Investigation of the (n, α) reaction on sm and nd isotopes in the neutron energy region below 1 keV

Nuclear Physics A154 (1970) 177--190; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permission from the publisher

I N V E S T I G A T I O N O F T H E (n, ~) R E A C T I O N O N Sm A N D N d I S O T O P E S I N T H E N E U T R O N E N E R G Y R E G I O N B E L O W 1 keV J. KVITEK and Yu. P. POPOV Joint Institute for Nuclear Research, Dubna, USSR

Received 29 April 1970 Abstract: Alpha widths for sixteen resonances in the 149Sm(n, ~) reaction have been measured on the

IBR Pulsed Reactor of the Joint Institute for Nuclear Research. The results obtained are discussed together with the improved data on ~-widths for the 14aNd, ~45Nd and ~4 7 S m resonances in the neutron energy region 0.04-900 eV. From the dependence of ~-widths on resonance spins, spin assignment of the ~4aNd resonances is made. The experimental values of the average ~widths for the given isotopes are compared with those calculated from the statistical nuclear model. The effect of neutron pairing correlations on the probability of ~-decay of excited states is discussed. Alpha-width fluctuations relative to the average value are considered. l E

NUCLEAR REACTIONS 149Sm, 14aNd, X*SNd(n, ~), (n,y), E = 0.04-900 eV; measured tr(E),/'~/F~,. 144.l,6Nd ' 14s. 150Sm deduced resonances, J, ~, -P~. Enriched targets.

1. I n t r o d u c t i o n

The investigation o f the (n, ~) reaction in the resonance neutron energy region opens up new specific possibilities to the experimentalist. On one hand, this permits highly excited nuclear states to be investigated with the resolution which can be obtained only by neutron spectrometry methods. O n the other hand, the mechanism o f ~-particle emission is rather simple and the theory o f ~-decay is fairly advanced, so one can hope for a more complete analysis o f nuclear levels at the binding energy o f the neutron in the nucleus and also o f the mechanism o f or-decay f r o m excited nuclear states. However, such experiments are difficult to perform, since the (n, ~) reaction cross section is very small and the competititve (n, 7) reaction b a c k g r o u n d is large. U p to now the (n, ~) reaction has been studied with thermal neutrons f r o m powerful reactors. The (n, ~) reaction cross sections and a-particle energy spectra for the rare-earth isotopes 14aNd [refs. x,2)], 1475m [ref. ~)], 149Sm [refs. 1 - 4 ) ] and 15XEu [ref. 2)] have been obtained by means of semi-conductor detectors and ionization chambers. It was shown that for the mentioned nuclei the ratio a(n, ~)/a(n, 7) is in the range of 1 0 - 5 - 1 0 - 9 . The or-particle spectra turned out to consist o f transitions to the g r o u n d a n d first excited states. Only the Macfarlane and Oakey experiment 2) which was an 177

178

J. KVITEK AND Yu. P. POPOV

appreciable improvement of the experimental technique yielded ten e-particle transitions for 1495nl(n, e) half of which go to so far unknown levels of the daughter nucleus. However, the interpretation of the data obtained is difficult, because several resonances with different parameters may often contribute to the thermal neutron capture. The measurements with resonance neutrons are capable of yielding more complete and unambiguous information on the (n, c~) reaction mechanism. But the available fluxes of such neutrons are several orders of magnitude lower than those of thermal neutrons. Therefore small area (several cm z) detectors are impractical here. In the first resonance neutron experiments the (n, e) reaction was studied with a natural samarium target 6). Measurements were made on the enriched isotopes a43Nd and 145Nd [ref. 7)] and recently on the isotopes 95Mo and 123Te [ref. 8)]. We developed a gaseous scintillation detector with a multilayer target 9). The detector had a high e-particle detection efficiency but it did not permit the e-particle energy analysis, therefore only total probabilities of e-decay of individual excited states were studied. The e-particle spectra for two resonances of 147Sm were measured 1o) by means of a double ionization chamber with a grid. In the present paper we describe and discuss the results of the investigation of the (n, e) reaction on ~49Sm and also the completed and corrected experimental data on the (n, e) reactions on ~43Nd, 145Nd and 1478m [refs. 6, 7)] in the neutron energy region 0.04-900 eV.

