01i3 0.01i4 --0.02±4 --0.01~3 0.04±4 --0.0122 --0.01i4 0.00+3 --0.02~3
0.17~2 -0.06±2 --0.03±3 0.03~3 --0.01±2 0.04±3 0.07±2 0.08~3 --0.06±2 --0.00±3
a4
--0.01£I
a2
Ea, = 3.02 MeV ~12
0.01i3
0.03±3 0.02~4 0.00±2 0.01~2
0.01i2
0.01±2
a4
0.01±3 0.02~2
0.03±5 0.05±3
0.05~3
0.05£3 0.01±3 0.02i2 --0.01±2 --0.03t2 0.01±3
0.11±3 0.19±2
0.04±1 0.00±2 0.06~1 - - 0 . 0 0 £ 2
--0.03±3 0.02±4 0.01+1 --0.01±1
0.16~2
0.01±2
gl2
/~p = 3.25 MeV
0.05±12 - - 0 . 0 9 ± 4
0.01~3 --0.02±5 0.02±2
0.04±3 0.06±4 -0.02±2 --0.05±9
- 0.01i3 0.0223
0.02±2 0.00£2
0.02±4 0.03±4 0.04±3 0.03±2
0.15~2 --0.20£2
--0.01±2 0.00±1
0.03£3 0.03±3 --0.02±2 0.01~2
0.14~2
0.01il 0.01i2
04
Ep = 3.10 MeV
-0.03£1 0.00±1
Legendre polynomial coemcients ofT-ray angular distributions
8
602
J . G . M A L A N e t al.
with Hauser-Feshbach calculations as an aid in making spin assignments (sect• 4). For the 1602, 1644 and 1662 keV levels the averaging was probably not adequate, due to the limitation in maximum proton energy. 3.3. G A M M A - R A Y A N G U L A R
DISTRIBUTIONS
Gamma-ray angular distributions were measured at /~p = 2.56, 2.89, 3.02, 3.10 and 3.25 MeV. The incident proton energies were chosen close to the thresholds of the individual levels studied. Best-fit normalized Legendre polynomial coefficients are presented in table 3. These values show little change with increasing proton energy, although absolute ~-ray intensities fluctuated appreciably. The variation of a2 values TABLE 4 Summary of spin and mixing-ratio determinations Energy level (keV)
Accepted values ")
Ev
J
(keV)
final state
748
595 657
]~-
~+ ~+
1022
1022
5-
.~¢-~
1140
392
~+
~+
988
~-
~+
1050
~-
~+
1140
5-
~+
./
6
>0
0.11±0.18
1155
1065
~-
1155
5-
G-) 2G-)
1362 1424
~~-
~~-
1515
5-
~-
855
~+
~-
1512
~-
~-
1603
5-
•643
•490
~-
~*
1662
1571
}-
½-
1515
1603
or
6.31+3"*tb-oo)
--a~ " 3~~ - -+0 .0.21 24
or
5.67 _+~92 .
--0.14+0.18
+1.51 or --3.49_7.94
~-
5+ ~-
5+
unrestricted 0.32±0.10 or unrestricted --
08
"
41 5.67_+ 1 3 .2.40
+0.7 --2.0
06+2.1 • --O.fi <0
not determined --0.29±0.11
+0.48 or --2.36_0.77
>0 ~) See text for arguments on spin assignments. b) Mixing-ratio errors were determined on the basis of the 0.1 o/ confidence limit relative to the best-fit Z 2 value.
49Ti(p, n)49V
603
outside the quoted errors for a few of the weaker y-rays is probably due to incomplete averaging in the compound nucleus or to slight contamination from angle-dependent background. At the higher incident proton energies some cascade feeding of the lowerlying levels occurred. This was serious only in the case of the 748 keV level, where at Ep = 3.25 MeV about 30 % of its total decay strength came from this source. Since the decay from this level was observed to be almost isotropic at all energies, even when no feeding occurs, this did not influence any of the conclusions. Cascades to other levels amounted to < 4 % (1022 keV level), < 7 ~o (1140 keV level) and < 1% (1155 keV level) of the total decay strengths respectively and were consequently disregarded in the analysis of the angular distributions. In order to improve averaging in the compound nucleus, measurements at 2.89 and 3.02 MeV, and again those at 3.10 and 3.25 MeV were averaged before proceeding with the Zz analysis. Spins were rejected on the basis of the 0.1 }~ confidence limit. Mixing ratios determined in the analysis are given in table 4.
4. Discussion of spin assignments Spin assignments to energy levels of 49V are presented in fig. I. Cognisance has been given to all available experimental data.
The 9round state, 91 and 153 keVlevels. An 1 = 3 stripping pattern for the ground state of 49V has been clearly established 6- ~t). For the 90 keV level an 1 = 3 pattern
49Ti (p,n)49V ffp = 2.06 MeV
B I-I
On= 0 °
I-t/~ Z U.I z
- -
EXPERIMENT H.F. THEORY
?-
5-
3-
7-
7-
3-
2
2
2
~
T
T
0
91
0
91
153
S2
~2
52
72
153 - J ~
Ex
UJ tz:
3-2
3" 2
Fig. 2. Intensity histograms for the ground and first two excited states of 49V indicating a spin sequence of ~2- ~ ~-, 2'~- for these levels.
604
J.G.
