Levels in 15N resulting from the proton capture of 14C

2•.C [

Nuclear Physics A128 (1969) 162--182; ~ ) North-Holland Publishin# Co., Amsterdam Not to be reproduced by photoprint or mmrofilm without written permission from the publisher

LEVELS IN lSN RESULTING F R O M THE PROTON CAPTURE OF t4C H. E. SIEFKEN t, p. M. C O C K B U R N and R. W. K R O N E

Department of Physics, University of Kansas, Lawrence, Kansas *t Received 17 January 1969

Abstract: The gamma-ray decay of the 14C(p,)')lSN resonances at Ep = 352, 527, and 634 keV has been extensively studied. Spins of d = ~ and J = { have been assigned to the upper and lower levels of the 9.16 MeV doublet respectively on the basis of angular correlation measurements. Similar measurements show the spin of the 8.31 MeV level to be J = ½. Mixing ratios for most of the gamma-ray transitions have also been determined. [NUCLEAR REACTIONS 14C(p, )'), E = 250-670 keV; measured E~, 7)" coincidence, yy(0, ~).1 E ] ~SN deduced levels, ),-branchings, J, multipolarities, res. strengths. Enriched targets.' ]

1. Introduction

A comparison of the lifetimes and decay properties of corresponding states in mirror nuclei is of much experimental and theoretical interest. Studies of this type are relatively easy in nuclei which have simple ground state configurations such as 15N and 150, where the last Ip shell particle is missing. Kurath 1), Halbert and French 2), Inglis 3), and Lane 4) have shown that the properties of the low-lying states of the mass-15 mirror pair can be reasonably understood in terms of the individual particle model. At higher excitation energies the interpretation is more difficult due to the many possible particle configurations. Shukla and Brown 5) have given an alternative description of the lower states in which the interaction of various particle-hole combinations with the 160 core has been considered. Most of the experimental information about the bound states in 15N has been obtained from studies 6-19) other than the two capture reactions 14N(n, V)lSN and 14C(p, 7)15N. Gamma-ray branchings for most of the levels below 10.5 MeV and multipole mixing ratios for some of the stronger transitions have been determined. However, conflicting evidence has been reported regarding the decay properties of the 9.16 MeV level. To explain the discrepancies it has been suggested that this level is an unresolved doublet. The spins of the 8.31 and 9.05 MeV states are also not known with certainty although (d, p) stripping data limit the possible values to J = 1 or ~. Particle-gamma correlations have been unable to distinguish between these possibilities. * Present address: Department of Physics, University of British Columbia Vancouver, British Columbia, Canada. tt This work has been supported in part by the United States Atomic Energy Commission. 162

15N LEVELS

The

163

14C(p, 7)15N reaction has previously been investigated by Bartholomew

22). A number of resonances were observed and the more prominent gamma-ray cascades in the resulting decay were identified. The availability of much better targets and vastly improved techniques in gamma-ray spectroscopy made it profitable to re-examine this reaction. In the present paper we report on measurements of the gamma-ray spectra from the four resonances observed. At each of the three stronger resonances, resonance strengths and branching ratios have been determined. Gamma-ray angular distributions and correlations measurements have confirmed the spins of most of the known states in 15N below 10.5 MeV. A second paper covering lifetime and polarization measurements is being prepared for later publication. et al. 20) and, more recently, by Hebbard and Dunbar 21) and Hebbard

2. Experimental procedure The University of Kansas 3 MeV Van de Graaff accelerator was used with the lower one third of the accelerating column grounded to provide better control and focusing of the beam in the low bombarding energy range 250 to 650 keV. The energy was stabilized to better than 0.09 ~ at the target by energy defining slits located at the focal point of a 25 ° analysing magnet. A three element electrostatic lens was used to focus the beam on the target which was mounted inside a water cooled aluminum chamber located at the center of the gamma-ray goniometer, previously described in detail by Lee et al. 23). A liquid nitrogen cold trap and an Ultek ion pump located near the target chamber minimized the build-up of contaminants on the target and maintained the operating pressure at approximately 10 -6 mm of Hg. Targets containing approximately 90 ~_+ 10 ~/o 14C, were prepared by chemically extracting the carbon from enriched BaCOn-. No significant background resulting from either 12C or ~3C was observed. Beam currents as large as 18/~A were used for periods of 75 hours before target deterioration became appreciable. Spectra were observed with both NaI(TI) and Ge(Li) detectcrs. The resolution of the latter is essential to distinguish gamma-ray transitions through the close lying first and second excited states of ~SN. Two Ge(Li) detectors were available; one, a 15 cm 3, coaxially drifted detector, has a resolution of 3.3 keV ( F W H M ) at 661 keV (a 3VCs) and 20.4 keV at 7.3 MeV. It was used to determine the decay schemes and measure the angular distributions of certain ?,-rays. The attenuation coefficient of this detector was determined by measuring the known angular distributions of several y-rays from the same experimental geometry. A 25 c m 3 coaxially drifted detector, in conjunction with two 17.6 cm diam× 7.6 cm thick NaI(T1) side detectors, was available as a three element pair spectrometer to measure the intensity of weak ?,-rays that were otherwise partially obscured by the background of the Compton distribution of higher energy 7-rays, or ?-rays which were unresolved by the NaI(T1) detectors. This t Targets supplied by Oak Ridge National Laboratory, Oak Ridge, Tennessee.

164

r~. E. $1EFKENet al.

system could also be arranged as an anticoincidence, C o m p t o n suppression spectrometer. In this mode, it provided a reduction of the background by a factor of approximately three for a 2.5 MeV v-ray. Angular distributions and triple correlations were observed in most cases with large volume scintillation detectors. A shielded, collimated, 23 cm diam x 10 cm thick NaI(Tl) detector, with its front face located 20 cm from the target, was used on the horizontal arm of the goniometer. Another shielded, collimated, 12.7 cm diam x 12.7 cm thick detector, located 14.4 cm from the target, was mounted on the vertical arm of the goniometer and served as a reaction monitor. All the scintillation detectors had a resolution of approximately 10 ~ ( F W H M ) for the 661 keV v-ray from 137Cs. Standard configurations of modular electronics were used throughout all phases of the experiment. These typically consisted of linear amplifiers, cross-over pick-off gates, single channel analysers and fast coincidence units. All coincidence measurements were made with a resolving time 2z = 50 ns to minimize the loss of true coincidences due to small variations in the cross-over point of the pulses. Pulse height distributions were recorded with a T M C 400-channel analyser and, when Ge(Li) detectors were used, with a N D 161 4096-channel analyser to provide the required resolution. The N D analyser was also used in the two-parameter mode to record coincidence events in the triple correlation measurements. Data reduction and analysis was facilitated by an IBM 1800 computer which is interfaced with the ND-161 analyser. This system was used for data display via a large screen oscilloscope and an on-line Calcomp plotter, for spectrum stripping, and for the analysis of all the angular distribution measurements. The more detailed calculations involved in the analysis of the triple correlation data were performed on the University of Kansas G E 625 computer. 3. Excitation curve and resonance strengths

