Some studies of T = 32 states in 17F

Nuclear Physics A322 (1979) 3 3 - 3 9 ; ~ ) North.Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

S O M E S T U D I E S O F T = a2 S T A T E S IN l~F F. M. K H A N , J. LOWE and J. M. NELSON

Department of Physics, University of Birmingham, Birmingham, B15 2TT, Enoland and A. R. BARNETT

Department of Physics, University of Manchester, Manchester, M13 9PL, England Received 15 January 1979 Abstract:Measurements have been made of some parameters of the second and sixth T = ~ states in 17F. F o r the second state, the resonance energy was found to be Ep = 12.707_+0.001 MeV (E x = 12.550___0.001 MeV), which agrees with and improves on the accuracy of earlier work. For the sixth T = ½ state, at Ep = 14.435 MeV, the ~,-decay was determined to be predominantly Y0 with a branch to the first excited state ofF(~)/F(yo) < 0.14. Together with ol~her work, this determines J~ to be ½-. The capture strength is found to be (23"+ 1)FpFy/F = 11.4+2.6 eV.

E

NUCLEAR REACTIONS 160(P--,.3)~/'EE,_ _capture= 12-15strength.MeV; measured a(Ep). 17E deduced

1. Introduction At least seven states with T = 3 are known 1) in the T3 = 1 nucleus 17F, five of which have been observed 1) in the proton capture on 160. All five of these are narrow; their widths are less than 50 keV, and the lowest two have widths of less than 2 keV. Some proton and alpha decays of these states have been studied by Skwiersky et al. 2), who determined the energies of several states to + 4 keV. G a m m a decays from T = 3 states have been studied by Harakeh et al. a). G a m m a rays leading to the ground and first excited (E x = 495 keV) states of ~7F were not resolved in their work. Harakeh et al. also present a detailed shell-model calculation in an attempt to account for the excitation energies and y-decay widths of the states. The present work was initiated in an attempt to improve on earlier work in two respects: (i) A recent energy calibration of our accelerator enabled us to measure the resonance energy with greater precision than in earlier work, for any reasonably strong resonance which is sufficiently narrow. The energy of the second T = 3 state, at Ex = 12.55 MeV, was measured to + 1 keV. 33

34

F.M.

KHAN

et al.

(ii) The resolution of our high-energy v-detector enabled us to separate the Vo and Vl decays on a strong resonance. The decay mode and the Vo/Vl branching ratio were measured for the state at E~ = 14.174 MeV. Together with proton scattering data, these results determine J~ unambiguously for this resonance. In addition, the capture strength, (2J+ 1)FpF~/F, was determined for the latter state, for comparison with earlier work. After our experiments were completed another determination of the energy of the 12.55 MeV state was published 4), which is in excellent agreement with our value.

2. Experimental method Proton beams from the coupled electrostatic accelerators at Oxford Nuclear Physics Laboratory were incident on a solid target. Gammas from the target were detected in a 25 cm diameter x 30 cm NaI(TI) detector surrounded by a NE102 anti-coincidence shield. The detector, together with its associated electronics and data acquisition system, has been described before 5, 6). Pulse-height spectra were accumulated in a multichannel analyser and transferred to a PDP10 computer for analysis. 2.1. T H E

E x = 12.55 M e V S T A T E

This state is known 1) to have a width of only 1.8 +0.5 keV. Thus the excitation energy can be determined to an accuracy of about 1 keV provided the accelerator energy calibration is known accurately enough. A precise knowledge of the energy is required, for example, for the study of Coulomb shifts between members of an isobaric multiplet. A precise calibration of the Oxford electrostatic accelerators was carried out recently by Chew et al. 7). The energy reference for this calibration was the resonance in 12C(p,y)13N at Ep = 14.23075+0.00020 MeV (the first T = ~ state in 13N), the energy of which was determined by Huegnes et al. 8). In ref. 7), it was found that, provided care is taken in cycling the magnet, the beam energy can be determined to better than 1 keV in this region. In the present experiments, the state at E x = 12.55 MeV was observed as a resonance in the Vo yield from a target of PbO, 200 #g. cm-2 thick (equivalent to 3_+ 1 keV energy loss). Two precautions were taken in determining the resonance energy: (i) The magnet was cycled to minimise differential hysteresis, and all measurements were carried out at the same point on the hysteresis cycle. (ii) During the same hysteresis cycle, the accelerator energy calibration measurement was repeated by remeasuring the energy of the T = ~ resonance in t3N at Ep = 14.23075 MeV. This technique provided a check on the long-term stability of the calibration, and also an improvement in the accuracy since the reproducibility

17F

35

between different hysteresis cycles did not contribute to the uncertainty in the calibration. Excitation functions were measured for seven points across each resonance. 2.2. THE E x = 14.174 MeV STATE

