Short range energy dependence of the neutron widths of 177Hf resonances

Nuclear Physics A264 (1976) 9 3 - 104; ~ North-Holland Publishino Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

SHORT RANGE ENERGY DEPENDENCE OF THE NEUTRON WIDTHS OF 17"/Hf R E S O N A N C E S G. ROHR and H. WEIGMANN

Central Bureaufor Nuclear Measurements, Geel, Belgium Received 15 December 1975 Abstract: Neutron widths of ~77Hf+ n resonances have been determined from neutron radiative capture and self-indication measurements. Together with the resonance spin classification of Coceva, the data have been used to study the spin and energy dependence of the neutron strength function. A statistically significant energy dependence of the strength function for spin 4- has been observed. Various statistical tests show the presence of two narrow structures with a "spreading width" of the order of the level spacing ( ~ 5 eV).

NUCLEAR ~x~ACTION 177Hf(n, 7), self-indication, E = 10-300 eV; measured a(E, E~). 17SHfresonances deduced F., So. Statistical test for intermediate structure,

I. Introduction The nucleus 177Hf is one of the few examples where an almost complete spin determination of a large set of neutron resonances has been achieved 1). In their paper describing this spin classification and a number of statistical tests on the resonance spacings distribution, Coceva et al. i) also point to an apparent energy dependence over 100 eV intervals o f the density of spin 4 - resonances in the compound nucleus 178Hf. Such narrow structures in neutron resonance data other than the level density, have recently been claimed to be observed in other nuclei, e.g. in ~lSin [-ref. 1)]. However, this case is less significant, because the observed structures do not seem to be clearly connected with a special c o m p o u n d nuclear spin. In order to study the case of iv 7Hf in more detail, we have measured the neutron widths of the resonances with assigned spin i.e. up to 300 eV neutron energy. Preliminary results have already been reported 3,4).

2. Experiment and results Neutron capture.cross-section and self-indication measurements have been made. A sample o f 0.28 g/cm 2 o f l i f O 2 with XVVHfenriched to 74 ~ was located 60 m from the target o f the C B N M electron linac which was used as a pulsed neutron source. The lime-of-flight resolution varied between 1.5 nsec/m at 300 eV neutron energy 93

94

G. ROHR AND H. WEIGMANN TABLE 1 Resonance parameters of aTVHf

E (eV)

gFnFy/F(meV)

10.95 13.67 13.96 21.97 22.26 23.44 25.62 27.01 31.57 32.80 36.08 36.95 43.03 45.12 46.21 48.79 49.57 54.75 56.34 57.01 59.26 62.17 63.45 66.75 70.00 71.35 72.25 75.60 76.02 82.37 84.58 85.29 86.77 88.52 93.16 97.09 99.01 103.08 104.10 111.40 111.89 114.44 115.11 121.26 122.64 123.74 126.14 131.25 134.12

0.25+0.02 0.42+0.03 1.72+0.11 1.04_+0.07 0.36+0.05 0.79+0.05 0.23_+0.02 1.27_+0.08 0.15 +0.02 0.76+0.07 1.49___0.11 4.70_+0,32 2.74+0,18 1.78-+0A2 3.65-+0.23 10.30_+0.92 2.79+0.22 7.30__+0.50 5.14-+0.33 2.18-+0.21 1.79_+0.13 0.61 +0.08 16.80+ 1.37 16.3 ___1.36 0.25 +0.04 6.50_+0.42 0.57+0.11 0.94+0.11 7.15-+0.49 0.28_+0.05 9.20-t-0.62 2.36+0.16 0.51 _+0.07 1.90+0.15 1.83_+0.14 8.13+0.54 0.48_+0.08 12.70-+0.84 0.63-+0.07 1.05_+0.08 2.08_+0.15 0.13_+0.03 1.94_+0.16 2.02_+0.18 0.31 _+0.03 3.70_+0.28 0.34_+0.07 14.20_+0.95 1.94_+0.15