Four resonances have been observed in the 14C(p, V)lSN reaction between an excitation energy of 10.208 MeV and the neutron threshold at 10.833 MeV. Fig. 1 shows the relative yields obtained in approximately 5 keV steps for E~ > 2.8 MeV. The resonances occur at Ep = 2 6 1 _ 1, 352+ 1, 527+_l, and 634+_1 keV and correspond to levels in 15N at 10.45, 10.54, 10.71, and 10.81 MeV respectively. N o other resonances were found in this energy range, in agreement with the results of Hebbard and D u n b a r 21). The strengths ~o~ = (J+½)FyFp/F of the three stronger resonances were calculated f r o m the measured yield at 0 -- 55 ° of a strong y-transition and the known decay scheme. A previous estimate 2~) of the proton and radiative widths indicates the target thickness is at least thirty times the natural width of the 527 keV resonance, which has the largest width of the resonances considered. Since the observed yield is approximately equal to the thick target yield when the ratio of energy loss in the target to the natural full width approaches ten 24) the use of the thick target expression for calculating the resonance strengths is considered valid.

ZSN LEVELS

165

T a b l e 1 shows the r e s o n a n c e strength for each resonance as f o u n d in this a n d p r e v i o u s investigations. It is e s t i m a t e d that the present values o f ~), are in e r r o r by n o t m o r e t h a n + 17 700. T h e largest c o n t r i b u t i o n to this e r r o r results f r o m the uncert a i n t y in the target c o m p o s i t i o n . Present techniques available for s p e c t r u m stripping p e r m i t the d e t e r m i n a t i o n o f yields with g o o d precision and, a l t h o u g h the accuracies o f the m e a s u r e m e n t s are different at each resonance, the m a x i m u m e r r o r due to I

1

I

I

6000 5000

z o u

4000

o~

3000 ~

2000 z I000

200

L

_

I 500 PROTON

400 ENERGY

500

,'

booOOo

600

( keY )

Fig. 1. The yield ofT-rays of energy greater than 2.8 MeV from the laC(p, 7)tSN reaction. The 261 keV resonance was observed with a different target than that used for the remainder of the yield curve.

TABLE 1 Strengths to7 (in eV) for three 14C(p, ),)tSN resonances Ev present experiment t Hebbard and Dunbar ~1) Hebbard ~)

352 keV

527 keV

634 keV

0.034 0.021 0.042

1.78 0.54

0.23 0.11

* All errors are estimated to be ~ 17 ,%. statistics a n d d e t e c t o r efficiency is believed to be less t h a n 4 ~ in all cases. T h e stopp i n g cross sections for p r o t o n s in c a r b o n were t a k e n f r o m the e x p e r i m e n t a l w o r k o f W h a l i n g 2s) a n d are e s t i m a t e d to c o n t r i b u t e not m o r e t h a n 3 ~o to the t o t a l uncertainty. The d i s c r e p a n c y for the 527 k e V resonance between the present result a n d t h a t o f H e b b a r d a n d D u n b a r 21) m a y at least in p a r t by u n d e r s t o o d b y the e x p e r i m e n t a l difficulties t h e y e n c o u n t e r e d in resolving a n d a n a l y s i n g the c o m p l e x v-ray s p e c t r a observed. In view o f this d i s a g r e e m e n t the yield m e a s u r e m e n t s were r e p e a t e d with a different target a n d were f o u n d to r e p r o d u c e the original results. M o r e o v e r , the value F / F = 0.55 o b t a i n e d recently by W a r b u r t o n et al. 8) is in g o o d a g r e e m e n t with the present results at Ep = 634 keV if mFp = 0.4 eV is used 21).

166

SIEFKEN e t al.

H.E.

4. Decay schemes and branching ratios Gamma-ray decay schemes and branching ratios from each of the three stronger resonances were determined from pulse-height spectra obtained with NaI(T1) and Ge(Li) detectors. These detectors were positioned at 55 ° with respect to the beam axis to minimize angular distribution effects due to the P2(cos 0) term in the Legendre polynomial expansion. The P4(cos 0) term is identically zero for the two J = resonances and is generally small at the J = ~, 352 keV resonance (see below).

z z

Ep = 261 keV O= 5 5 *

600

u

o_

2 615 (TkC")

400

tn "-i Q u

• '

.

[I

hi;./\ ~i

200

• """ ~" ~I ........,.,....~.

"f o

50

:.

......

..~. /-..<...:.:...~

.tOO 't50 CHANNEL NUMBER

. .......~: .....~.._l... ~:"

200

eso

Fig. 2. A typical),-ray pulse-height spectrum measured at the 261 keV resonance with a 12.7 cm × 12.7 cm NaI(TI) crystal. The off-resonance background shown has not been subtracted. All energies are quoted in MeV. 88

BOO0

I

527

Ep =352 keV 324 3,~

0=55"

5000 u W

4000

Z o

2000

, 50

'

, 100

i

~

2 150

200

250

CHANNEL NUMBER

Fig. 3. A typicaly-ray pulse-height spectrum measured at the 352 keV resonance with a 23 c m × 10 cm N a I ( T I ) crystal. The solid curve is the computer best fit using the component line intensities shown. All energies are quoted in M e V .

15N L E V E L S

527

167

544

IEO00 Ep = 527 keV e= ,55" W

9000

W

6000

IO 71 • 3s

Z Q U

3000 632

91

73O

0

50

0

@32

EO0

150

I00

250

CHANNEL NUMBER

Fig. 4. A typical x-ray pulse-height spectrum measured at the 527 keV resonance with a 23 cm x 10 cm N a I ( T I ) crystal. The solid curve is the computer best fit using the component line intensities shown. All energies are quoted in MeV.