MALAN
e t al.
was also observed a), limiting the spin values of these two levels to ) - and ~-. Assuming the known spin value o f ~ - for the 153 keV level ~), only the spin sequence -~-, ~- and ~- is compatible with the measured relative neutron intensities (fig. 2). This sequence is in agreement with the generally assumed values and with the results of (p, 7) measurements 4, 5). The 748 k e V level In both stripping and pick-up reactions 8-1t) evidence was found for an 1 = 2 transition to this level, allowing spins of z2-+ and ~+. Proton capture and pick-up reactions 5, s) favour the value ~r. From the 7-ray angular distributions of the present work it is impossible to distinguish between the two alternatives,
4 9Ti (p,n)49 V ~ (748 keY) 512" 512-
..... 150
+ ÷
100 312+-
b
50 112"-
2,2
2,4
2.8
2.6 Ep
3,0
3,2
---
Fig. 3. Relative neutron intensities for the 748 keV level compared with Hauser-Feshbach calculations.
since the almost isotropic 595 and 657 keV distributions (table 3) allow all spin values < 5. However, a comparison of the excitation strength of this level relative to that of adjacent levels of known spin completely rejects the 5+ value, while -2 ~+ gives good agreement with the Hauser-Feshbach theory (fig. 3). The 1022 k e V l e v e L In the 5°Cr(t, ~) reaction 8) a rather flat angular distribution for the transition to this level was observed, suggesting a high spin value. This is corroborated by its single decay to the ~- ground state. In the present work the consistently anisotropic 7-ray angular distribution is compatible only with spin values of 11_+ (pure M2 or E3 radiation) and -x2-L- (pure E2 radiation) (fig. 4). However, it should be pointed out that a 11_- state at an excitation of about 1 MeV is predicted by both the Coriolis coupling model 23) and a pure (14)" calculation 24) and has been shown to exist in a number of nuclei in this mass region a' 12, 2s). A low-lying _lj_+ state is excluded from shell-model considerations and has not been observed in 2
'~9Ti(p, n)49V
605
relevant experiments. The spin of the 1022 keV level is therefore proposed to be -~-and this value has been assumed in all calculations of relative excitation strengths.
I
I
Ey = 1022 keY ,oo
ooL/
y"
I
[~p= 2,56 MeV
L,
'"~[---
" ~- S0
I
-60 °
-30" ~o ARCTAN (~
30° -~
1000
0.5 COS 2 e
I
- Ey ='1022 keY
'
E~/=1022 keV
~'p= 2.9; MeV1
"~
I
1.0
~0°
60
-
_~NN~ I\*'
W(e)=l+O'209P2(COSe)
- 0.006P4(COS0 ) -90 °
.....
- / o
I
0,0
I
~p = 2,95 MeV j~
Io22
100
1 ,°°J', .;I >-
J=7
/\
,-,,
90
',i/
,-I
~o
..J
-7
~-
It"
80
W(, 0_)1,0 _ ~
11
- 0.005 p4 (cose)
1 -
90°
I
-60 •
I
-30 o 0o ARCTAN 8
'
30°
60°
90'>
i
1.0
i
0.5
<~
i
0.0
-'------ C O S 2 O
Fig. 4. Angular distributions and Zz diagrams for the 1022 keV ground state y-decay. The Z z axes have been shifted for display purposes. A spin value of ~ is indicated for this level. The 1140 k e V level. Spin values of ~ or -~ are suggested for this level by (p, 7) experiments 5). No clear evidence for any particular spin value has emerged from other reactions exciting this level. The observed branching of its 7-decay indicates spin values of:~- or ~z, whereas the almost isotropic angular distributions allow spin values < 5. A comparison of the relative excitation strength of this level with theory (fig. 5) shows that its spin is either ~+ or ~-.
606
J . G . MALAN e t al.
The 1155 keVlevel. This level is not populated in the (p, 7) reaction, which might indicate a high spin value. Particle transfer reaction~ contribute no information on its probable spin. The observed v-decay to the-~-- 1022 keV level 3) and to the ground as well as first excited states limit its probable spin values to ~- and 9-. Relative intensity measurements (fig. 5) are compatible with both these values whereas ~-, (~-) and ~- are allowed by the slightly anisotropic angular distributions. Although a spin value of ~- is therefore not completely rejected, the present results indicate a clear preference for the ~- value. This assignment is supported by a comparison with the situation in nearby nuclei, where the -~2 -~-- level is always accompanied by a ~-- state [refs. ~' ~2, 25)], as indeed it should be from theoret~caJ considerations 23,24).
1155 keY LEVEL
1140 keV LEVEL 1,5
1514 keV LEVEL
1,5 7/2 "
I
~ 9 / 2 -
1,0
~
b"-
7
/
t
2
712"
O
Jl-,-
y
L,') [¢1
,4"
b
712"
0,5
1,0
"
0.5
0,5
b~
5•2-
~ < 512+
512
512"
3/2-
b"
~
~312" I
i
I
I
I
2,6
2,8
3,0
3,2
3.4
Ep (MeV) - - - - . = - "
0
I
I
2,6
2,8
I
I
3,0 3,2 Ep (MeV)
3/2" 312 °
I
[
J
I
3,4
3,0
3,2
3,4
Ep (MeV)- ' - D -
Fig. 5. Comparison of experimental and theoretical relative excitation strengths for the 1140, 1155 and 1514 keV levels. Spin values of ~, ~ or ~, and ~ are preferred for these levels respectively.
The 1515 ke V level. The observed decay of this level to the ground and first two excited states suggests spin values of ~-, 2 -~ or ~-. In the 48Ti(p, V) work of Legg et al. [ref. 5)] the possible spin values are given as ½, ~ or ~. In the present experiment large anisotropies were observed for the 1361 and 1424 keV v-rays, while the 1515 keV decay was almost isotropic. Assuming that the M2, E1 mixing ratio is ~ 0, the Z2 analysis (fig. 6) selects the spin value s - , in agreement with intensity measurements shown in fig. 5. The reason for the relatively poor fit to the distribution for the 1424 keV v-ray is not clear but might be due to incomplete averaging in the compound nucleus or to a slight contamination of the relevant peak by angle-dependent background. The 1602 k e V level In a (t, ~) experiment 8) a rather doubtful l = 5 stripping pattern was observed for this level. However, its decay as shown in fig. 1 favours the
49Ti(p, n)49V
607
spin values ~ - and ~-+. The angular distributions, although not excluding 2 + at the 1 ~o confidence level, indicate preference for the former value. The excitation strength o f this level was deduced from measurements at only two proton energies and suffers from incomplete averaging. Nevertheless, intensity measurements favour the spin value 2. Both the values } - and ~+ are therefore possible for this level. 100
E.y= 1361 keY
'
1
[~p= 3,17 MeV
Ep = 3,17' MeV
jIT l
t~=--,
10
.J
><
~~ .