J

L

F. (meV)

3 4 3 4 3 4 3 3 3 4 3 4 4 4 4 3 4 4 3 4 3 3 4 3, 4 4 4 3 3 4 4 4 3 4 3 3 4 4 3

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0.57+ 0.75+ 4.16+ 1.90___ 0.83_+ 1.43_+ 0.52_+ 3.02+__ 0.34_+ 1.38_+ 3.57_+ 9.72+ 5.31 + 3.34-+ 6.96-t32.80-+ 5.42+ 15.80+ 13.90-+ 4.15+ 4.33-+ 1.42_+ 65.30_+

0 0 0 0 0 0 0 0 0 0 0 0 0 0

0.44-+ 0.07 14.50+ 0.56 1.32-+ 0.25 2.21 ___ 0.26 16.27_+ 1.01 0.50_+ 0.09 22.77_+ 1.51 5.81_+ 0.37 0.92_+ 0.12 4.61-+ 0.36 4.43+ 0.32 19.20+ 1.51 0.86_+ 0.14 54.20_+ 3.32

3 4

0 0

2.48_+ 0.18 3.94+ 0.27

3 3

0 0

4.71_+ 0.36 4,92_+ 0.43

3 4 3 4

0 0 0 0

9.54_+ 0.61 _+ 61.40-+ 3.66_+

0.05 0.06 0.35 0.13 0.11 0.10 0.05 0.20 0.05 0.12 0.26 0.27 0.33 0.21 0.16 0.89 0.41 0.84 0.54 0.38 0.30 0.19 2.20

0.65 0.13 4.96 0.27

Fr (meV)

83.7+15.7

56.2+ 7.5

77.7-+21.0 81.5+ 4.8 68.8_+ 12.6 69.6_+ 13.7

55.1+ 1.4 unresolved doublet 54.2_+ 7.8

58.60- 14.2 63.2_+ 4.2

67.1_+ 5.6

NEUTRON WIDTHS OF 177Hf

95

TABLE 1 (continued) E (eV)

gF~FJF(meV)

J

L

136.24 137.53 141.17 143.18 143.81 145.62 148.59 151.00 152.85 156.08 160.08 163.05 167.38 171.13 174.37 176.15 176.92 181.13 184.70 192.75 194.18 199.22 201.90 208.70 210.14 212.16 217.18 219.69 222.40 223.37 224.74 226.80 229.22 232.88 236.36 238.74 240.92 248.96 259.68 264.64 267.74 272.36 273.32 284.89 288.13 294.50 298.70 299.74

0.31 4-0.05 5.33___0.38 8.844-0.59 2.04_+0.21 4.82_-t-0.44 2.81 4-0.22 7.25-+0.50 0.23 4- 0.07 1.03_+0.07 1.34-+0.13 1.81 +0.16 11.10-+0.72 3.26_+0.25 4.674-0.41 5.504-0.43 14.20_+ 1.60 13.60-+ 1.59 2.51 4-0.26 0.75+0.20 3.39+0.32 3.31 +_0.36 8.57_+0.67 6.454-0.74 13.40+ 1.01 1.87_+0.27 0.92+_0.27 3.124-0.35 4.33-+0.46 1.57+0.26 3.204-0.33 17.20+ 1.13 4.86_+0.53 2.954-0.32 0.234-0.24 4.45_+0.53 16.00_+ 1.21 7.71 _+0.69 9.02_+0.84 1.00 + 0.28 17.20--+ 1.30 13.00_+ 1.40 13.40-+ 1.59 7.28_+ 1.52 24.00 4- 1.38 4.434-0.53 1.96_+0.35 21.20 _+2.35 4.05-+0.62