20000 8 =55" J W

~15000 I

w

~I0000

N

q

Q U

5000

0

632

0

50

100

730

150

832

916

200

L

.~50

CHANNEL NUMBER Fig. 5. A typical x-ray pulse-height s p e c t r u m m e a s u r e d at the 634 keV resonance with a 23 c m x 10 c m NaI(T1) crystal. T h e solid curve is the c o m p u t e r best fit u s i n g t h e c o m p o n e n t line intensities s h o w n . T h e two line s h a p e s at 5.5 a n d 5.3 M e V were used to r e m o v e the intensities o f the 5.54, 5.51 a n d 5.27, 5.30 M e V x-rays respectively. All energies are q u o t e d in MeV.

PAIR SPECTROMETER

150

Ep = 6 3 4 k e V e:55"

4 'Z83 B0

.j

3 5O6

Z

z -
II ,,I

i/,

ry

i00

200

300

400

BOO

900

Ld

n

s 27o 529~

160

U'I

5 507

Z El U

r

80

Ii '5 5 3 6 •

500

600

I1

700

EHANNEL NUMBER

F i g . 6. A p o r t i o n o f t h e 7 - r a y p u l s e - h e i g h t s p e c t r u m m e a s u r e d a t t h e 634 k e V r e s o n a n c e w i t h a 25 c m s G e ( L i ) d e t e c t o r u s e d as a p a i r s p e c t r o m e t e r . T h e d o u b l e e s c a p e p e a k s are l a b e l e d w i t h t h e y - r a y energies quoted in MeV. 10.8 I 10.7 I

~

I ~

I I

5

7 9 8 S <1 & Z

I

I

J I I II

•

I

i

i

i

i

5/2" 5/Z~J =7/2

I

10.5& 10.b.5

5/2* V2(3/2) 7/2 5/2 =.5/2 5/2

10.07 9.95 9.85

9.76 9.22 916 9.05 8.57 8.51

5/,2 + 3/2 + I/2 +

7.56 7.50 7.15

7/2 + 5/,2+ 5/2 +

6.52 5.50

t/2 ÷

5.27

5/'2 ÷

r ¸'

"

I/2 -

~sN Fig. 7. E n e r g y level d i a g r a m and y - r a y decay schemes f o r ZSN. M e a s u r e d b r a n c h i n g ratios are s h o w n for the r e s o n a n c e levels a n d s o m e l o w e r levels.

lSN LEVELS

169

All ?-ray intensities have been corrected for absorption due to the target backing and crystal efficiency. Figs. 2-5 show the spectra obtained at each of the four resonances. Even though the 261 keV resonance is very weak, the presence of 4, 5, and 6 MeV ?-rays is clearly distinguishable from the off-resonance background. Several previously unreported ?-rays are observed in the pulse-height spectra from the three stronger resonances. In some cases these originate from transitions between states for which the spin and parity assignment is either uncertain or completely unknown. Previous measurements of the ?-ray cascades from each resonance level to the doublet levels at 5.3 MeV did not resolve the four possible ?-rays of nearly equal energy. Fig. 6 shows a portion of a spectrum obtained with the pair spectrometer in which the two cascades from the 10.81 MeV level are clearly resolved. Due to the low background present, this spectrum was also used to determine the relative intensities of the primary ?-rays. Similar spectra obtained at the other two resonances show the presence of ?-rays from a single cascade through the first excited state. A two step cascade through the first excited state at Ep = 10.54 MeV results in two ?-rays of nearly the same energy. It is evident that the ?-ray intensities of the spectra can only be accounted for by including this cascade in the decay scheme. The fact that no apparent broadening is observed in the Ge(Li) spectrum indicates that the energy difference is less than 5 keV. Fig. 7 summarizes the results obtained. A number of new cascades have been observed and they account for most of the proposed changes in the branching ratios from those reported in previous measurements s, 2~, 22). The 9.16 MeV level has been split indicating that there is now conclusive evidence for a very close lying doublet, which confirms the conjecture made by various recent investigators 9, 10.3,).

5. Angular distributions Angular distribution measurements were made at each of the three stronger resonances and the coefficients in the Legendre polynomial expansion W ( 0 ) l + AEP2(cosO)+A4P4(cosO) were determined by a least squares procedure. The A 4 coefficients were found only at the J = ~, 2 352 keV resonance. The data were analysed in terms of a multipole mixing parameter defined as X - ( J ' l l Z + lllJ) (J'IILIIJ)

'

with the phase convention of Ferguson 26). For two step cascades the values of the mixing parameter for the corresponding spin sequences of the primary transition were found at the Z2 minima and were held fixed during the analysis of the secondary transition. The secondary transition was also analysed in terms of an unobserved primary transition in which a two dimensional display of the Xpri--Xse c surface was printed out.

170

H . E . SIEFKEN et al.

_u

o

it'

i

I

i

i

O~}SERVEDA2 COEFFICIENT

...1

o

~

z

I---

I r - 80 *

-

I

I

O*

&O *

4.0"

I

80*

arctg Xpm

Fig. 8. A plot o f the theoretical A 2 coefficient versus arctg Xpr 1 o f the 5.27 M e V y - r a y in the 10.54 --~ 5.27 --->0 cascade. T h e m i x i n g p a r a m e t e r s o f the 10.54 ~ 7.15 ~ 5.27 a n d 5.27 --~ 0 transit i o n s f o u n d in this e x p e r i m e n t were used in the calculation. A similar plot involving the A4 coefficient p r o d u c e d results consistent with the r a n g e o f allowed values o f [Xprl[-

TABLE 2 A n g u l a r distribution coefficients corrected for finite detector size for y-rays at Ep = 352 keV T r a n s i t i o n (MeV) 10.54 10.54 10.54 10.54 10.54

-+ 8.57 ~ 7.30 -~ 7.15 --> 6.32 --~ 5.27

.4~

A4

--0.654-0.18 --0.61 4-0.02 0.424-0.01 --0.474-0.02 0.48+0.01

T r a n s i t i o n (MeV)

--0.164-0.24 --0.014-0.02 --0.004-0.01 --0.044-0.03 0.004-0.01

7.15 6.32 7.30 8.57

--~ 5.27 --> 0 --> 0 ~ 0

A2 0.284-0.02 --0.454-0.01 --0.354-0.01 0.064-0.01

A, 0.01 4-0.02 0.024-0.01 --0.01 4-0.01 --0.114-0.07

TABLE 3 Mixing parameters a n d m i n i m u m Z 2 values f o r y - r a y a n g u l a r distributions at Ep = 352 keV T r a n s i t i o n (MeV)

Jt

10.54 10.54 10.54 10.54 10.54 8.57 7.30 7.15 6.32

~ ~ ~ ~ ~ ~ ~~ .~

~ 8.57 ~ 7.30 "-->7.15 ~ 6.32 ~ 5.27 ~ 0 a) --~ 0 a) --> 5.27 a) --->0 a)

J~

Xprl

Xsec

0.174-0.09

½ ½ ½

Z2 0.38

0.11 ±0.01

1.89

0.034-0.01 0.04±0.01 --0.464-0.19

0.59 1.12 --0.25-4-0.03, --0.034-t-0.01, 0.02 4-0.02, 0.044-0.01,

a) T h e s e c o n d a r y was analysed h o l d i n g the value o f Xpri fixed.