%
1.0
~
o,1"~CONF.LIMIT
islA
100
,
=-
9o
(,.. _z
80 W(01=1.0,116
P2 ( C O 5 0 1
* 0 , 0 0 3 P4 (COS 01 1,O .90 o
,
-- 60 °
I -- 30 o
[ 0o
I 30 °
ARCTAN ~'
-~-
,oooFE.y= 1424 keV ,
I 60 °
7C 90 °
I 1.0
P 0,5
I 0,0
COS2O
]
Ep'= 3,17'MeV
ffp= 3,17 MeV
I
I
gl
120
100
100
~> % 10
60
-
"
-
b-5;i -ff6~
Uh-~
W(0)
=1-0,077
P2 ( C O S 0 )
- O. 001 P4 (COS 1
- 90 °
,
~
I
-60 °
-30 o
0o
ARCTAN (~
~
,
30 °
60 ° =-
60
/
90 °
I 1,0 ~,
I 0,5
O )
1
0.0
COS20
Fig. 6. Angular distributions and X2 diagrams for the 1361 and 1424 keV 7-decay of the 1514 keV level at Ep = 3.17 MeV indicating a spin of 6- for this level. The 1643 k e V level. The spin of this level is known to be ½+ from (p, 7) and particle-transfer reactions 1). The weakness of its excitation in the present experiment, as well as the fairly isotropic angular distribution of its 1491 keV v-decay (considering the rather large experimental errors) is consistent with this assignment.
608
J . G . MALAN et al.
The 1662 ke Vlevel. This level is also weakly excited in the present work, indicating spin values of ½ or I. Analysis of the nearly isotropic 1571 keV 7-ray angular distribution shows preference for the value 1 - although the other spins are not rejected at the 1 ~/o confidence level, A spin o f ~ - or ½- is supported by the I = 1 stripping pattern observed in particle-transfer reactions and the results of (p, 7) experiments 1). A comparison of the experimental level scheme with calculations of low-lying negative-parity states shows better agreement with the Coriolis coupling model of Malik and Scholz 23) than with the pure (lf~)" calculation of McCullum et al. 24). With a deformation of - 0 . 3 9 the former model reproduces the low-lying ~ - , ) - , 3triplet and correctly predicts the ~ - and -12 -~-- states near 1 MeV, albeit in reverse order. A!though a clear correspondence with calculated states does not exist for the other experimental levels, the present work does suggest candidates for all the negative-parity states predicted below an excitation of 2 MeV. The proposal 3) that the 1140 and 1602 keV levels are members of a rotational-like band based on the 748 keV ld ~- hole state is neither supported not discredited by this work.
References l) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21 ) 22) 23) 24) 25)
Nuclear Data Sheets 4 (1970) 397 J. N. Mo., B. Cujec, R. Dayras, I. M. Sz6ghy and M. Toulemonde, Nucl. Phys. A147 (1970) 129 P. Blasi, M. Mand6, P. P. Maurenzig and N. Taccetti, Nuovo Cim. 4A (1971) 61 H. V. Klapdor, Nucl. Phys. A134 0969) 419 J. C. Legg, D. G. Megli, D. R. Abraham, L. D. Ellsworth and S. Hechtl, Phys. Rev. 186 0969) 1138 P. N. Maheshwari, C. St.-Pierre and D. Doutriaux, Nucl. Sci. Appl. 3, no. 2 0967) 16 C. St.-Pierre, P. N. Maheshwari, D. Doutriaux and L. Lamarche, Nucl. Phys. A102 0967) 433 D. Bachner, R. Santo, H. H. Duhm, R. Bock and S. Hinds, Nucl. Phys. A106 0968) 577 D. J. Pullen, B. Rosner and O. Hansen, Phys. Rev. 166 0968) 1142 D. J. Pullen, B. Rosner and O. Hansen, Phys. Rev. 177 (1969) 1568 B. Cujec and I. M. Sz6ghy, Phys. Rev. 179 0969) 1060 J. W. Tepel, J. G. Malan and J. A. M. de Villiers, Nucl. Phys. A158 (1970) 129 A. J. Elwyn, H. H. Landon, S. Oleksa and G. N. Glasoe, Phys. Rev. 112 0958) 1200 G. J. McCallum, A. T. G. Ferguson and G. S. Mani, Nucl. Phys. 17 (1960) ll6 E. E. Chupp, J. W. M. DuMond, F. J. Gordon, R. C. Jopson and H. Mark, Phys. Rev. 112 (1958) 532 P. Blasi, P. R. Maurenzig, R. A. Ricci, N. Taccetti, R. Giacomich, M. Lagonegro and G. Poiani, Nuovo Cim. 51B (1967) 241 N. Coetzee and J. A. M. de Villiers, private communication J. W. Tepel, Nucl. Instr. 40 (1966) 100 C. A. Engelbrecht, H. Fiedeldey and J. W. Tepel, South African Atomic Energy Board report PEL 202 (1972) C. A. Engelbrecht and H. Fiedeldey, Ann. of Phys. 42 0967), and private communication E. Barnard, J. A. M. de Villiers, D. Reitmann, A. B. Smith, E. M. Pennington and J. F. Whalen, to be published P. A. Moldauer, Rev. Mod. Phys. 36 (1964) 1079 F. B. Malik and W. Scholz, Phys. Rev. 150 0966) 919 J. D. McCullum, B. F. Bayman and L. Zamick, Phys. Rev, 134 (1964) B505 J. E. Wiest, C. Robertson, F. Gabbard and M. T. McEllistrem, Phys. Rev. C4 (1971) 2061
1
Nuclear Physics A195 (1972) 596--608; (~) 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 f r o m the publisher
ENERGY LEVELS OF
49V F R O M
49Ti(p,
n)49V
J. G. MALAN, E. BARNARD, J. A. M. DE VILLIERS, J. W. TEPEL and P. VAN DER MERWE Atomic Energy Board, Pelindaba, Transvaal, Republic of South Africa
Received 17 April 1972 (Revised 12 June 1972) A bstraet: Energy levels of 49V were populated by the 49Ti(p, n) reaction at proton energies between 2.03 and 3.25 MeV. Spins were determined within the framework of the statistical theory from the angular distribution of de-excitation v-rays measured at five incident proton energies. Excitation strengths and decay branching ratios of the various levels were taken into account. A complete level and decay scheme for 49V up to an excitation of 1.7 MeV is presented. [ E
NUCLEAR REACTIONS 49Ti(p, n), (p,n~'),E = 2.0-3.3 MeV; measuredo'(E; E,L o'(E; E~,, 07.), E~,, 1~.. 49V deduced levels, J, =, )'-branching. Enriched targets.