2. Experimental procedure The investigation of the (n, e) reaction on rare-earth isotopes was carried out on the IBR Pulsed Reactor with a microtron as an injector. The neutron energy was determined by the time-of-flight method. The time resolution was 0.03-2 ps/m, the flight paths were 30 and 100 m. An e-particle xenon-filled scintillation detector 9) with a multilayer target was used. X e n o n was placed in an electrostatic field of 800 V/cm; this allowed the xenon light output to be increased by two orders of magnitude. The e-particle detector had the following parameters: (i) The total target area was 0.7 m z. (ii) The 7-ray detection efficiency in the radiation field of 108 7/s was er < 10-8. (iii) The detection efficiency for e-particles from the (n, e) reaction was e~ = 0.3-0.4. (iv) The detector background counting rate was about 20 pulses per rain. The e-particle detector was constructed to allow the multilayer target to be mounted so that the scintillation gaps with the isotope in question alternated with the scintillation gaps without the isotope. Scintillations in both types of gaps were recorded independently and so it was possible to measure simultaneously the (n, e) reaction and the neutron beam background. In one set of measurements (see table 1) additional grid electrodes were utilized in the middle of the scintillation gaps. Due to the increase

sm Nd (n, a) REACTION

179

o f interplate distances, the total target area was decreased by 30 ~ . A t the same time, this detector construction change made it possible to reduce the detector's proper b a c k g r o u n d and its sensitivity to v-rays almost by one half. Targets o f n e o d y m i u m and samarium on an aluminium backing were used. The deposition procedure was described by T o m i k o v a 11). Table 1 presents the characteristics o f the samples o f a natural mixture o f samarium and n e o d y m i u m isotopes and those o f enriched 14aNd, 145Nd and 149Sm isotopes. TABLE 1 Characteristics of targets and operating conditions in individual series Target

Target weight (g)

Enrichmerit (~o)

Layer thickness (mg/cmz)

Number Resolution Measurement of by neutime layers tron energy (h) ~s/m)

Detector a) operating conditions

Nd

30.5

4.6

12

2.0 0.10

21.5 110

B

Sm

16.6

4.75

6

73.2

14SNd

17.7 11.8 32.3

2.66 2.66 4.87

12 8 12

56 100 140 75 74

A

14aNd

0.10 0.60 0.03 0.10 0.10

x49Sm

33.35

94.6

6

0.10

120

A

84.6

10.0

B B, C B

a) The detector was operated A - with background gaps; B - without background gaps; C -with additional grid electrodes.

In our experiment we studied simultaneously the g-particle yield versus the neutron energy and the y-rays o f the target which were recorded by a M o x o n - R a e detector 12) insensitive to the shape o f the y-ray spectrum of individual resonances. The yields o f g-particles and y-rays in an individual resonance were defined by the following factors: N: =

t: ¢ A r : / r ,

(1)

N~, = e r ~o~ t~ d p A F J F ,

where e~ and e~ are the detection efficiencies for the (n, a) and (n, y) reactions, respectively, co~ and coy are the solid angles at which g-particles and v-rays are recorded by the corresponding detectors, t~ and t~ are the times o f measurements by g-particle a n d v-ray detectors, ¢ is the neutron flux, A is the so-called resonance area in transmission measurements, and F, Fv, F~ are the total, radiation and g-particle resonance widths, respectively. I n this case the following relation holds: = kNJNv,

(2)

180

S. K V I T E K A N D YU. P. P O P O V

where the coefficient k involves only the characteristics of the e-particle and y-ray detectors and the time of the measurements. The coefficient k can easily be determined by a simultaneous measurement of eparticle and y-ray counts in the thermal energy region where the (n, a) and (n, 7) reaction cross sections are known from other investigations: (a(n, e) N~,) k = \a(n, y) ~ th"