3 4 4 3 4 3 3

0 0 0 0 0 0 0

4 3 4 3 3 3 4 3 4 4 4 4 3 4 3 4 3 3 4 3 3 4 3 4 3 4 3 3 4 3 4 3 4 4 3 3 4 3 4 3

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

F. (meV) 0.71 + 11.32__ 23.90_+ 4.97410.0547.03_+ 21.35-+

0.13 0.72 3.33 0.49 0.81 0.51 1.34

1.89+ 0.12 3.194- 0.30 3.40-+ 0.29 44.90_+ 6.59 8.28+ 0.59 12.47_+ 0.97 11.76_+ 0.81 57.81 _+ 6.60 41.464- 4.56 4.83+ 0.48 1.364- 0.35 6.72+ 0.57 8.42+ 0.84 20.664- 1.46 18.41 4- 1.78 45.80-+ 10.80 4.53_+ 0.61 2.16+ 0.62 6.134- 0.63 11.424- 1.07 3.77-+ 0.61 6.304- 0.60 83.87_+ 12.35 10.15_+ 0.97 7.41 _+ 0.74 0.41 -+ 0.42 11.79_+ 1.23 72.30_+ 9.23 17.94_+ 1.38 28.57__+ 2.31 1.83 4- 0.50 77.20+21.90 38.42-+ 3.93 40.42+ 4.4l 21.464- 3.56 148.00 4- 43.00 9.ll+_ 0.95 4.76-+ 0.82 107.62 + 28.26 10.584- 1.43

F 7 (meV)

47.2_+ 12.9

58.24- 10.8

48.8+ 12.5

80.1 +23.0

87.3 _+ 14.4

The F, values of resonances for which no individual F;. is given were calculated from #F,FJF assuming an average F = 74 meV for ~pin-3 and F ~ 58 mcV for spin-4 re'~onancc~.

96

G. ROHR AND H. WEIGMANN

and 10 nsec/m at 10 eV. Capture events were detected by a modified M o x o n - R a e capture detector• F o r the cross-section measurements, the relative neutron flux was determined with a 1oB slab viewed by a NaI(T1) detector. Absolute calibration was obtained by the black resonance technique, using resonances in Ag at 5.2, 16.3, 51.4, and 70.9 eV. In the self-indication measurements, a sample of natural H f (metallic disk of 0.69 g/cm 2) was used. Detector and detecting sample were the same as in the cross-section measurement• A combined area analysis of the capture cross-section and self-indication data has been performed, using a modified version of the T A C A S I 5) code. Neutron widths of 98 resonances in the energy range 10--300 eV, and radiative widths for some strong resonances have been obtained that way. The resonance at 255.1 eV was not analysed since its area was strongly influenced by resonances of other isotopes (176'178'179Hf). Furthermore, the resonance at 66.76 eV assigned i) as spin 3has been reanalysed by the authors of ref. 1) using a shape fitting code and additional information from capture y-ray spectra; according to their analysis 6), the resonance is a doublet of different spins and nearly equal contribution to the capture area. Thus, in the present analysis the capture area of the unresolved doublet at 66.76 eV has been attributed to equal parts to two resonances with spin 3 - and 4 - . The results of the resonance analysis are presented in table 1. Errors on the widths as given in table 1 are standard deviations and contain a normalization uncertainty of + 4 % in addition to the statistical uncertainties; both contributions have been added quadratically. The data of table 1 have been communicated to B N L and have been taken into account in the latest resonance data compilation 15). A comparison with older data ~) shows reasonable agreement up to a neutron energy of about 70 eV. Above that energy, we observe relatively more resonances which indicates that in the earlier data a steadily increasing fraction of resonances is missing. Very recently, another measurement on 177Hf has been reported by Liou e t al. 16). In the range o f overlap o f both sets o f data, there is good agreement• 3. D i s c u s s i o n

3.1. STRENGTH FUNCTION F r o m the resonance parameters given in table 1 [below 10 eV neutron widths of TABLE 2

S-wave neutron strength functions of 177Hffor different energy intervals Energy interval (eV)