]
1. Introduction The level structure of 49V has been studied through a variety of nuclear reactions. The results of these investigations have been summarized and evaluated t). Recently two more high-resolution studies of 49V appeared, viz. a 46Ti(~, pT)49V experiment by Mo et al. 2) and, during the course of the present experiment, a 49Ti(p, nT)49V experiment by Blasi et al. a). Together with the earlier work, this results in a fairly comprehensive and accurate level and decay scheme for 49V up to an excitation of 2.5 MeV. The spin values of a number of the observed levels are however uncertain or completely unknown. Information on spins has been furnished mainly by the (p, ~,), (3He, d) and (t, ~) experiments 4 - t l ) . In the (p, ~) experiments, the decays of resonances with probable spins of ½ or ~ were studied, effectively limiting information on low-lying levels to those with spin < ~. Since stripping and pick-up experiments were performed on spin-zero targets, similar limitations apply to levels studied by these methods. In addition the latter reaction mechanisms are somewhat selective. Highspin states in f~ shell nuclei are often particularly suited to investigation by means of (p, n) and (p, nT) reactions ~2). The non-selective nature of these compound-nuclear processes ensures an excitation probability for any particular level solely dependent on its spin/parity value and independent of any complicated structural considerations. Previous investigations of the 49Ti(p, n) and (p, ny) reactions 3, 13-16) were concerned mainly with level energies and the decay scheme of 49V. The present investigation was therefore initiated in order to obtain more complete data on the spins o f 596
49Ti(p, n)*gV
597
low-lying49V levels, primarily by way of a comparison of y/-ray angular distributions at various proton energies with the predictions of the statistical theory. The information simultaneously obtained on the relative excitation probabilities of the different levels and their decay characteristics, as well as data from other experiments, were taken into account in making spin assignments. It was considered prudent to verify the generally assumed spin values of the ground and first two excited states of 49V, to which most of the higher levels decay, before proceeding with the analysis. This was done by measuring relative neutron intensities to these levels over an adequate energy range, and comparing the results with the predictions of the statistical theory.
2. Experimental procedure and data analysis The experimental arrangement was similar to that described previously 12). A pulsed ( ~ 1 ns pulse length) proton beam of < 12 #A was provided by the AEB 3 MV Van de Graaff accelerator at Pelindaba. The maximum proton energy was 3.25 MeV, limiting the excitation of 49V via the (p, n) reaction (Q = - 1.383 MeV) to 1.80 MeV. The targets consisted of up to 320/~g/cm z of TiO2, enriched to 66 ~ in 49Ti, evaporated onto 0.25 m m thick tantalum discs. The water- and air-cooled target was exposed to the proton beam at an angle of 45 °, giving an effective target thickness of ~ 40 keV in the energy range used. The neutron intensity measurements were performed with a standard time-of-flight system employing a plastic scintillator. The relative efficiency of the detector was calibrated against that of a standard long counter. For the y-ray angular-distribution measurements a 40 cm 3 Ge(Li) detector was fixed at 90 ° as a monitor, while another 40 cm 3 detector was moved between 0 ° and 90 ° in 15 ° steps. Monitoring in this way on the same 7-ray observed in the angular distribution, or on the total decay of the level from which the y-ray originates, ensured reliable results. Monitoring by means of the total neutron or y/-ray flux (or by means of a known y-ray angular distribution) proved to be unsatisfactory. This was probably due to slight drifts in proton beam energy giving rise to fluctuations of a few percent in the relative intensities of y-rays from different levels. When the latter methods of monitoring were used, every angle was usually repeated at least twice, and a few of the distributions were completely remeasured as a check on the results. The resolution of both Ge(Li) detectors was ~ 2.8 keV at E~ = 1.33 MeV. The front of each detector was shielded by 2.4 m m of lead which suppressed the high flux of low-energy 7-rays, but also effectively precluded the observation of weak ~-rays of energy < 200 keV from the target. Efficiency and energy calibrations were performed with standard 7-ray sources. Time-of-flight gating methods were used in order to eliminate background due to neutron effects in the Ge(Li) detector, which were especially troublesome in the vicinity of the 595 keV y/-ray transition in 49V. As most of the other y/-ray peaks of interest were undisturbed by the neutron-induced back-
598
J.G. MALAN
et al.
ground, some angular distributions were obtained without time-gating. The target to detector distance was about 35 cm for the time-gated, and somewhat less for the ungated runs. Spectra were recorded with a dispersion o f < 1 keV per channel in a 2048-channel analyser. The data were analysed for peak energies and areas on a C D C 1700 and an I B M 360/40 c o m p u t e r with the p r o g r a m m e s L I Z ~7) and P E A K 18) respectively. The latter p r o g r a m m e was slightly modified in order to facilitate the handling o f 7-ray spectra. TABLE l Optical potentials used in the Hauser-Feshbach calculations Parameter
Potentials neutron A
neutron B
proton
45.85 4.845 0.62
44.36 4.543 0.62
47.08 4.845 0.62
13.92 5.345 0.50
9.0 4.797 0.50
13.48 5.345 0.50
R e a l well
depth (MeV) radius (fm) diffuseness (fro) I m a g i n a r y Gaussian s u r f a c e
depth (MeV) radius (fm) diffuseness (fm) Spin-orbit
depth (MeV)
7.0
7.0
7.0
The energy dependence of the potential depths is given in ref. 20). Values shown are for E o = 3.0 MeV and E. = 0.5 MeV. Saxon-Woods and Thomas radial shapes are used for the real well and the spin-orbit term respectively.