(3)

It should be noted that the coefficient k can also be obtained for an isotope with unknown ath(n, e). In this case it is essential to measure simultaneously the aparticle and y-ray yields from the isotope in question and from another isotope with a known k-value. The unknown calibration coefficient is defined by the ratio

kj = k, e(E,,) B 1

e(E,j) B,'

(4)

where the index j refers to the quantities of the isotope under study, i denotes the isotope with the known calibration coefficient k, B is the binding energy of the final neutron in the nucleus. Eq. (4) was derived by using the fact that for the Moxon-Rae detector the detection efficiency for the slow neutron radiative capture is proportional to the neutron binding energy. Such a calibration method was used to determine the value of k for 14SNd resonances from the 143Nd calibration coefficient, and for 147Sm resonances from the ~49Sm calibration coefficient. The calibration measurements were made on natural isotopic mixtures. In both cases it was shown by calculation that the e-particle detection efficiency during transitions to the ground and first excited states is approximately the same for the isotopes considered. In the case where the neutron energy resolution is insufficient and the peaks of the (n, e) and (n, y) curves comprise several resonances, the ratio of the e-particle and y-ray yields in a peak is

Z (~Ar,/r), Z (N,), k r, Z (CA~r),

Z (N~),

1

'

(5)

where i numbers the resonances contributing to the peak. If Fn ~ const, or F. >> F~ and the neutron flux changes only slightly within a given resonance group, then (c~A/F), = const, and we have (6)

Z i

kL

Sm Nd

(n, or) REACTION

181

3. Experimental results As an example of our measurements, we present the results of the investigation of the (n, ~t) reaction on the enriched isotopes of 149Sm and 14aNd. The ~-particle counting rates plotted against neutron energy are shown in figs. 1 and 2 (lower curves). The upper curves represent the yield of 7-rays from the (n, 7) reaction obtained under the same conditions. ,

3000

I

I

i

lilJ

I

I

I

I

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I

I

I III 1

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I

I

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149

Sm

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(n,~)

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III

300

200

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i ?

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10 2

Fig. l. Recorded numbers o f or-particles (lower curve) andT-quanta (upper curve) for against the neutron energy. The dashed line indicates the background.

I

1 I0 J

149Sm,plotted

For a more precise determination of the (n, 7) reaction resonance areas and the refinement of the average level spacing value for the neodymium isotopes, additional measurements of the 7-ray yield dependence on the neutron energy were performed with a resolution of 6 ns/m. The results of our measurements of the (n, e) and (n, 7) resonance areas with samarium and neodymium isotopes are given in table 2. Multiplying the measured values of F~/r~ by the F~ values from refs. 13.14) or by the average values of F~ (see table 3), we obtained the values of F~.

182

J. K V I T E K A - N D Y u .

P. POPOV

The error in the values of F~ includes only the errors in the determination of the resonance areas of the a-particle and ~-ray counting curves. The errors in the average radiation width (about 20 9/0)and also the errors in the calibration coefficient k (10 9/o for Sm isotopes and about 25 ~ for Nd isotopes) are ignored. These errors are the same for all widths of a given isotope, and their introduction would conceal the awidth fluctuations. 1500

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NEUTRON ENERGY (eV) Fig. 2. Recorded number of a-particles (lower curve) and~'-quanta (upper curve) for 1,3Nd' plotted against the neutron energy. It might be useful to make some remarks on the experimental data. It should be noted that in our first measurements of a-widths for neodymium isotopes 7), when normalizing by the thermal cross section of l*3Nd, we neglected the contribution of the neutron capture by the impurity samarium nuclei to the value of N~ (formula (3)) and this introduced an error. In this connection, all the values of a-widths for the 143Nd and 145Nd isotopes were corrected. In so doing, four methods of normalization were employed: (i) by formula (3) but with allowance for the contribution of ~-rays from 0.5 9/o impurity samarium nuclei; (ii) by using the parameters of the resonance Eo = 4.37 eV [ref. 13)], the known energy dependence of the neutron flux