This is the lowest T = 3 resonance for which the spin had not been unambiguously determined from earlier work. Harakeh et al. 3) give J" = ½- or a-, with a preference for the latter from elastic proton scattering work. They also established that the y-decay was to 70 and/or 71, but were unable to resolve these gammas. In the present experiment, the state was again excited in proton capture. In this case an organic target was used, since neutron background from Pb rendered the PbO target unsuitable. Foils of both mylar (CtoHsO4, 1.7 mg" cm-2) and mannitol ( C 6 0 6 H 1 4 , 1 mg. cm -2) evaporated onto a C-foil were used. The former is more stable under bombardment, but contains less oxygen. In both cases, target deterioration proved a problem and it was found necessary to make several short on-resonance runs interspersed with off-resonance runs. The former were made at a target-centre energy of 14.435 MeV, and the latter alternately 100 keV higher and 100 keV lower than this. The spectra showed, in addition to the 160(p, y)lYF lines under study, gammas from 12C(p, p'Y12.71)12C and 12C(p, yo)13N, which provided a continuous monitor of the detector energy calibration. The y-detector efficiency was determined by observation, with the same geometry, of the thick target yield of 15.07 MeV gammas from the resonance at Ep = 14.23075 MeV in 12C(p, yo)13N. The y-yield from this resonance has been determined by Marrs et al. 9), and it provides a convenient efficiency calibration standard for the detector. 3. Results 3.1. THE 12.55 MeV STATE

A typical pulse-height spectrum from the NaI(T1) detector is shown in fig. 1. The ground-state gamma from 160(p, y)l~F can be seen clearly. The count in the y-ray peak was extracted using the method described before 5,6), in which the spectrum is represented as a sum of y-peaks, of known line shape, and a background. The parameters of the background, and of the strength of each y-ray were varied to fit the experimental spectrum. Fig. 2 shows the yield of the capture gammas both for the ~TF state, and for the 13N calibration state. The ~3N resonance appears at Ep = 14.2303 ±0.0005 MeV. This result is in good agreement with the known value s) of 14.23075 ± 0.00020 MeV confirming the stability of the tandem calibration over several months. The ~TF resonance energy is Ep = 12.707±0.001 MeV, from which the excitation energy is E x = 12.550±0.001 MeV.

36

F . M . KHAN et al.

18C 160 (p,EI17F

OQ i

Ep=12.708 MeV I--

z

•

90

.

60 °

O (..)

~ 11

v

•

4.

I+11+

• •

•

•

• •o I + %1•

ol

+~

II

•

•

•

•

•

•

10 +•+

• • • • i i +~

• • i•t

i

O' 0

i

200

'

2:~0

'

I

•

! J

2~.0CHANNEL260

Fig. 1. Pulse-height spectrum in the NaI(TI) detector from protons of energy 12.707 MeV on the Pb0 target.

12C(p,~,)13N .J LU

,..I, 12.696

,.I,,,I,,

,2.702

,I,.

,I ,. , l , j

,2.7o6 E p|Me'2"710v|

. . , I . . . I , . . I , , , I , , , I J , , V.,2.2 t. '14.226 14.232

I lt..236

E plMeV)

Fig. 2. Gamma yield for T = ~ resonances in 160(p, 70)17F and t2C(p, yo)t3N as a function of proton energy at the target centre.

3.2. THE 14.174 MeV STATE

Pulse-height spectra on and off resonance for this state are shown in fig. 3. The spectra shown are those from the mannitol target, which gave a better yield of t 7F gammas. The energy calibration is adequately determined from the gammas from 12C and 13N at 12.71 and 15.19 MeV respectively, originating from carbon in the target. The following conclusions can be drawn from the spectra: (i) The detector resolution is good enough to demonstrate that it is ?0 rather than 7t that resonates here. The spin and parity of the resonance have been restricted to J ~ = ½- or 3- from proton scattering data 2,3). Also the 17F ground state has J~ = -~+, and the radiative width (see below) implies a dipole transition. Thus the state must have J~ = -~-, in agreement with the preferred value from ref. 2).

1?F

37

• ON RESONANCE o OFF RESONANCE

56O

12C1(~:~]12C

*.

MeV

~

•

420

j

160(p,I}1~

-r E 200

I, ~4qtv

*

% ~ iIiw

13ate~

i,

~ v . , ,2C(p,rPN •

+

15.19

",,4.

MeV

°

I--

I

I

I

~

'alL-

I

--.

i,

. . . . .

CHANNEL

Z

."

0

( j mo

% •

1' ' ,

; •

•

dl, e

%6*

• •

•

. ...,... . 0

DIFFERENCE

*

• 5C

~°

•

I •

••

~ • o• • .

.:.,,...,-'....:,.,.....,..:.

I

CHANNEL -50,

Fig. 3. Pulse-height spectra on and off resonance for the Ep --- 14.435 MeV resonance in s60(p, y)lTF for the same integrated beam charge. The lower curve shows the difference between the on and off resonance spectra.