Statistical-model calculations were performed with the c o m p u t e r code P E L I N S C A [ref. ~9)]. Transmission coefficients for p r o t o n and neutron channels are calculated in this p r o g r a m m e f r o m the energy-dependent optical-model parameters r e c o m m e n d ed by Engelbrecht and Fiedeldey 2o). This potential (A in table 1) gives a g o o d account o f the s-wave neutron strength function which peaks strongly for A ~ 50 but predicts a (p, n) transition strength too large by a factor o f 3 or more for the 748 keV level in 49V (spin ~+ or ~-+, see sect. 4). However, in a recent study 21) o f elastic and inelastic neutron scattering from T i a slightly modified set o f parameters (B in table 1 ) with the same energy dependence as in ref. 2o) gave a much better fit to the experimental data, although not reproducing the s-wave strength function satisfactorily. The reason for the discrepancy is not clear, but may in part be due 22) to the omission o f level width fluctuation corrections in the usual Hauser-Feshbach calculations performed by P E L I N S C A . In this work all calculations are based on potential B in table 1.
'*9Ti(p, n)4°V
599
3. Experimental results 3.1. LEVEL AND DECAY SCHEME T h o u g h the present experiment was primarily concerned with spin assignments, some useful data o n the decay of 49V were also obtained, as s u m m a r i z e d in table 2. TABLE2 Level energies and branching ratios in 49V Level energy a) (MeV)
E3' (MeV)
748.2 ±0.2
595.4i0.2 657.5 ±0.2
1021.7~0.2
1021.7,,0.2
100.0
1140.44-0.2
392.2520.2 987.5320.3 1049.74-0.2
5.9-4-0.9 18.44-0,5 26.8 4-1.1 48.9 ±0.6
5.24-0.6 16.64-1.5 26.4 320.9 51.8 4-1.3
1140.44-0.2
Branching ratios present work
ref. 3)
54.0 ±0.8 46.0 --0.8
55.4 z~0.6 44.6 ±0.6
1155.3 320.2
1064.6 --0.2 1155.3 4-0.2 (133) b)
22.8 320.3 77.2 4-0.3
22.4 -4-0.4 74.7 4-0.7 2.94-0.6
1514.6 4-0.2
1361.5 4-0.3 1423.94-0.3 1514.8 4-0.3
56.0-4-1.2 11.3 -4-0.6 32.7-- 1.5
56.3 321.0 11.3 4-0.4 32.4 4-0.7
1602.74-0.2
447.4-4-0.3 462.2-4-0.2 854.5±0.3 1512.04-0.3 1602.6.4.0.2
0.9320.4 7.5 4-0.9 8.64-1.8 55.64-1.1 27.4-4-1.1
6.9 -4-0.6 8.04-0.8 59.55_ 1.3 25.64-0.6
1643.34-0.4 1661.8320.2
1490.44-0.4 c) 1571.1--0.2 (1508) d)
64 ± 4 36 4-4
") The level energies of the first and second excited states were taken as 90.7 and 152.9 keV respectively l). b) Not observable in the present experiment (see text). ~) Very weak transition. Background structure in this area makes a reliable determination difficult. d) Not observable in the present work due to proximity of the very strong 1512 keV line. 111 c o m p a r i n g these results with the work of Bias± et al. 3, 16) and M o et al. 2) it must be b o r n e in m i n d that the present experimental layout precluded the study of the decay o f the first two excited states a n d of the proposed weak 133 keV t r a n s i t i o n between the 1155 a n d 1022 keV levels 3). Due to their low spin values a n d the l i m i t a t i o n in m a x i m u m p r o t o n energy, the 1643 a n d 1662 keV levels were rather weakly excited. A very weak 898.2 +_0.5 keV ?-ray of u n k n o w n origin is observed in the present work.
600
J . G . M A L A N et al.
This y-ray was tentatively allocated to a 1643-748 keV transition by Blasi et aL 3), but in view of energy balance considerations this assignment cannot be supported. The proposed 1508 keV y-ray in the decay of the 1662 keV level 3) was not observed in the presence of the very strong 1512 keV line. The rest of the decay scheme and branching ratios are in excellent agreement with the results of ref. 3) and agrees substantially with ref. 2). A previously unreported 447 keV y-ray is allocated to a transition between the 1603 and 1155 keV levels. This assignment is made on the basis o f energy sums and its observed threshold. Assuming that this line originates from the 1603 keV level, its branching ratio remains constant over the energy range studied. 1661,8~ 1643,3 1602,7 J 1514,6
27569 8 1 33 1156
1155,3 ~
5027186
11 ~ , 0 , 4 ~ "
II
I
I II I
1
i
4654
112-, 312" - 172 ÷ ~__ 712 +
I I I t
7723
I
1oo
1021,7
748,2
f
I
912",(71
2- )
"~- 512 1112~'~
3•2 +
o
r.
152,9 90,7 712"
0
Fig. 1. Level and decay scheme of 49V.
A complete decay scheme for 49V up to and including the 1662 keV level ispresented in fig. 1. Transitions not observed in the present work but reported by Blasi et al. 3) are included as dashed lines. 3.2. R E L A T I V E I N T E N S I T I E S
Neutron intensity measurements for the groups feeding the ground and first two excited states of 49 V were made at 30 keV intervals between 2.03 and 2.12 MeV. N o prominent resonance structure or intensity fluctuations were observed in this energy region, indicating adequate averaging over levels in the compound nucleus. F r o m isolated neutron-intensity measurements at higher energies, as well as from y-ray angular distributions supplemented by yield measurements at 55 °, relative excitation strengths for all observed levels were deduced. All known 7-ray branching ratios and cascades from this and previous work were taken into account. After averaging over sufficiently large energy intervals, the relative intensities are compared
595 657
1022
392 988 1050 1140
1065 1155
1362 1424 1515
462 855 1603
1490
1571
748
1022
1140
1155
1515
1603
1643
1662
~14
--0.01±3 0.04±3 0.03~2
(/2
0.03£:2 - 0.03i2
0.19±2
Ev ~ 2.56 MeV
Quoted errors pertain to the last decimal digit.