183

sm Nd (n, u) REACTION TABLE 2 Alpha-particle widths o f neutron resonances in Sm and N d isotopes

Target nucleus

1498m

Resonance energy E (eV)

Resonance spin jr

Fy, (meV)

0.098 0.87 4.98 6.48 9.0 12.2 14.9 17.1 25.2; 26.1 30.9 34.0 40.2 41.3 45.1 51.8 57-68 70- 75 90-100

44444-

63 60 67

3.4 18.3 29.8 32.1 49.4 58 83.5 183

34-

61 41

X4SNd

4.36 43.1 102

333-

l'taNd

--6 55.8 127 135 159 180 187 408 705

147Sm

67

55

3(4-) 33(4-) 3(4-) 33-

51 63 86 80 94 70 83

~-particle yield in a resonance N~ 170504-2050 930± 120 170+ 30 110± 20 250-4- 30 170-4- 30 110£: 25 344- 18 4304- 40 2854- 55 574- 27 7104- 60 6004- 50 554- 44 140± 75 250+ 40 4604- 200 5904- 40 17904- 150 4804- 30 954- 22 624- 22 1304- 20 67-4- 15 2154- 25 5604- 40 394047604160-4-

90 60 40

980004- 300 304- 15 19804- 160 52504- 240 404- 25 3804- 45 ~ 10 17604- 130 2804- 50

/'/_r'y X 106

_P~x 107 (eV)

1.0040.67± 0.6441.2040.7 43.7 ± 1.0040.9 43.9 -b 4.6 42.2 416.5 411.1 -b 0.5 41.9 41.5 -q3.8 43.5 4-

0.25 0.11 0.13 0.25 0.1 0.8 0.25 0.5 0.5 1.0 1.1 2.2 1.5 0.5 1.0 0.3 1.7 0.5

0.634- 0.16 0.404- 0.07 0.434- 0.09 0.764- 0.16 0.47+ 0.06 2.4 ± 0.5 0.64-4- 0.16 0.54+0.30 2.4 4- 0.3 3.9 4- 0.9 1.4 4- 0.7 10.2 4- 0.4 6.9 4- 0.9 0.7 4- 0.7 1.2 ± 0.6 0.9 4- 0.2 2.3 4- 1.0 2.2 4- 0.3

41 5.8 8.0 3.0 17.0 10 33 56

6 0.6 2.2 1.0 3.5 3 4 8

25 2.4 4.8 1.6 10 6 20 27

-4-4444444-

8.6 4- 0.5 2.3 4- 0.3 3.3 -4- 1.0 59 4- 16 4 -4- 2 91 4- 13 480 4- 45 ~ 2.5 28 4- 6 _--<6 5504160 225 4-100

-4-+444-444-

3 0.3 1.3 0.6 2 2 3 4

4.4 41.4 42.0 4-

0.8 0.2 0.8

51 3.2 86 336 ~ 2 17 ~ 4 420 170

4- 13 4- 1.6 4- 12 4- 30 4-

4

-4-120 4-100

a n d the m e a s u r e m e n t o f the e-particle yield in the r e s o n a n c e a n d t h e r m a l regions; (iii) b y u s i n g t h e s i m u l t a n e o u s m e a s u r e m e n t s o f t h e 0~-particle y i e l d i n t h e 4.37 e V 1 4 5 N d a n d 3.4 e V 1 4 7 S m r e s o n a n c e s b y a d o u b l e i o n i z a t i o n c h a m b e r l o ) ; (iv) b y u s i n g t h e m e a s u r e m e n t s o f t h e 0~-particle a n d y - r a y y i e l d s f o r t h e : 4 a N d i s o t o p e s i n the thermal and resonance regions.