3.484-0.03 1.884-0.01 1.96 4-0.02 1.60/:0.01

1.88 0.30 0.79 1.29

15N LEVELS

171

TABLE 4 Angular distribution coefficients corrected for finite detector size for ~-rays at Ep = 527 keV Transition (MeV) 10.71 10.71 10.71 10.71 10.71 10.71

A~

--> 0 --~ 9.05 --->8.31 ~ 7.30 --> 6.32 --+-5.27

Transition (MeV)

--0.734-0.01 0.384-0.08 --0.114-0.18 0.52+0.05 0.35±0.02 --0.04 4-0.02

9.22 9.1 8,31 7.30 7.15 6.32 5.27

~ 6.32 --~ 0 a) --+ 0 ~ 0 ~ 5.27 --+ 0 ~ 0

A~ 0.074-0,18 0.054-0.06 --0.064-0.06 --0.204-0,04 0.234-0,05 --0.11 4-0,02 0.39+0.02

a) A combination of the 9.05 --->0 and 9.22 --> 0 transitions.

TABLE 5 Mixing parameters and minimum Xz values for the v-ray angular distributions at Ep = 527 keV Transition (MeV) Yl

J~

Xprl

10.71 --->0 10.71 --* 9.05

½ ½ .~ ½ ~ ½ ~ {~ ½

0.154-0.01 or 1.28±0.01 --0.474-0.04 or 14.3 0.01 4-0.05 or 3.73 +0.05 --0.21 _-t-0.09 or 3.0710.08 0.344-0.14 or 14,3 --0.094-0.03 or --2.754-4-0.03 --0.034-0.01 or --4,334-0.01 0.0510.02 or 3.734-0.02

10.71 --~ 8.31

} ~ ~}

10.71 10.71 10.71 8.31

~ ~ ~ ½

--->7.30 --->6.32 --> 5.27 --->0 a) --~ 0 a) -+ 0 a) --> 0 a) ~ 5.27

.~ .g2 ~ ~

Z2

0.0 0 . vnl +°'51 1- 73 +2"~s -o.31 or -1.11 0.584-0.42 or --0.474-0.06 0.044-0.06 or 1.604-0.06 0.024-0.02 or 2.744-0.02 0.09 to 2.14

½

7.30 6.32 5.27 7.15

Xsee

½ ½ ½ ~

1.72 0.61 0.39 0.61 0.61 1.0 0.84 1.37 0.83 0.59 0.28 1.02 0.32 0.39

a) The secondary was analysed while holding the value of Xpri fixed. Mixing parameter errors have been tabulated unless the mixing parameter is too large to be significant or the method of the angular distribution analysis does not permit an accurate estimate of the error.

T a b l e 2 lists t h e a n g u l a r d i s t r i b u t i o n c o e f f i c i e n t s , A2 a n d Jr4, o b t a i n e d a t 352 k e V r e s o n a n c e . T h e y h a v e b e e n c o r r e c t e d f o r t h e finite size o f t h e d e t e c t o r . T h e p r e s e n t r e s u l t s a r e i n g o o d a g r e e m e n t w i t h t h o s e o f H e b b a r d 22) w i t h t h e e x c e p t i o n o f a 3.30 M e V y - r a y w h i c h is, i n f a c t , t w o u n r e s o l v e d y - r a y s c o r r e s p o n d i n g t o t r a n s i t i o n s f r o m t h e 10.54 M e V s t a t e t o t h e 7.30 a n d 7.15 M e V s t a t e s , r e s p e c t i v e l y . T h e t h e o r e t i c a l a n g u l a r d i s t r i b u t i o n o f t h e t w o 5.27 M e V 7-rays i n t h e 10.54 -~ 5.27 0 cascade

were calculated

from

the mixing

parameters

of the

10.54--+ 7.15,

7.15 ~ 5.27, a n d 5.27 --, 0 t r a n s i t i o n s a n d t h e i r r e s p e c t i v e i n t e n s i t i e s . S i n c e t h e o n l y unknown

is t h e m i x i n g p a r a m e t e r

o f t h e 5.27 M e V p r i m a r y 7 - r a y , t h e t h e o r e t i c a l

172

H.E. SIEFKEN e t aL

TABLE 6 Angular distribution coefficients corrected for finite detector size for y-rays at Ep = 634 keV Transition (MeV) 10.81 10.81 10.81 10.81 10.81 10.81 10.81 10.81 9.16

Aa

-~ 0 --~ 5.27 --~ 5.30 --~ 6.32 --~ 7.15 ~ 7.30 --0,8 . 3 1 --~ 9.16 a) --~ 0 ~)

Transition (MeV)

--0.47±0.01 --0.704-0.07 0.474-0.03 0.494-0.02 0.064-0.02 0.584-0,04 --0.704-0.04 0.144-0.03 --0.104-0.03

9.16 9.16 9.16 7.15 8.31 7.30 6.32 5.30 5.27

--~ 7.15 --~ 6.32 -+ 5.27 -+ 5.27 ~ 0 -+ 0 -+ 0 --~ 0 ~ 0

A~ 0.45:k0.07 --0.64+0.11 0.504-0.13 0.244-0.02 0.004-0.02 --0.074-0.01 --0.134-0.01 --0.044-0.04 0.20:t:0.03

a) This is the angular distribution of two 1.65 MeV y-rays. b) This is the angular distribution of the 9.05 and 9.16 MeV y-rays. TABLE 7 Mixing parameters and minimum Z~ values for the y-ray angular distributions at Ep = 634 keV Transition (MeV) Jl 10.81 ~ 0 10.81 --o-5.27 10.81 ~ 5.30 10.81 ~ 6.32 10.81 ~ 7.15 10.81 --~ 7.30 10.81-~- 8.31