SO(J = 3-) SO(J = 4-)

0-300

0-100

100--200

200-300

2"~ ~5+°'58 -0.45 2•30+°'s2 -o.41

+1.45 4"03-0.98

2 "07 +0.87 -0.56

+1.28 2'66-0.~7 +0.64 1"21-0.37

3.v n4+1"51 -0.90 + 1.o9 1-77-0.6o

97

N E U T R O N W I D T H S O F 177Hf

ref. 7) have been used] strength functions have been determined for the complete energy range 0-300 eV as well as for partial energy ranges of 100 eV. They are listed separately for spin 3- and 4- in table 2. The errors given in the table have been determined according to the prescription of Liou and Rainwater 8) The spin independent value, (9 , ~ + 0.38~

S O ~- ~. . . .

0.32/X

10 - 4 ,

J=3

L~ 50

10 100

200

En leVI

3O0

1,4 -- + 0.99

50 +0.84

10

En [eV] Fig. 1. Cumulative sum of reduced neutron widths as a function o f energy.

98

G. ROHR AND H. WEIGMANN

is in good agreement with optical model calculations including collective motion 9). Further, the values for the two spin states as obtained from the complete energy range 0-300 eV, are not significantly different. However, the strength function for spin 4 - shows a marked energy dependence: F o r instance, in the ranges 0-100 eV and 100--200 eV there is a change by more than a factor of three. An illustration o f the statistical significance of the energy dependence of the strength function is presented in fig. 1: The cumulative sum of reduced neutron widths as a function of energy is plotted separately for spin 3- and 4 - . Whilst the staircase function for spin 3 - is well fitted by a straight line the slope of which gives the strength function, / X

50

N

10

~6o

260

360

En I~V]

X

50

J

N

,0// o

16o

20o

360

En [.v]

Fig. 2. Cumulative sum of levels as a function of neutron energy. Crosses indicate values obtained from missed level analysis. The line marked "a" indicates the average spacing of spin 4- level (missed levels included) as obtained in the 100-200 eV interval.

N E U T R O N W I D T H S O F 177Hf

99

this is not the case for spin 4 - . In this case the slopes of the straight lines indicate the strength function and its error limits as obtained in the neutron energy range 100-300 eV. Compared to this strength function is the cumulative sum of reduced widths up to 85 eV as well as the error on this sum based on a 68 % confidence interval [~+0.99", (p+0.84) - 0 . 1 6 and on a 98 % confidence interval ~P-o.0U. As seen from the figure, the cumulative sum of reduced widths up to 85 eV falls away from the value expected from the strength function observed in the range 100-300 eV, even if one allows for a 99 % confidence limit. In order to check this result further, additional statistical tests have been performed and are described in the following.

3.2. LEVEL S P A C I N G

In fig. 2 a graph similar to that presented by Coceva et al. ~) of the cumulative number of levels is shown as a function of neutron energy, separately for spin 3and 4-. As has already been pointed out in ref. 1), there is an apparent change in the density of spin 4 - level at about 100 eV neutron energy. The question arises whether this is caused by a particularly large number of missed levels in the region above 100 eV, where the measured spin 4 - strength function is small. Therefore, a missed level analysis of the kind described by Fuketa and Harvey 10) is performed, using the neutron widths obtained in the present experiment. In this analysis, Porter-Thomas distributions for the reduced neutron widths are assumed for the energy range in question and it is estimated that resonances with £ 11° smaller than 2 × 10 -7 E~ .3 escape detection. Table 3 shows the result of this analysis which is also indicated in fig. 2 in the form of crosses which give the cumulative numbers of resonances which would have been observed up to 200 eV and 300 eV if the number of missed levels per 100 eV interval was constant and equal to three. The figure shows that also with the inclusion o f the missed level analysis there seems to be a change in the density of spin 4 - levels at 100 eV. A surplus o f spin 4 - resonances below 100 eV is supported by the fact that the ratio of level densities observed for both spins below 100 eV would yield a spin cut-off parameter of ~r -- 15 with a lower 84 % confidence limit of a > 7. It should be mentioned that it is excluded that the TABLE 3 Results of missed level analysis Energy range (eV) 0-300