184

J. KVITEK AND Yu.P.

POPOV

For the 4.37 eV resonance the following values of ~-widths in peV were obtained by these methods: 0.34, 0.40, 0.42 and 0.68. The weighted average of F~ was taked as 0.44+0.08 peV and the ~-widths of all the other resonances of 143Nd and 14SNd isotopes were normalized using this value. The 149Sm nucleus. For neutron energies below 100 eV we observed twentzy-seven resonances in the (n, ~) reaction. Above 57 eV the energy resolution was not sufficient to separate individual resonances and the experimental curve represents the summed yield N, for several resonances. Fortunately, resonances within each group have neutron widths which do not differ much. Therefore, according to eq. (6), the ratio of the peak areas in the ~-particle and 7-ray counts for resonance groups will be close to the ratio of the corresponding average widths. The values of F, for four low-lying resonances obtained from the measurements with a natural samarium isotopic mixture 6) were improved in the measurements with the enriched isotope. The value of F, for the 17.1 eV resonance 6) was incorrect, probably due to an inaccurate calculation of the contribution of the strong 147Sm resonance with Eo = 18.3 eV. The 1478m nucleus. The values of ~-widths for eight resonances were obtained from measurements of the (n, ~)reaction on natural samarium 6). The results of the measurements with x49Sm showed that the peak of the ~-particle counting curve at E ~ 40 eV in the measurements with natural samarium belongs mostly to the resonances of 149Sm with Eo = 40.2 eV and 41.3 eV rather than to the 147Sm resonances with Eo = 39.9 eV, as was earlier supposed 6). The 145Nd nucleus. The 145Nd (n, ~) reaction was observed for the first time. Alpha-widths for three resonances were measured. It was assumed that all the ~particle counts in the peak corresponding to the unresolved 102 eV and 103 eV resonances should be attributed to the 102 eV resonance with a favourable spin J " = 3 (the 103 eV resonance has J~ = 4 - ) . The total contribution of the investigated resonances to the thermal cross section of the (n, ~) reaction is 0.04+0.01 mb, and this agrees with an upper limit of the cross section, cr(n, ~)th < 0.1 mb, given in ref. 1). The 143Nd nucleus. Five resonances are seen in the (n, ~) reaction in the energy range below 410 eV whereas in the (n, 7) reaction fourteen resonances appear. At higher energies the sole resonance with Eo = 705 eV is clearly seen. 4. Discussion When resonance neutrons with zero orbital angular m o m e n t u m are captured by the target nuclei of 143Nd, 145Nd, 147Sm and 149Sm, doubly even compound nuclei are formed in highly excited states with spin and parity J~ = 4 - or 3 - . In most cases the excited states decay by v-ray emission. The small value of the ~-decay probability is generally due to the low penetrability of the Coulomb barrier to ~-particles.

Sm Nd (n, ~¢) REACTION

185

The probability of ~-decay (or e-widths) can be represented as a product of two factors 15): W = F,,/h = f P ,

(7)

w h e r e f i s the probability of e-particle formation at the nuclear surface. It is determined by the structure of the nuclear high-lying excited levels. Its calculation presents great difficulties for the existing nuclear models. The statistical model associates this factor with the average level spacing in the vicinity of the decaying state by a simple expression D f 2nh ° (8) The second factor P in expression (7) takes account of the probability of the e-particle penetration through the Coulomb and centrifugal nuclear barriers. Since the quantity P is an exponential function of the e-particle energy, the most intense e-transitions go to the lowest levels of the daughter nucleus. The values of P for e-decay of the states excited during the slow neutron capture by various nuclei were calculated by Griffioen and Rasmussen 1 6 ) and by Dadakina 17). The measured values of e-widths characterize the total probability of e-particle transitions to various states of the daughter nucleus from the same state of the compound nucleus. Table 2 summarizes the results of the e-width measurements for the decay of about forty resolved excited states of Sm and Nd nuclei and of several groups of resonances. It is of interest to compare the experimental values of e-widths averaged over all nuclear resonances with the calculations from the statistical theory /~,, =