~ ~ ~t -~ ~t ] ~

9.16 ~ 7.15

½ ~t

9.16 ~ 6.32

½ .~

9.16 -+ 5.27

½

8.31 --+0

½

7.30 -9- 0 a) 6.32 --~ 0 a) 5.30 ~ 0 a)

~ ~ ½

Jr ½ ]] ½ ~r ~ ~½ ~ ] ~ ~ ~ ~ .~ { @ ~ ½ ½ ½ ½ ½

Xprl --0.024-0.01 or 1.804-0.01 --0.634-0.04 or --2.754-0.04 --0.55:k0.02 --0.074-0.01 or --3.084-0.01 0.144-0.03 or 2.694-0.11 --0.12:k0.02 or --3.37±0.02 0.124-0.03 or 1.32-+-0.03 1.304-0.13 undetermined IXprl]~ 2.15 Ixpr,I ~ 0.577 undetermined [Xpr~I~ 11.4 [Xprt/~ 0.577 undetermined Ixp~,l ~ 1.73 IXpr~[~ 0.84

Xsee

undetermined --0.577 ~ Xsec ~ --3.73 --0.087 ~ Xsec ~ 1.19 undetermined --0.577 ~< X~oe --~ --1,00 0.268 ~ Xsee < 1,73 undetermined --0.364 < Xsec _~ --5.67 --1.73 < Xsee ~ 0.0 0.0 --0.264-0.03 or 3.734-0.27 --0.074-0.03 or 2.05 4-0.03 0.124-0.04 or 1.334-0.04 0.0

gz 1.36 0.99" 1.58 0.89" 1.67 1.23 1.23. 4.03 14.2 1.65 1.65 8.91 0.94 0.89 7.40 2.10 2.10, 0.42 0.42 1.23 2.31 1.22

a) The secondary was analysed holding the value of Xpri fixed. v a l u e s o f t h e A 2 a n d A4 c o e f f i c i e n t s c o u l d b e c a l c u l a t e d a n d c o m p a r e d w i t h t h e m e a s u r e d v a l u e s as s h o w n i n fig. 8. T a b l e 3 s u m m a r i z e s t h e r e s u l t s o b t a i n e d a t t h e 352 k e V r e s o n a n c e . F o r t r a n s i t i o n s b e t w e e n s t a t e s f o r w h i c h s p i n v a l u e s h a d p r e v i o u s l y been assigned, only the value o f the mixing p a r a m e t e r c o r r e s p o n d i n g to the c o r r e c t

I~N LEVELS

173

spin sequence is shown. Unless specifically pointed out, all other spin sequences were eliminated, unless they satisfied the 0.1% confidence limit 27). Tables 4 and 5 list the values of the A2 coefficients and the mixing parameters respectively for ?-rays observed in the decay of the 527 keV resonance. Only the angular distribution of the ground state ?-ray has been reported previously. The value A2 = -0.63-t-0.04 obtained by Bartholomew et al. 20) compares with the present value of Az = - 0 . 7 3 + 0 . 0 1 . The uncertainty of the data does not allow a statstically significant interpretation of the angular distribution measurements for some of the low intensity y-rays. Consequently, only the spin assignments for the 5.27, 6.32, 7.30, and 10.71 MeV states were confirmed at this resonance. The weak 9.05 and 9.22 MeV ?-rays could not be resolved with the NaI(Tl) detector used and the value listed in table 4 is therefore the composite result of both 7-rays. The 634 keV resonance was studied in detail to determine the properties of several states, notably tbe 8.31 MeV and the 9.16 MeV doublet states. Tables 6 and 7 summarize the angular distribution results obtained at this resonance.

6. Triple correlations Of the three stronger resonances studied, only the 634 keV resonance level decays with appreciable intensities to lower states with uncertain spin values. To investigate the 9.05 and 9.16 MeV states, triple correlation measurements were taken with a multiparameter analyser using the octant geometry 26) with the primary ?-ray detected by the fixed detector located at 0 = 90 ~. The resolving time of 2z = 50 ns introduced a chance coincidence count rate less than 2 % of the true coincidence count rate and hence no subtraction of chance counts was made in the spectra. Following the projection of the two parameter surface onto the fixed detector axis and the extraction of the ?-ray intensities at each angle, the data were fit using the theoretical formalism of Harris, Hennecke and Watson 28) for various spin sequences. The results of these measurements will be discussed in a later section.

7. Discussion of results 7.1. T H E 5.27 A N D 5.30 MeV L E V E L S

A strong branch through the 5.27 MeV level is observed at both Ep = 352 and 527 keV, however, only at the latter resonance is the analysis simple. The contribution from the cascade through the 7.l 5 MeV state was neglected since the intensity of the ?-rays involved is smaller by two orders of magnitude than the 10.71 -=, 5.27 primary transition. Values ofXpr j were independently determined from the angular distribution of the primary for various spin sequences and then held fixed for corresponding spin sequences used in the analysis of the secondary. The results shown in fig. 9 confirm the J = ~ assignment for the 5.27 MeV level and show the g a m m a ray to the ground state is almost pure magnetic dipole. Our value Xsec = 0.02+0.02 agrees with the

174

n . E . SIEFKEN et al.

measurements by Warburton and Olness 9 ) , b u t falls outside the value Xse~ = - 0 . 0 8 _+0.02 proposed as the best average value from two earlier measurements i 2.14), which used the particle-7 correlation method of Litherland and Ferguson. In the present experiment a transition to the 5.30 MeV level is seen only at the 634 keV resonance. Although the J = ~ assignment is eliminated, the statistical uncerCOS 2 0

1.0

0.75

050

0.25

0.0

I

I

I

r

I

I

I

230 210 o

,~

190

x

170

~__m

z

15G 130

I

/ 1+52

10o

1071 - -

--

3'2+

X Pml F~XEO

x2

1o 527--

J

--

0.1% XsEe 10% 0

12-

l

I -80 °

I -40"

0°

arctg

I

1

40*

80 +

X

Fig. 9. Plot of Z~ versus arctg Xse e curves resulting from the fit o f the theoretical angular distribution to the data points s h o w n for various spin values o f the 5.27 MeV state.