0-100

100-200

200-300

3-

obs. resonances missed resonances

50 16

19 3

16 6

15 7

4-

obs. resonances missed resonances

49 20

24 3

14 10

11 7

100

G. ROHR AND H. W E I G M A N N TABLE 4 Correction of average resonance parameters of 177Hfdue to missed level analysis

AE (eV)

0-100

100-200

100-300

200-300

Average values for spin J = 3 - resonances in different energy intervals

( D ) (eV)

~) b)

5.11 4.42

6.07 4.50

6.56 4.61

7.00 4.69

( F °) (meV)

") b)

1.06 0.92

1.61 1.21

1.83 1.33

2.12 1.45

a) b)

2.07 2.08

2.66 2.68

2.84 2.88

3.04 3.10

S x 104

Average values f o r spin J = 4 - resonances in different energy intervals

( D ) (eV)

a) b)

4.20 3.73

6.72 4.02

7.78 4.68

9.00 5.45

(Fn°) (meV)

") b)

1.69 1.50

0.81 0.51

1.16 0.73

1.59 1.00

a) b)

4.03 4.03

1.21 1.26

1.49 1.54

1.77 1.83

S x 104 a) Observed resonances. b) Missed levels included.

p-wave levels cause the apparent excess of levels below 100 eV, since the probability for the smallest resonance to be p-wave is < 10-3 The influence of the missed level analysis on the parameters D, ( F °) and S is shown in table 4, separately for both spins and different energy intervals. 3.3. MONTE CARLO CHECK OF AVERAGE PARAMETERS

In order to check whether the observed energy dependence of the strength function is caused mainly by variations of the average width or the level density, a Monte Carlo test, based on the data of table 4, is done in the following way: Resonances in the 0-100 eV range are generated from the average parameters D and S as obtained for either the 100-200 eV or the 100-300 eV region. Average parameters DM, (F°)M, and S Mare calculated from the Monte Carlo generated resonance sets and are compared to the experimental values De, (F°)e and S e as measured in the interval 0-100 eV. Table 5 contains the probabilities P for conditions as indicated in the first column of the table, to be fulfilled, again separately for both spin values and without and with inclusion of missed levels. As is obvious from the table, the behaviour of the parameters of spin 3- levels is entirely normal, at least if missed levels are included. In contrast, calculated probability values for spin 4- resonances are generally very small, except for the level

N E U T R O N W I D T H S O F 177Hf

101

TABLE 5 Probabilities P ( ~ ) for accidental occurance of observed average resonance parameters in the interval 0-100 eV, if expectation values are taken from intervals 100-200 eV and 100-300 eV Expectation values taken from 100-200 eV

100-300 eV

R e s u l t f o r 3 × 104 histories with resonances J = 3 -

(D)M < ( D ) ,

a) t,)

P = 10.5 P = 47.1

P = 3.2 P = 38.8

~)

(r°) >--(c°L

~)

P = 84.1 e = 78.0

P = 89.6 P = 85.0

S u > S~

a) b)

P = 71.6 P = 75.9

P = 74.2 P = 81.4

R e s u l t f o r 3 x 104 histories with resonances J = 4 a)

P =

(D)M < ( D ) e

b)

P = 25.9

P =

0.00 2.1

(F°) > (F°)e

") b)

P= P=

1.01 0.18

P= P=

12.7 0.46

S M > S~

") b)

P= P=

0.08 0.18

P = P=

0.30 0.17

0.01

P=

For definitions of symbols see text. a) Observed resonances. b) Missed levels included.