D

~ P., 7~-

(9)

where D is the average level spacing of the compound nucleus with a given value of J~ at the excitation energy equal to the neutron binding energy in the nucleus, and Pu is the nuclear barrier penetrability for e-particles with the orbital angular momentum l for the transition to the/-state of the daughter nucleus. Unlike the usual e-decay (e-decay from the nuclear ground state) in the case of the (n, e) reaction the value of D is either well known from neutron spectrometry data (for example refs. 13.14)) or was determined by our measurements of the cross section of the (n, ~') reaction on enriched isotopes with a resolution of 6 ns/m. Table 3 presents the theoretical and experimental values of r~ for four isotopes of Sm and Nd. The number of resonances over which the experimental data were averaged is given in parentheses. Unlike previous papers 6, 7), in this table the values of F~ were averaged over the resonances with both spin values. For some nuclei the experimental results were obtained by averaging over a number of resonances insufficient for statistical analysis

186

J. KVITEK AND Yu. P. POPOV

(for example in the case of 146Nd averaging was only over six resonances). Yet to a first approximation, agreement can be noted between the calculations from the statistical model and the experiment. TABLE 3 Averaged ~t-particle widths for Sm and Nd isotopes

Compound nucleus

['y (meV) [refs. 14.13)]

D (eV)

Fa (~ueV) theory [ref. 17)] [ref. 16)]

144Nd 146Nd 148Sm 1s OSm

76 60 59 62

80 34 14 6

18 0.18 4.7 0.70

23 4.3 0.44

experiment 7.1 0.13 0.6 O.19

(13) (6) (11) (28)

To obtain _Fa the measured or-widths were summed up in the energy ranges below 410 eV (144Nd), 103 eV (t46Nd), 83 eV (14SSm) and 100 eV (15°Sm), then the sum was divided by the total number of

resonances known in this energy range (from ref. la) or our measurements with a y-ray detector).

However, if the experimental values o f t , are averaged over all investigated isotopes, it can be said that the experimental averaged e-width value is smaller than that calculated from the statistical model. An analogous result was obtained by comparing the experimental data with values calculated from the optical model. In the case of doubly even compound nuclei the observed discrepancy can possibly result from the effect of nucleon pairing in the compound nucleus which also shows up in the eparticle spectra of the (n, ~) reaction and which is considered in greater detail in ref. 10). In our case the nucleon pairing effect results in a decrease of f i n formula (7), since for a-particle transitions to the ground and excited states with Eexc < Epair it is essential thatfincludes not simply the probability of the a-particle formation at the nuclear surface, but also the probability of the ~-particle formation in the presence of paired nucleons in the remainder of the compound nucleus. Fluctuations of the reaction widths from resonance to resonance are usually described by the Porter-Thomas distribution 18) with the number of degrees of freedom v as a parameter. It is well known that v varies from 1 for neutron widths to 20-50 for total 7-widths. The data on ~-widths fluctuations are lacking so far. To estimate the theoretical value of v one can take advantage of the results of the paper by Wilets 19) concerning the statistical analysis of the reaction width distribution in the case of partially open channels. When applied to the (n, ~) reaction, the effective number of open channels is veff = ( ~ e l l ) 2 / Z p 2 .

li

If

(10)

1.193 3* (6") F

__

l~

E

,oN

0° IIIIIIIlllllllllll /

0

146.

2*

/

0,456

I~4~

"~rn(n,o[)

/

4" 443"

E

0

1.597

f~ 140

O~'lllllllllllllllll

1.910 21

//

/

/

9..7 t8

/

55

144 60Nda4

i

3-

4"

E (eV) jn 127 3"

8.//~~----- -6

"~Nd(n, o()

Fig. 3. Alpha-decay modes from excited states of ~44Nd and lS°Sm.

B

6.5 5.0 0.87 009#

E (eV) j~

~o

z

?