tainty in the data for the 10.81 ~ 5.30 transition does not allow a definite choice to be made between the J = ½ and ½ values. 7.2. T H E 6.32 MeV LEVEL

This state is of interest since it is the lowest excited state with negative parity as shown by the l = 1 stripping pattern from the l+N(d, p)lSN reaction. This, together

15N LEVELS

175

with the J = ~2 assignment from angular correlation studies 12) suggests that this state is described by a p~ hole in an otherwise closed shell. Assuming that the 5.27 MeV state has the principle configuration (p~)2 d~, our failure to observe a 6.32 -~ 5.27 transition in the present experiment supports the p~ hole picture for the 6.32 MeV state since the El matrix element vanishes for this transition. The E2/M1 mixing amplitude of the ground state transition has been calculated by Rose and Lopes 29) to be X = 0.08. The attempt to explain the larger experimental value Y = 0.13+0.01 by including a collective term 3o) shows that the contribution from such a term is approximately an order of magnitude smaller than the single-hole contribution. Transitions to the 6.32 MeV level are observed at all three resonances with approximately equal intensity. In all three cases the J = ~2 assignment is confirmed and the average E2/M1 mixing parameter required is X -- 0.06+_0.04 or 1.51 +_0.04. This value is in excellent agreement with the results of Warburton et al. 12) and within experimental error of the average value reported by Poletti et al. 31). 7.3. THE 7.15 MeV LEVEL Decays to this level occur at all three resonances. The 7.15 ~ 5.27 transition accounts fer nearly the entire intensity from the 7.15 MeV level and therefore places an upper limit of 3 ~ on the intensity of the (unobserved) dilect transition to the ground state. Angular distribution measurements confirm the J = ~ assignment and the value of the E2/M1 mixing parameter for the 7.15 --, 5.27 transition shows that the 1.88 MeV y-ray is nearly pure M1, in agreement with previously reported values (refs. 14-, 16)). 7.4. THE 7.30 MeV LEVEL The present measurements confirm the J = -~ assignment and indicate that this state decays 100 ~ to the ground state. Angular distribution measurements fix the M2/EI mixing amplitude as X = - 0 . 0 5 + 0 . 0 3 or 1.97+0.03. The smaller value is in agreement with the results of Warburton et al. 8) who report that the 7.30 MeV y-ray is predominantly El, and also with the work of Hausser et al. 16) who find X = - 0.07___ 0.02. 7.5. THE 8.31 MeV LEVEL By comparing the intensities o f the primary and secondary transitions through this state we find an 85 °/o branch to the ground state compared with the values 83 % and 70 % which were obtained from the 14N(d, p)lSN and 13C(3He,p)lSN reactions re;pectively. The assignments J = 1+ or ~+ follow from particle-y correlation measurements 14), (d, p) stripping measurements 13, 15), and the E1 multipolarity measurement of the ground state v-ray 8). However, in spite of these measurements, the rigorous experimental exclusion of one of the spin choices has not been possible due to the fact

176

H . E . $IEFKEN et al.

that, in the case of the particle-y experiments, both spins fit equally well with an appropriate choice of the mixing parameter in the A2 coefficient. Fig. 10 shows the strongly anisotropic angular distribution of the 2.49 MeV y-ray feeding the 8.31 MeV state. The analysis eliminates the J = ~ choice and establishes the J = 5-assignment well below the 1 ~ confidence limit. This conclusion is supported O

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Fig. 10. P l o t o f z 2 versus arctg R'pr I curves r e s u l t i n g from the fit o f the t he ore t i c a l a n g u l a r d i s t r i b u t i o n to the d a t a p o i n t s s h o w n for the spin values J = ½ or ] o f the 8.31 MeV state.

by the angular distribution and triple correlation measurements involving the 8.31 -~ 0 transition which may be fit by J = ½ only by using an M2/E1 mixing amplitude of 0.27 or 3.73 which is in contradiction to the previous El measurement 8). The J" = ½+ assignment suggests that this is the mirror level to the 7.55 MeV state in XSO.

l°N LEVELS

177

7.6. THE 8.57 MeV LEVEL The angular distribution o f the 1.97 MeV y-ray resulting f r o m the 10.54 ~ 8.57 transition is best fit by assuming J = ~ for the 8.57 MeV level. The M2/E1 mixing parameter for the 8.57 ~ g.s. transition is X'sec = -0.25_+0.03, which agrees within experimental error with the value X = - 0 . 1 3 _ + 0 . 2 5 previously reported 14). This appreciable c o m p o n e n t o f M2 radiation was not observed in the measurement by W a r b u r t o n et al. 8). cos 20

cos 4)

cos 2 e

0.75 0.500.25 0.0 -I.0-O.71 0.0 00 0.25 0500.75 300: I I I I I I I I I I I A2 GEOMETRY D GEOMETRY C2 GEOMETRY

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I J31/2

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i

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.

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.

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Fig. 1I. Plot of Z~ versus arctg Xseo curves resulting from the fit of the theoretical triple correlation to the data points shown for various assumed spin values of the 9.05 MeV state. 7.7. THE 9.05 MeV LEVEL The difficulties in making a definite spin choice for this level are similar to those encountered in the study o f the 8.31 MeV level. A l t h o u g h the particle-7 angular correlation w o r k lo) has shown the distribution o f the g r o u n d state transition to be isotropic, it is not possible to distinguish between the J = ½ or ~r assignments. Fig. i1 shows the theoretical fits to the triple correlation data involving the 10.81 ~ 9.05 --* 0 cascade. A J = ~ assignment is clearly preferred although the primary mixing parameter for this choice requires an E2 transition probability o f 5 W.u. The M2/E1 mixing parameter for the g r o u n d state decay is Xseo = - 0 . 6 4 + 0 . 0 7 which is m u c h larger than the pure El f o u n d in a previous measurement a). The transition speeds for E1 and M2 radiation are consistent with the lifetime limit o f • < 0.5 ps for this

178

H . E . $IEFKEN e t al.