E~.i (keV)
Energy level (keV)
T A BL E 3
0.02£3 0.02±2 0.00±2 0.05£2 --0.01±2
--0.00±2 0.02±1
--0.00±2
--0.02i3 --0.02±3
a4
0.02£3 --0.04±2 -0.00±2
0.19±1
0.03±2 --0.01£2
a2
Ep = 2.89 MeV
0.0122 -0.01±2 0.01i3 0.01i4 --0.02±4 --0.01~3 0.04±4 --0.0122 --0.01i4 0.00+3 --0.02~3
0.17~2 -0.06±2 --0.03±3 0.03~3 --0.01±2 0.04±3 0.07±2 0.08~3 --0.06±2 --0.00±3
a4
--0.01£I
a2
Ea, = 3.02 MeV ~12
0.01i3
0.03±3 0.02~4 0.00±2 0.01~2
0.01i2
0.01±2
a4
0.01±3 0.02~2
0.03±5 0.05±3
0.05~3
0.05£3 0.01±3 0.02i2 --0.01±2 --0.03t2 0.01±3
0.11±3 0.19±2
0.04±1 0.00±2 0.06~1 - - 0 . 0 0 £ 2
--0.03±3 0.02±4 0.01+1 --0.01±1
0.16~2
0.01±2
gl2
/~p = 3.25 MeV
0.05±12 - - 0 . 0 9 ± 4
0.01~3 --0.02±5 0.02±2
0.04±3 0.06±4 -0.02±2 --0.05±9
- 0.01i3 0.0223
0.02±2 0.00£2
0.02±4 0.03±4 0.04±3 0.03±2
0.15~2 --0.20£2
--0.01±2 0.00±1
0.03£3 0.03±3 --0.02±2 0.01~2
0.14~2
0.01il 0.01i2
04
Ep = 3.10 MeV
-0.03£1 0.00±1
Legendre polynomial coemcients ofT-ray angular distributions
8
602
J . G . M A L A N e t al.
with Hauser-Feshbach calculations as an aid in making spin assignments (sect• 4). For the 1602, 1644 and 1662 keV levels the averaging was probably not adequate, due to the limitation in maximum proton energy. 3.3. G A M M A - R A Y A N G U L A R
DISTRIBUTIONS
Gamma-ray angular distributions were measured at /~p = 2.56, 2.89, 3.02, 3.10 and 3.25 MeV. The incident proton energies were chosen close to the thresholds of the individual levels studied. Best-fit normalized Legendre polynomial coefficients are presented in table 3. These values show little change with increasing proton energy, although absolute ~-ray intensities fluctuated appreciably. The variation of a2 values TABLE 4 Summary of spin and mixing-ratio determinations Energy level (keV)
Accepted values ")
Ev
J
(keV)
final state
748
595 657
]~-
~+ ~+
1022
1022
5-
.~¢-~
1140
392
~+
~+
988
~-
~+
1050
~-
~+
1140
5-
~+
./
6
>0
0.11±0.18
1155
1065
~-
1155
5-
G-) 2G-)
1362 1424
~~-
~~-
1515
5-
~-
855
~+
~-
1512
~-
~-
1603
5-
•643
•490
~-
~*
1662
1571
}-
½-
1515
1603
or
6.31+3"*tb-oo)
--a~ " 3~~ - -+0 .0.21 24
or
5.67 _+~92 .
--0.14+0.18
+1.51 or --3.49_7.94
~-
5+ ~-
5+
unrestricted 0.32±0.10 or unrestricted --
08
"
41 5.67_+ 1 3 .2.40
+0.7 --2.0
06+2.1 • --O.fi <0
not determined --0.29±0.11
+0.48 or --2.36_0.77
>0 ~) See text for arguments on spin assignments. b) Mixing-ratio errors were determined on the basis of the 0.1 o/ confidence limit relative to the best-fit Z 2 value.
49Ti(p, n)49V
603
outside the quoted errors for a few of the weaker y-rays is probably due to incomplete averaging in the compound nucleus or to slight contamination from angle-dependent background. At the higher incident proton energies some cascade feeding of the lowerlying levels occurred. This was serious only in the case of the 748 keV level, where at Ep = 3.25 MeV about 30 % of its total decay strength came from this source. Since the decay from this level was observed to be almost isotropic at all energies, even when no feeding occurs, this did not influence any of the conclusions. Cascades to other levels amounted to < 4 % (1022 keV level), < 7 ~o (1140 keV level) and < 1% (1155 keV level) of the total decay strengths respectively and were consequently disregarded in the analysis of the angular distributions. In order to improve averaging in the compound nucleus, measurements at 2.89 and 3.02 MeV, and again those at 3.10 and 3.25 MeV were averaged before proceeding with the Zz analysis. Spins were rejected on the basis of the 0.1 }~ confidence limit. Mixing ratios determined in the analysis are given in table 4.
4. Discussion of spin assignments Spin assignments to energy levels of 49V are presented in fig. I. Cognisance has been given to all available experimental data.
The 9round state, 91 and 153 keVlevels. An 1 = 3 stripping pattern for the ground state of 49V has been clearly established 6- ~t). For the 90 keV level an 1 = 3 pattern
49Ti (p,n)49V ffp = 2.06 MeV
B I-I
On= 0 °
I-t/~ Z U.I z
- -
EXPERIMENT H.F. THEORY
?-
5-
3-
7-
7-
3-
2
2
2
~
T
T
0
91
0
91
153
S2
~2
52
72
153 - J ~
Ex
UJ tz:
3-2
3" 2
Fig. 2. Intensity histograms for the ground and first two excited states of 49V indicating a spin sequence of ~2- ~ ~-, 2'~- for these levels.
604
J.G.