188

.L KVITEK AND Yu. P. POPOV

It should be noted that formula (I0) was derived under the assumption that the probability of s-particle formation at the nuclear surface for a given resonance is independent of the s-particle orbital angular momentum and takes no account of pairing correlations. Before comparing the calculations by formula (I0) with the experimental data, let us consider the level schemes of the compund and daughter nuclei and ~-particle transitions for the l*9Sm(n, ~) and 1*aNd(n, a) reactions (fig. 3). According to the laws of conservation of parity and the rules of summation of angular momenta, a-transitions from 3 - states to the ground and excited states of the daughter nucleus are possible. Alpha transitions from 4 - states ot 0 + states are forbidden, only transi-

$ o

4 3 o

2 I

o

0

2

4

$

8

I0

~ "I0 7

Fig. 4. Experimental distribution of or-widths for 15 resonances with spins 3 and 4 in the l'*9Sm(n, ~)~46Nd reaction (see the text) and theoretical fitting of the differential distribution in terms of the sum of two Porter-Thomas type distributions with v(4) = 60,/~(4) = 0.05 peV and v(3) = 2.8 and r~(3) = 0.4/~eV (solid line). tions to the excited states of the daughter nucleus are possible, i.e. with a smaller aparticle energy. As a consequence of this, the probabilities of a-decay from L3 - states should exceed those from 4 - states and the more so, the higher the first excited level with non-zero spin of the daughter nucleus. Since / ~ , ( 3 - ) ~ F , ( 4 - ) , the or-width fluctuations for the resonances with spins 3 - a n d 4 - are to be treated separately. F r o m expression (10) we calculated that for X43Nd ( 3 - ) Vef f = 1.0, for 14aNd ( 4 - ) Veff = 1.8 and for the other investigated isotopes vaf lies in the range 1.6-2.3. Unfortunately, the available experimental data (table 2) cannot be exploited to a full extent due to the lack of spin assignments for the majority of resonances. For 143Nd ( 3 - ) the experiment yields Veff = 1.8+__1.0 (Vaf = 2F,,/(F~,-2 2 --F,,)).-2 Fig. 4 shows the ~-width distribution for 15 resonances of 149Sm with spins 3 and 4 - . The experimental distribution (dots) was plotted taking account of the errors in ~-widths (table 2). Each value of F~i has a corresponding triangle with its top at the point F~i, a base equal to 4 AF~a and a height equal to X2AF~a. Thus the area of such a triangle equals unity and 75 ~ of the triangle area lies within one statitical error +_-AFar.

Sm Nd

(n, ~) REACTION

189

The number of resonances is as yet insufficient for reliable determination of the values ofverr and/~, for J~ = 3- and 4 - , respectively. Therefore an attempt was made to describe the experimental a-width distribution by the sum of two Porter-Thomas type distributions with weights ~-~ (J = 4) and ~ (J = 3). The parameters/~, (4) = 0.05/~eV and P,(3) = 0.4 /~eV were also fixed on the basis of the value of /~ = 0.2 ~leV averaged over all resonances and the relation F~(3)/ff~(4) = 8 calculated from the known penetrabilities 17). This fitting gave v(3) = 2.8 and v(4) = 60 (the solid curve in fig. 4). Attention is drawn to the five lowest resonances of 149Sm: they all have the same values of spin and parity d~ = 4 - and the values of r , coincide within the measurement error. This is rather surprising, because the a-particle spectrum measured for one of these resonances (E0 = 0.098 eV) consists practically of only one a-transition to the first excited state of the daughter nucleus with I n = 2 + [refs. 4, 5)]. This fact is possibly due to the similar nature of the excitations of all five levels. In this connection, it would be interesting to make more precise measurements of a-widths, to study c~-particle and y-ray spectra for the resonances mentioned above, and to compare partial a-particle and ~,-ray widths. In a number of cases correlations between F~ and the resonance spin showed up in the experiment. Such correlations should be most pronounced in the 143Nd(n, a) 14°Ce reaction, since the magic neutron nucleus 14°Ce has its first excited level at 1.6 MeV. Calculations from (9) give ff~(3-)/F,(4-) ~ 100 in this case. The experiment (see table 2) indicates two groups of a-widths, the first one with F, > 1.7/~eV and the second one with F~ < 0.4/teV. The spin of the negative energy resonance is unambigously determined from the measurements of the a-particle spectra after thermal neutron capture 1, 2). Such a difference in a-widths enables us to draw conclusions as to the resonance spins (see table 2). The proposed method of spin assignment by the value of a-width is not always unambiguous and is sensitively dependent on the a-width distribution (veff) and the energy of the first excited level of the daughter nucleus. In the case of 1*aNd, even if it is assumed that v~ff = 1 for jR = 4 - the probability that the 180 eV resonance will have spin 4 - (i.e. F, = 15 r , ) is 10 -4 and for the other resonances it is still lower. Consequently, spin 3 - is reliably ascribed to strong resonances. At the same time, the probability of observing a resonance with J~ = 3- and F, = 0.01 r , accounts for several percent. Therefore spin 4 - is ascribed to weak resonances only as most probable. Such spin assignment of the levels of the compound nucleus is difficult for the Sm isotopes and Z45Nd, because the first excited level of the daughter nucleus lies considerably lower than in the case of 14°Ce. This results in a marked overlapping in the a-width distributions for the 3- and 4 - levels. However, for 147Sm (see table 2) the a-width of the resonance with Eo = 3.4 eV [J~ = 3 - , ref. 2o)] is ten times as large as that of the resonance with Eo = 18.3 eV [J~ = 4 - , ref. 20)]. This spin assignment of the 1475m resonances is inconsistent