level, however, a measurement with better statistics is needed to verify the J = assignment. 7.8. TI-[E 9.16 MeV LEVEL The properties of this level have been the subject of considerable controversy. First observed in the 14N(d, p)lSN reaction 15) and more recently is) in the 11B(6Li, d)lSN and 11B(TLi' t)lSN reactions, this state became of interest when several measurements yielded apparently contradictory results. The experiments of Warburton et al. 8) have ruled out J~ = 71 + or ½- ; the first, because the ground state transition is not pure El, the second because the lifetime limit (r < 0.5 ps) of the 9.16 MeV y-ray precludes M2 or E3 radiation for the 9.16 ~ 7.15 transition. Charged particle-y-ray correlation measurements by Phillips et al. 10) using the 13C(3He ' py)15 N reaction, have subsequently fixed the spin to be ~. Their branching ratios are in disagreement, however, with those observed by Warburton who used the same reaction at a different bombarding energy. Moreover, both results disagree with experiments using the 14N(n, y)lSN reaction 32,33). TO explain this inconsistency it has been suggested that there may be two close lying states at 9.16 MeV with one level or the other preferentially populated in the different reactions used. An attempt to resolve the proton groups from t h e 1 3 C ( 3 H e , py)lSN reaction 1~) with a magnetic spectrograph has shown that the two levels are not separated in energy by more than 8 keV. More recently, the 14N(d, p)l 5N reaction has been re-examined 1o) to search for structure in the proton spectra and to determine the branching ratios of the y-radiation resulting from the 9.16 MeV level. No evidence was found for the existence of more than a single proton group. The branching ratio measurements reaffirmed the results of Warburton, but all are in disagreement with those of Phillips. The most positive indication for a close lying doublet is contained in the analysis of experiments by Greenwood 34) who measured the neutron binding energy from the 14N(n, y)lSN reaction. This analysis showed that the value obtained, using the two step cascade through the 9.16 MeV state, differed by 2.2+_1.3 keV from the average of several other two step cascades. This result indicates that the ground state transition must originate from the lower member of the doublet. In the present experiment a 5.6 ~o branch to the 9.16 MeV state is observed at Ep = 634 keV. The analysis of the data obtained provides conclusive evidence for the existence of two close lying states. Angular distribution and correlation measurements fix the spin of the upper (U) state as J = s and that of the lower (L) state as J = ~. Fig. 12 shows a portion of a spectrum obtained with the 25 c m 3 Ge(Li) detector operated in the Compton suppression mode. A comparison of the shape of the 1.650 MeV y-ray with that of the 1.884 MeV and four other y-rays (not all shown) indicates that the former has a F W H M of 10.3___0.1 keV whereas the others average 8.3+_0.3 keV. The broadened peak, which was observed at 0 --- 55 °, was analysed in terms of two Gaussian shapes with the widths fixed by the average value of neighboring peaks. The result of the analysis indicates that the two peaks are separated by

I~'N LEVELS

179

3 keV which confirms the results of Greenwood and falls well within the upper limit set by Gallman. In a recent publication Steerman and Young 35) have incorporated the branching ratio measurements from several reactions into an explanation that is consistent with two levels. In the present experiment the branching ratios of the four 7-rays observed in the decay of the 9.16 MeV states coincide with corresponding transitions observed from other reactions at a value o f f ~ 0.40 (Steerman and Young notation) which

1600

Ep:634 keY O=0-

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100

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.

.

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I

800

I

I

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CHANNEL NUMBER Fig.

portion of the y-ray pulse-height spectrum measured at the 6 3 4 k e Y resonance with a Ge(Li) detector operated in the Compton pulse suppression mode. The decay energies which identify the peaks are quoted in M e V .

12. A

25 cm 8

TABLE 8 Experimental branching ratios of the 9 . 1 6 M e V "level" in ZSN

Transition (MeV) 9.16 9.16 9.16 9.16

-4 ~ ~ ~

0 7.15 6.32 5.3

Branching ratio % 33±3 39±3 13-+-2 15~2

means the lower member of the doublet is populated with approximately 40 % of the 10.81 ~ 9.16 intensity. The relative intensities for the decay of the 9.16 MeV "level" are shown in table 8. The spins of the two levels wele established from triple correlations measurements. To study the lower level the 10.81 --, 9.16 ~ 0 cascade was observed in the octant

180

H.E.

$IEFKEN e t al.

o

_ 'I

I~I

-

i

i,[~i, ,

{

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0

~

~

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/

89

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o I- ~ i

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SINFIO~ _-I0 ~ZlSHQN

~6 o.~

~

15N LEVELS

181

g e o m e t r y . Fig. 13 shows the result o f a s s u m i n g v a r i o u s spins for the lower 9.16 M e V level. O n l y J = ~: gives a satisfactory fit, which confirms the results o f Phillips et al. x0). T h e possible values Xp~ = 0 . 0 6 + 0 . 0 4 a n d X~,¢ = 0 . 0 9 + 0 . 0 3 are in excellent a g r e e m e n t with the p a i r c o r r e l a t i o n m e a s u r e m e n t s which s h o w e d the 9.16 M e V y-ray m o s t likely to be M1. This w o u l d indicate t h a t the 9.16 L M e V level is tbe s e c o n d excited negative p a r i t y state in ~SN a n d c o r r e s p o n d s to the J~ = ~ m i r r o r level at ~sO at 8.98 MeV. TABLE 9

Mixing parameters and minimum Z2 values for assumed spin sequences obtained from the triple correlation measurements at Ep = 634 keV Transition (MeV)

A

./2

Ja

Xpri

10.81 ~ 8.31 ~ 0 10.81 --~ 9.05 ~ 0

~ ~t ~

½ ~ ½ ~

½ Jr ½ ½

10.81 ~ 9.15 --~ 0

~t ~

~ ~

½ ½

~t

~ ~ ½ ~ ~ ~

½ ½ ~ ~ ~ ~

tmdetermined --0.13 undetermined --0.36±0.04 --3.17:k0.08 0.27 i 0 . 0 4 --0.06i0.04 --1.30i0.04 --4.33 --1.43±0.12 undetermined 4.33 0.094-0.05 --22.9

10.81 ~ 9.15 ~ 7.15

~

Xsec 0.0 --0.374-0.01 or 3.734-0.25 0.0 --0.644-0.06 or 22.9 --0.084-0.02 or 2.794-0.03 --0.134-0.03 or 4.51 0.09-4-0.03or 1.43±0.03 --0.784-0.03 or --7.96±0.03 --0.36±0.06 0.09±0.03 undetermined --0.184-0.04 0.04±0.05 0.13±0.03