MALAN
e t al.
was also observed a), limiting the spin values of these two levels to ) - and ~-. Assuming the known spin value o f ~ - for the 153 keV level ~), only the spin sequence -~-, ~- and ~- is compatible with the measured relative neutron intensities (fig. 2). This sequence is in agreement with the generally assumed values and with the results of (p, 7) measurements 4, 5). The 748 k e V level In both stripping and pick-up reactions 8-1t) evidence was found for an 1 = 2 transition to this level, allowing spins of z2-+ and ~+. Proton capture and pick-up reactions 5, s) favour the value ~r. From the 7-ray angular distributions of the present work it is impossible to distinguish between the two alternatives,
4 9Ti (p,n)49 V ~ (748 keY) 512" 512-
..... 150
+ ÷
100 312+-
b
50 112"-
2,2
2,4
2.8
2.6 Ep
3,0
3,2
---
Fig. 3. Relative neutron intensities for the 748 keV level compared with Hauser-Feshbach calculations.
since the almost isotropic 595 and 657 keV distributions (table 3) allow all spin values < 5. However, a comparison of the excitation strength of this level relative to that of adjacent levels of known spin completely rejects the 5+ value, while -2 ~+ gives good agreement with the Hauser-Feshbach theory (fig. 3). The 1022 k e V l e v e L In the 5°Cr(t, ~) reaction 8) a rather flat angular distribution for the transition to this level was observed, suggesting a high spin value. This is corroborated by its single decay to the ~- ground state. In the present work the consistently anisotropic 7-ray angular distribution is compatible only with spin values of 11_+ (pure M2 or E3 radiation) and -x2-L- (pure E2 radiation) (fig. 4). However, it should be pointed out that a 11_- state at an excitation of about 1 MeV is predicted by both the Coriolis coupling model 23) and a pure (14)" calculation 24) and has been shown to exist in a number of nuclei in this mass region a' 12, 2s). A low-lying _lj_+ state is excluded from shell-model considerations and has not been observed in 2
'~9Ti(p, n)49V
605
relevant experiments. The spin of the 1022 keV level is therefore proposed to be -~-and this value has been assumed in all calculations of relative excitation strengths.
I
I
Ey = 1022 keY ,oo
ooL/
y"
I
[~p= 2,56 MeV
L,
'"~[---
" ~- S0
I
-60 °
-30" ~o ARCTAN (~
30° -~
1000
0.5 COS 2 e
I
- Ey ='1022 keY
'
E~/=1022 keV
~'p= 2.9; MeV1
"~
I
1.0
~0°
60
-
_~NN~ I\*'
W(e)=l+O'209P2(COSe)
- 0.006P4(COS0 ) -90 °
.....
- / o
I
0,0
I
~p = 2,95 MeV j~
Io22
100
1 ,°°J', .;I >-
J=7
/\
,-,,
90
',i/
,-I
~o
..J
-7
~-
It"
80
W(, 0_)1,0 _ ~
11
- 0.005 p4 (cose)
1 -
90°
I
-60 •
I
-30 o 0o ARCTAN 8
'
30°
60°
90'>
i
1.0
i
0.5
<~
i
0.0
-'------ C O S 2 O
Fig. 4. Angular distributions and Zz diagrams for the 1022 keV ground state y-decay. The Z z axes have been shifted for display purposes. A spin value of ~ is indicated for this level. The 1140 k e V level. Spin values of ~ or -~ are suggested for this level by (p, 7) experiments 5). No clear evidence for any particular spin value has emerged from other reactions exciting this level. The observed branching of its 7-decay indicates spin values of:~- or ~z, whereas the almost isotropic angular distributions allow spin values < 5. A comparison of the relative excitation strength of this level with theory (fig. 5) shows that its spin is either ~+ or ~-.
606
J . G . MALAN e t al.
The 1155 keVlevel. This level is not populated in the (p, 7) reaction, which might indicate a high spin value. Particle transfer reaction~ contribute no information on its probable spin. The observed v-decay to the-~-- 1022 keV level 3) and to the ground as well as first excited states limit its probable spin values to ~- and 9-. Relative intensity measurements (fig. 5) are compatible with both these values whereas ~-, (~-) and ~- are allowed by the slightly anisotropic angular distributions. Although a spin value of ~- is therefore not completely rejected, the present results indicate a clear preference for the ~- value. This assignment is supported by a comparison with the situation in nearby nuclei, where the -~2 -~-- level is always accompanied by a ~-- state [refs. ~' ~2, 25)], as indeed it should be from theoret~caJ considerations 23,24).
1155 keY LEVEL
1140 keV LEVEL 1,5
1514 keV LEVEL
1,5 7/2 "
I
~ 9 / 2 -
1,0
~
b"-
7
/
t
2
712"
O
Jl-,-
y
L,') [¢1
,4"
b
712"
0,5
1,0
"
0.5
0,5
b~
5•2-
~ < 512+
512
512"
3/2-
b"
~
~312" I
i
I
I
I
2,6
2,8
3,0
3,2
3.4
Ep (MeV) - - - - . = - "
0
I
I
2,6
2,8
I
I
3,0 3,2 Ep (MeV)
3/2" 312 °
I
[
J
I
3,4
3,0
3,2
3,4
Ep (MeV)- ' - D -
Fig. 5. Comparison of experimental and theoretical relative excitation strengths for the 1140, 1155 and 1514 keV levels. Spin values of ~, ~ or ~, and ~ are preferred for these levels respectively.
The 1515 ke V level. The observed decay of this level to the ground and first two excited states suggests spin values of ~-, 2 -~ or ~-. In the 48Ti(p, V) work of Legg et al. [ref. 5)] the possible spin values are given as ½, ~ or ~. In the present experiment large anisotropies were observed for the 1361 and 1424 keV v-rays, while the 1515 keV decay was almost isotropic. Assuming that the M2, E1 mixing ratio is ~ 0, the Z2 analysis (fig. 6) selects the spin value s - , in agreement with intensity measurements shown in fig. 5. The reason for the relatively poor fit to the distribution for the 1424 keV v-ray is not clear but might be due to incomplete averaging in the compound nucleus or to a slight contamination of the relevant peak by angle-dependent background. The 1602 k e V level In a (t, ~) experiment 8) a rather doubtful l = 5 stripping pattern was observed for this level. However, its decay as shown in fig. 1 favours the
49Ti(p, n)49V
607
spin values ~ - and ~-+. The angular distributions, although not excluding 2 + at the 1 ~o confidence level, indicate preference for the former value. The excitation strength o f this level was deduced from measurements at only two proton energies and suffers from incomplete averaging. Nevertheless, intensity measurements favour the spin value 2. Both the values } - and ~+ are therefore possible for this level. 100
E.y= 1361 keY
'
1
[~p= 3,17 MeV
Ep = 3,17' MeV
jIT l
t~=--,
10
.J
><
~~ .