spacing, where P = 26 ~ and 2.1 ~ (missed levels included), if the 100--200 eV and 100-300 eV intervals are used for comparison, respectively. These figures indicate that the apparent change in level spacing may still be of statistical origin. On the other hand, the very small probability of P = 0.18 ~ for the strength function may be regarded as definitely significant for a nonstatistical variation, and the similarly small P-values for the average reduced width show that the high value of the strength function in the 0-100 eV interval is primarily caused by unusually strong resonances in this energy range. It should be pointed out that the significance of the structure of the strength function is independent of whether or not a missed level analysis is applied. 3.4. STATISTICAL TEST F O R I N T E R M E D I A T E S T R U C T U R E

Finally, we want to check whether a typical intermediate structure behaviour is present in the neutron widths o f the spin 4 - resonances below 90 eV. Such a structure can be defined as a violation o f the statistical distribution o f F ° values in a limited

102

G. R O H R A N D H. W E I G M A N N

.1=/4

-¢

rn" - t h r e s h o l d

E

....... o b s e r v e d

resonances

_ . _ I-r~ > 0.2 m e V r r ~ > 1.0 meV

I

,

ib

/j

,, i ...... %,~,.~xj

\,~4

!

S -

ji

'~

... sb

'

' Eo [eVl

Fig. 3. Reduced neutron widths of spin 4 - resonances in the energy range 0-90 eV. The different lines connect values above threshold as given in the figure.

energy range. Several statistical methods have been proposed to evaluate the, significance level. As proposed by James ~), we apply a Wald and Wolfowitz runs test to the F ° values given in fig. 3 in dependence on the neutron energy, indicated by crosses. The basis of this method is to test the hypothesis that for a sequence of F ° values the probability of a given F ° value being higher (h) or lower (l) with respect to an assumed reference remains constant throughout the sequence. Choosing a reference value of R = 3 meV we obtain the sequence hhlllllllllllhhllllll with m = 4 higher values and n = 17 lower values. The n u m b e r of runs in this sequence, where a run is defined as an unbroken sequence of the h and l, is U = 4. This compares to an expectation value for U of e(U) = 1 + 2 m n / ( m + n) = 7.48 for a statistically distributed set, with a standard deviation of a(U) = [2mn(2 m n - m - n)/((m + n)2(m + n - 1))]~ = 1.33. Thus the observed n u m b e r of runs U = 4 differs from the expectation value by 2.6 standard deviations, corresponding to a probability of < 1%. This result is caused by the two pairs of high resonances with F ° values at about 5 and 60 eV neutron energy, which are responsible for a large strength function in the 0-100 eV range. The pairs of resonances could be interpreted as two intermediate structures of very small spreading widths (,,~ 5 eV) produced in a higher hierarchy of the c o m p o u n d process. Additionally, we apply the test for "runs up and d o w n " as proposed by BodinetRobinet and M a h a u x 22). A " r u n u p " and a " r u n d o w n " of length d are defined as a group of d consecutive values F i. . . . /'i+d-~' such that Fi+ j_ 1 > Fi+~ for a "run

N E U T R O N W I D T H S O F 177Hf

103

TABLE 6 N u m b e r s K(d, N) o f runs up and down and their probabilities P as obtained from M o n t e Carlo calculations for different thresholds of F ° (b) r ° > 0.2 meV

(a) Observed resonances

d

K(d, 21)

1 2 3

11 3 1

P 0.10 0.25 0.41

(c) F ° > 1.0 meV

d

K(d, 17)

P

1 2 3 4 5

7 2

0.14 0.25

1

0.023

d I 2 3 4

K(d, 11)