Z~0.35 0.35 7.10 1.84 4.60 0.76 4.01 20.8 11.0 1.62 1.61 1.60

T o d e t e r m i n e the p r o p e r t i e s o f the 9.16 u M e V level the 10.81 --* 9.16 v - , 7.15 cascade was investigated in the A 2 a n d C2 geometries with the 2.01 M e V ~,-ray o b s e r v e d in the m o v i n g detector. Fig. 14 shows the d a t a o b t a i n e d a n d the plots o f Xz versus arctg X for J = 3, ~, a n d 5. A l t h o u g h all three spin values fall well below the 0.1 O//oconfidence limit, only J = ~ can be fit with small values o f the mixing p a r a m e t e r s (table 9). This result is consistent with the a n g u l a r d i s t r i b u t i o n measurements o f the three y-rays f r o m the decay o f the 9.16 U M e V state. Table 7 shows t h a t the J = 3 spin choice requires large values o f Xpr i in each case investigated. T h e a n g u l a r d i s t r i b u t i o n o f the 2.83 M e V g a m m a lequires, for instance, [Xpri] > 11.4 which implies an E2 t r a n s i t i o n pl o b a b i l i t y o f 340 W.u. o r a n M2 t r a n s i t i o n p r o b a b i l i t y o f 10" W.u. 7.9. THE 10.54, 10.71 AND 10.81 MeV LEVELS These states have been assigned J~ = ~(+), 3 + a n d ~(+~ respectively f r o m a n g u l a r d i s t r i b u t i o n a n d elastic scattering m e a s u r e m e n t s 2o,21,22). T h e definite p a r i t y assignment for the 10.71 M e V level follows f r o m the elastic scattering m e a s u r e m e n t which shows the state is f o r m e d by d-wave p r o t o n s a n d the large p r o t o n r e d u c e d width suggests the state is m a d e up p r i m a r i l y o f a 14C core a n d a d~_ p r o t o n .

182

H.E. SIEFKEN e t al.

A l t h o u g h t h e 10.71 a n d 10.81 M e V s t a t e s b o t h h a v e p a r t i c l e c o n f i g u r a t i o n s w h i c h p r o d u c e J = ~, t h e d e c a y s f r o m e a c h a r e d i s t i n c t l y d i f f e r e n t as s h o w n b y t h e t r a n s i t i o n s t o t h e 5.3 M e V d o u b l e t levels a n d b y t h e a m o u n t o f m u l t i p o l a r i t y m i x i n g p r e s e n t in t h e g r o u n d s t a t e t r a n s i t i o n f r o m e a c h level.

References I) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24~ 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35)

D. Kurath, Phys. Rev. 101 (1956) 216 E. C. Halbert and J. B. French, Phys. Rev. 105 (1957) 1563 D. R. lnglis, Rev. Mod. Phys. 25 (1953) 390 A. M. Lane, Rev. Mod. Phys. 32 (1960) 519 A. P. Shukla and G. E. Brown, Nucl. Phys. A l l 2 (1968) 296 F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. 11 (1959) 1 A. E. Evans, Phys. Rev. 155 (1967) 1047 E. K. Warburton, J. W. Olness and D. E. Alburger, Phys. Rev. 140 (1965) B1202 E. K. Warburton and J. W. Olness, Phys. Rev. 147 (1966) 698 G. W. Phillips, F. C. Young and J. B. Marion, Phys. Rev. 159 (1967) 891 A. Gallman, P. Fintz, J. B. Nelson and D. E. Alburger, Phys. Rev. 147 (1966) 753 E. K. Warburton, J. S. Lopes, R. W. Ollerhead, A. R. Poletti and M. F. Thomas, Phys. Rev. 138 (1965) BI04 E. K. Warburton and J. N. McGruer, Phys. Rev. 105 (1957) 639 D. Pelte, B. Povh and W. Scholz, Nucl. Phys. 78 (1966) 241 R. D. Sharp, A. Sperduto and W. W. Buechner, Phys. Rev. 99 (1955) 632 O. Hausser, R. D. Gill, J. S. Lopes and H. J. Rose, Nucl. Phys. 84 (1966) 683 D. E. Alburger, C. Chasman, K. W. Jones and R. W. Ristinen, Phys. Rev. 136 (1964) B913 R. L. McGrath and R. R. Carlson, Bull. Am. Phys. Soc. 10 (1965) 443 A. Bussiere, N. K. Glendenning, B. G. Harvey, Jeanette Mahoney and J. R. Meriwether, Phys. Lett. 16 (1965) 296 G. A. Bartholomew, F. Brown, H. E. Gove, A. E. Litherland and E. B. Paul, Can. J. of Phys. 33 (1955) 441 D. F. Hebbard and D. N. F. Dunbar, Phys. Rev. 115 (1959) 624 D. F. Hebbard, Nucl. Phys. 19 (1960) 511 F. D. Lee, R. W. Krone and F. W. Prosser, Jr., Nucl. Phys. A96 (1967) 209 W. A. Fowler, C. C. Lauritsen and T. Lauritsen, Rev. Mod. Phys. 20 (1948) 236 W. Whaling, Handbuch der Physik (Springer-Verlag, Berlin, 1958) Vol. 34 A. J. Ferguson, Angular correlation methods in gamma-ray spectroscopy (North-Holland Publ. Co., Inc. Amsterdam, 1965) A. H. Wapstra, G. H. Nijgh and R. van Lieshout, Nuclear spectroscopy tables (North-Holland Publ. Co., Inc., Amsterdam, 1959) D. D. Watson and G. I. Harris, Tables of coefficients for angular correlation of radiative transitions from aligned nuclei (Aerospace Res. Lab., Wright-Patterson Air Force Base, 1966) H. J. Rose and J. S. Lopes, Phys. Lett. 22 (1966) 60l J. S. Lopes, O. Hausser, H. J. Rose, A. R. Poletti and M. F. Thomas, Nucl. Phys. 76 (1966) 223 A. R. Poletti, E. K. Warburton and Dieter Kurath, Phys. Rev. 155 (1966) 1096 H. T. Motz, R. E. Carter and W. D. Barfield, Pile neutron research in physics (IAEA, Vienna, 1962) p. 265 R. E. Carter and H. T. Motz, Internat. Conf. Nuclear physics with pile reaction neutrons, ed. by F. E. Throw (Argonne Nat. Lab., Argonne, Illinois, 1963) p. 179 R. C. Greenwood, Bull. Am. Phys. Soc. 12 (1967) 1199; and R. C. Greenwood, private commumcation C. E. Steerman and C. F. Young, Phys. Lett. 27B (1968) 8