190

~. KVITEKAND Yu. P. POPOV

with the results o f Cheifetz et al. i), w h o a s s u m e d t h a t the t h e r m a l cross sectiort o f the (n, ct) r e a c t i o n is d e t e r m i n e d b y the 3.4 eV, 18.3 eV a n d 27.1 eV resonances. O n the basis o f the s p e c t r u m o f ~-particles e m i t t e d after t h e r m a l n e u t r o n c a p t u r e b y the 147Sm nucleus they ascribed spin 4 - t o the 3.4 eV r e s o n a n c e a n d spin 3 - to t h e 18.3 eV resonance. However, the c o n t r i b u t i o n o f these levels to the t h e r m a l cross section o f the (n, ~) r e a c t i o n calculated using the values o f F , f r o m table 2 is o n l y 30 %. I n the r e a c t i o n (n, ~) these levels also explain a small f r a c t i o n o f the t h e r m a l cross section. Therefore, to a c c o u n t for the 147Sm t h e r m a l cross section, it is necessary to assume the presence o f a b o u n d level. Thus, no conclusion can be d r a w n as to the ~-particle spectra for the resonances with E = 3.4 eV a n d 18.3 eV a n d their spin values f r o m the ~-particle spectra in the t h e r m a l n e u t r o n energy region. W h a t is more, recent m e a s u r e m e n t s o f the energy spectra o f ~-particles f r o m the (n, ~) r e a c t i o n for the 147Sm resonances [ref. l o ) ] suggest a m o r e c o m p l e x structure o f the s p e c t r u m t h a n Cheifetz et al. 1) have s u p p o s e d in their analysis o f the ~-particle s p e c t r u m in the t h e r m a l n e u t r o n energy region, a n d these m e a s u r e m e n t s confirm the spin values d e t e r m i n e d in ref. 20). I n conclusioo, the a u t h o r s wish t o express their sincere t h a n k s to Professor F. L. S h a p i r o for c o n t i n u o u s interest in this w o r k a n d valuable advice, t o D o c t o r V. N. A n d r e e v for interesting discussions, to A. D a d a k i n a for furnishing the C o u l o m b b a r r i e r p e n e t r a t i o n calculations a n d also to the m e m b e r s o f the L a b o r a t o r y o f N e u t r o n Physics f o r their help d u r i n g the experiment. T h e a u t h o r s are grateful to V. S. Z o l o t a r e v a n d his c o w o r k e r s for furnishing separ a t e d isotopes.

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