(ii) From the observed resonant yield, the Breit-Wigner formula gives (2J + 1) Fp°Fr° = 11.4+ 2.6 eV. F In extracting this quantity, allowance has been made for the fact that the known 1) width, F = 30 + 5 keV, is not small compared with the target thickness of 30 keV. The total y-yield was determined from the 90° cross section using the value 3) for the Legendre coefficient a2 of -0.1. The error arises mainly from the extraction of the y-yield in the spectra of fig. 3. Spectra from the mylar target gave a result consistent with this but with a larger error. The ratio Fpo/F has been determined from proton scattering 2) to be 0.04-t-0.02. Using this value and J = ~ gives Fr = 72+37 eV. (iii) The peak at the energy corresponding to 160(p, yl)17F in fig. 3 in fact arises mainly from the one-escape peak from the Y0 line. The intensity of the one-escape peak has been estimated from the known detector line shape. Subtracting this contribution gives an upper limit (at 2 standard deviations) for the branching ratio of

F~,/Fvo ~_ 0.14.

38

F.M. KHAN et al.

4. Discussion Previous determinations of the energy of the second T =-32 state are 2) E x = 12.551_+0.004 MeV and a) E x = 12.554+0.006 MeV. The present result, E x = 12.550_+0.001 MeV, is consistent with these and improves on their accuracy. After our experiments were completed a result with comparable accuracy to ours, E x = 12.5508 +0.0023 MeV, was published by Hinterberger et al. 4), who observed the resonance in elastic proton scattering. Again the agreement is excellent. For the E x = 14.174 MeV state, the radiative width determined here, Fr = 72_+37 eV, agrees with the value of Fr = 81 _+54 eV determined by Harakeh et al. 3). The resonance is observed as a p-wave resonance in elastic proton scattering, and was assigned J~ = -32- by Swiersky et al. 2). According to Harakeh et al., both J~ = ½and 3- are consistent with the proton scattering data (i.e. only the/-value is determined), and they express a preference for ½- from shell-model calculations. However, the dipole decay mode found here determines the 3- assignment unambiguously. Several shell-model calculations have been published for the T = -32states of 17F. For example, Soga 10) constructed the odd-parity states from the 2p-lh configurations (p~ ld~) and (p~ ld~2s~). Detailed wave functions are not quoted by Soga, but the lowest predicted 3- state occurs at E x = 12.72 MeV, and presumably corresponds to the known state at 12.550 MeV. The wave function is mainly the (p~ld~) component, which cannot decay to the ground or first excited states of 17F by E1 y-emission. The second J~ = ~a- , T = 3 state is predominantly the (p~ldt2s½) configuration which can decay to the ground but not the first excited state by E1 decay, in agreement with the branching ratio found here. Gamma decay widths have been calculated for Soga's wave functions by Harakeh et al. a) who find a predicted value of F~ = 90 eV, in good agreement with experiment. Harakeh et al. 3) carried out a calculation in which all the lp and 2s-ld shell nucleons are active. The predictions for E1 y-widths for several decays of T = 3 states agree well with experiment with the sole exception of the 3- state whose decay was studied here. They give several predictions for F~ for different parameter choices, but all are at least a factor of 10 below the experimental value. The energy of this state is also not well fitted and they speculate that the J" = -~- assignment, which was not well established at that time, may be in error. The decay mode established in the present work, however, excludes this possibility. Thus the small E1 width predicted by their otherwise very successful calculation remains a puzzle. The authors would like to thank Professor K. W. Allen for use of the facilities of the Oxford Nuclear Physics Laboratory, and the SRC for the award of a research grant.

lVF

39

References 1) 2) 3) 4) 5) 6) 7)

F. Ajzenberg-Selove, Nucl. Phys. A281 (1977) 1 B. M. Skwiersky, C. M. Baglin and P. D. Parker, Phys. Rev. C'9 (1974) 910 M. N. Harakeh, P. Paul and K. A. Snover, Phys. Rev. C11 0975) 998 F. Hinterberger, P. yon Rossen, B. Schuller, J. Bisping and R. Jahn, Nucl. Phys, A263 (1976) 460 S. H. Chew, J. Lowe, J. M. Nelson and A. R. Barnett, Nucl. Phys. A286 (1977) 451 S. H. Chew, Ph.D. thesis, University of Birmingham 0975) unpublished S. H. Chew, J. Lowe and H. R. McK. Hyder, Oxford Nuclear Physics Laboratory report 6 75 (1975). unpublished 8) E. Huegnes, H. Rosier and H. Vonach, Phys. Lett. 46B (1973) 361 9) R. E. Marrs, E. G. Adelbcrger and K. A. Shover, Phys. Rev. Cl6 (1977) 61 10) M. Soga, Nucl. Phys. 89 (1966) 697