P

3

0.008 0.19

1

0.078

up" and El+j+ 1 < Fi+ j for a "run down" (j = 0, 1 . . . . d - 1). T h e n u m b e r o f " r u n s up" a n d " r u n s down" K(d, N) of different length d (N is the number of resonances) may be read directly from fig. 3. The probability P for the occurence of K(d, N) is given in table 6a and has been calculated from 3 x 104 Monte Carlo generated resonance sequences, using a Porter-Thomas distribution for the F ° values. These values do not indicate an intermediate structure. However, if a bias is introduced and only resonances above the bias taken into consideration, the picture changes : The results for two bias values, 0.2 and 1 meV, are shown in tables 6b and c, respectively. At least for the bias of 1 meV, probabilities become very small and are indicative for the presence of intermediate structure. The introduction of a bias is essentially equivalent to the subtraction of a statistical contribution from the superimposed structure.

4. Conclusion A statistically significant energy dependence of the strength function for spin 4has been observed, and it has been shown that this is mainly due to a nonstatistical behaviour of the average reduced neutron width, although a variation of the level density cannot be excluded. Tests for intermediate structure have shown, to a significance level of > 99 ~o, a positive result due to two groups of strong resonances at about 5 and 65 eV neutron energy. These two groups could represent narrow "intermediate" structures with a spreading width which is of the order of the level spacing ( ~ 5 eV). Presently, there is no theoretical explanation for "intermediate" structures with a width as small as a few eV (apart from the case of the fission, where one is dealing with a very different situation). However, a similar narrow structure has recently been observed in the case of spin 3- resonances in ~43Nd + n [ref. 13)]. In this case, nonstatistical features have also been observed in the y-decay of these resonances to low lying collective states, indicating that collective excitations might play an important role in the reaction process. In the present case of 177Hf y-ray spectra

104

G. ROHR AND H. WEIGMANN

m e a s u r e m e n t s have recently been m a d e by Coceva et al. 14) a n d are in the process o f being analysed. W e are i n d e p t e d to the U S A E C for l o a n o f the enriched 177Hf, to Dr. V. V e r d i n g h for the p r e p a r a t i o n o f the sample a n d to the linac o p e r a t i o n staff. W e t h a n k Dr. R. Batchelor a n d Mr. K. H. B o c k h o f f for their careful r e a d i n g o f the m a n u s c r i p t .

References 1) C. Coceva, F. Corvi, P. Giacobbe and M. Stefanon, Proc. Int. Conf. on statistical properties of nuclei, Albany, 1971, ed. J. B. Garg (Plenum Press, N3(, 1972) 2) C. Coceva, F. Corvi, P. Giacobbe and M. Stefanon, Phys. Rev. Lett. 25 (1970) 1047 3) G. Rohr and H. Weigmann, Conf. on nuclear structure study with neutrons, Budapest, 1972, p. 52 4) G. Rohr and H. Weigmann, Proc. Int. Conf. on nuclear physics, Munich 1973, ed. J. de Boer and H. J. Mang (North-Holland, Amsterdam, 1973) 5) F. H. Fr6hner, General Atomic Report GA-6906 (1966) 6) P. Giacobbe and C. Coceva, private communication 7) BNL 325, 2nd ed., suppl, no. 2 8) H. I. Liou and J. Rainwater, Phys. Rev. C6 (1972) 435 9) B. Buck and F. Percy, Phys. Rev. Lett. 8 (1962) 444 10) T. Fuketa and J. A. Harvey, Nucl. Instr. 33 (1965) 107 11) G. D. James, Nucl. Phys. A170 (1971) 309 12) Y. Bodinet-Robinet and C. Mahaux, Phys. Lett. 42B (1972) 392 13) G. Rohr, T. van der Veen, H. Weigmann and J. Winter, 2nd Int. Symp. on neutron capture gamma ray spectroscopy and related topics, Petten 1974, p. 306 14) C. Coceva, private communication 15) S. F. Mughabghab and D. I. Garber, BNL 325, 3rd ed., vol. 1 16) H. I. Liou, J. Rainwater, G. Hacken and U. N. Singh, Phys. Rev. CI1 (1975) 2022