%
1.0
~
o,1"~CONF.LIMIT
islA
100
,
=-
9o
(,.. _z
80 W(01=1.0,116
P2 ( C O 5 0 1
* 0 , 0 0 3 P4 (COS 01 1,O .90 o
,
-- 60 °
I -- 30 o
[ 0o
I 30 °
ARCTAN ~'
-~-
,oooFE.y= 1424 keV ,
I 60 °
7C 90 °
I 1.0
P 0,5
I 0,0
COS2O
]
Ep'= 3,17'MeV
ffp= 3,17 MeV
I
I
gl
120
100
100
~> % 10
60
-
"
-
b-5;i -ff6~
Uh-~
W(0)
=1-0,077
P2 ( C O S 0 )
- O. 001 P4 (COS 1
- 90 °
,
~
I
-60 °
-30 o
0o
ARCTAN (~
~
,
30 °
60 ° =-
60
/
90 °
I 1,0 ~,
I 0,5
O )
1
0.0
COS20
Fig. 6. Angular distributions and X2 diagrams for the 1361 and 1424 keV 7-decay of the 1514 keV level at Ep = 3.17 MeV indicating a spin of 6- for this level. The 1643 k e V level. The spin of this level is known to be ½+ from (p, 7) and particle-transfer reactions 1). The weakness of its excitation in the present experiment, as well as the fairly isotropic angular distribution of its 1491 keV v-decay (considering the rather large experimental errors) is consistent with this assignment.
608
J . G . MALAN et al.
The 1662 ke Vlevel. This level is also weakly excited in the present work, indicating spin values of ½ or I. Analysis of the nearly isotropic 1571 keV 7-ray angular distribution shows preference for the value 1 - although the other spins are not rejected at the 1 ~/o confidence level, A spin o f ~ - or ½- is supported by the I = 1 stripping pattern observed in particle-transfer reactions and the results of (p, 7) experiments 1). A comparison of the experimental level scheme with calculations of low-lying negative-parity states shows better agreement with the Coriolis coupling model of Malik and Scholz 23) than with the pure (lf~)" calculation of McCullum et al. 24). With a deformation of - 0 . 3 9 the former model reproduces the low-lying ~ - , ) - , 3triplet and correctly predicts the ~ - and -12 -~-- states near 1 MeV, albeit in reverse order. A!though a clear correspondence with calculated states does not exist for the other experimental levels, the present work does suggest candidates for all the negative-parity states predicted below an excitation of 2 MeV. The proposal 3) that the 1140 and 1602 keV levels are members of a rotational-like band based on the 748 keV ld ~- hole state is neither supported not discredited by this work.
References l) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21 ) 22) 23) 24) 25)
Nuclear Data Sheets 4 (1970) 397 J. N. Mo., B. Cujec, R. Dayras, I. M. Sz6ghy and M. Toulemonde, Nucl. Phys. A147 (1970) 129 P. Blasi, M. Mand6, P. P. Maurenzig and N. Taccetti, Nuovo Cim. 4A (1971) 61 H. V. Klapdor, Nucl. Phys. A134 0969) 419 J. C. Legg, D. G. Megli, D. R. Abraham, L. D. Ellsworth and S. Hechtl, Phys. Rev. 186 0969) 1138 P. N. Maheshwari, C. St.-Pierre and D. Doutriaux, Nucl. Sci. Appl. 3, no. 2 0967) 16 C. St.-Pierre, P. N. Maheshwari, D. Doutriaux and L. Lamarche, Nucl. Phys. A102 0967) 433 D. Bachner, R. Santo, H. H. Duhm, R. Bock and S. Hinds, Nucl. Phys. A106 0968) 577 D. J. Pullen, B. Rosner and O. Hansen, Phys. Rev. 166 0968) 1142 D. J. Pullen, B. Rosner and O. Hansen, Phys. Rev. 177 (1969) 1568 B. Cujec and I. M. Sz6ghy, Phys. Rev. 179 0969) 1060 J. W. Tepel, J. G. Malan and J. A. M. de Villiers, Nucl. Phys. A158 (1970) 129 A. J. Elwyn, H. H. Landon, S. Oleksa and G. N. Glasoe, Phys. Rev. 112 0958) 1200 G. J. McCallum, A. T. G. Ferguson and G. S. Mani, Nucl. Phys. 17 (1960) ll6 E. E. Chupp, J. W. M. DuMond, F. J. Gordon, R. C. Jopson and H. Mark, Phys. Rev. 112 (1958) 532 P. Blasi, P. R. Maurenzig, R. A. Ricci, N. Taccetti, R. Giacomich, M. Lagonegro and G. Poiani, Nuovo Cim. 51B (1967) 241 N. Coetzee and J. A. M. de Villiers, private communication J. W. Tepel, Nucl. Instr. 40 (1966) 100 C. A. Engelbrecht, H. Fiedeldey and J. W. Tepel, South African Atomic Energy Board report PEL 202 (1972) C. A. Engelbrecht and H. Fiedeldey, Ann. of Phys. 42 0967), and private communication E. Barnard, J. A. M. de Villiers, D. Reitmann, A. B. Smith, E. M. Pennington and J. F. Whalen, to be published P. A. Moldauer, Rev. Mod. Phys. 36 (1964) 1079 F. B. Malik and W. Scholz, Phys. Rev. 150 0966) 919 J. D. McCullum, B. F. Bayman and L. Zamick, Phys. Rev, 134 (1964) B505 J. E. Wiest, C. Robertson, F. Gabbard and M. T. McEllistrem, Phys. Rev. C4 (1971) 2061