The 19F(p,p') reaction at 140 MeV

Nuclear Physics 55 (1964) 353--363; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

THE 19F(p, p') R E A C T I O N A T 140 M e V D. NEWTON, A. B. CLEGG and G. L. SALMON t Nuclear Physics Laboratory, Oxford Received 24 January 1964 Abstract: Measurements are presented of the states excited in the inelastic scattering of 140 MeV protons from the X°Fnucleus. It is shown that they can be interpreted as a strong electric quadrupole transition exciting the J = j state of the ground state rotational band, two strong electric octupole transitions exciting J = t and ½ states of the negative parity rotational band and a strong E4 transition exciting the J = ~- state of the ground state rotational band. This interpretation adds considerably to the information available about the rotational bands of leF. E[

I

NUCLEAR REACTION F19(p, P'7'), Ep = 140 MeV; measured err, Ey, I~,. F 19 deduced levels, J, zt.

1. Introduction The low-lying states o f 19F apparently have a very simple interpretation as rotational bands in a deformed nucleus. Paul 1) has shown h o w the low lying positive parity states o f 19F can be understood as a rotational band, with rapid electric quadrupole transitions between the states 2, 3). Similarly the low-lying negative parity states, including those whose spins and parities have been recently identified 4), apparently f o r m a further rotational b a n d 5). It has also been f o u n d 3) that the electric octupole transition between the J = ~2- state o f the negative parity rotational b a n d and the J = ½+ g r o u n d state of the positive parity b a n d is rapid. In this paper we will report some measurements o f g a m m a rays in coincidence with inelastic scattering o f 140 M e V protons f r o m 19F. The small scattering angle o f the protons, 25 °, enabled us to observe strong electric quadrupole and electric octupole inelastic scattering, and possibly also strong electric 24-pole scattering. The results enable us to extend the rotational m o d e l interpretation o f the 19F energy level structure in some details. O u r analysis also shows the need for some m i n o r changes in the original model o f Paul 1).

2. Experimental Techniques and Results The experimental techniques have been described in detail in earlier papers 6) so only a brief description will be given here, emphasizing details which are particularly relevant to these measurements. 150 MeV protons f r o m the Harwell synchrocyclotron were focussed at approximately 17 m f r o m the cyclotron on to a target o f lithium fluoride sealed in a thin-walled aluminium can. The target was o f thickness 4.47 gm • c m -2, corresponding to the protons having a mean energy o f 140 MeV t Now at Princeton University, New Jersey, U.S.A. 353 July 1 9 6 4

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at the centre of the target. Inelastic scattering was identified by detecting a proton of such an energy that it could be the result o f elastic or inelastic scattering only. In coincidence with these protons we observed g a m m a rays and measured their energies so' that we were able to obtain information about the states excited in laF. The protons were detected, at a scattering angle of 0p = 25 °, by two plastic scintillation counters separated by 4.6 cm of aluminium, a thickness chosen to pass protons leaving 19F with an excitation of 11 MeV or less. The scattering geometry was determined by the front proton counter which was 3.8 cm square and placed 25.4 cm from the target. The gamma-rays were detected in the scattering plane at 120 ° to the incident beam, and on the side of the beam opposite to the proton counter, by a wellshielded sodium iodide crystal, 12.5 cm in diameter and 15 cm long, with its front face at 15 cm from the centre of the target. A triple coincidence, (resolving time 2~ ~ 14 nsec), between counts from the two proton counters and the gamma-ray counter, opened a gate which passed the corresponding gamma-ray counter pulse to a pulse-height analyser. The energy calibration of the gamma-ray counter was established with gamma-rays of known energy from radioactive sources. G a m m a ray counter e~ciencies, needed for calculating cross-sections, were taken from the calculations of Miller and Snow 7). The experimental measurements were made in two series of runs, the first using the cyclotron with a 1 ~o duty ratio, and the second using a 20 ~ duty ratio obtained by means of a rotating system of tungsten targets inside the cyclotron s). The results of these two different sets of measurements were consistent with each other, despite quite different accidental coincidence rates in the two cases. The proton beam intensity was monitored by a thin-walled ionization chamber placed 60 cm in front of the lithium fluoride target. This monitor was calibrated by measuring the known cross-section for elastic scattering of protons through 9 ° from 12C a, 1o). Typical coincidence pulse-height spectra obtained are shown in figs. 1 and 2. The 0.48 MeV peak is due to a gamma-ray from the first excited state of 7Li: lithium fluoride was chosen as a target as this is the only gamma-ray that could be produced by inelastic scattering from lithium, while there are no gamma-rays from 19F which are close to this energy, so there is no ambiguity. As the 0.48 MeV state of 7Li has J = ½ the gamma-ray must be produced isotropically, so that we are able to deduce the proton differential cross-section from the g a m m a ray yield at one angle, finding a value of 1.3+0.1 rob" sr -1. This is in excellent agreement with our previous measurement 11) at a scattering angle of 25 °, where we found a cross-section of 1.19 + 0.08 m b • sr-1. In that previous measurement we used a smaller sodium iodide crystal (5 cm diameter and 5 cm long). Thus the excellent agreement between the two measurements gives us confidence in our knowledge of gamma-ray counter efficiencies. Four other gamma-rays were observed in the pulse-height spectra. The peak at 1.3 MeV in fig. 1 is too wide to be due to a single gamma-ray. (The width of a peak due to a single gamma-ray at this energy should be 10 channels in fig. I. We are

1IF(p, p') REACTION

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able to check that there has been no instrumental smearing by measuring the width of the 0.48 MeV peak in the same spectrum). We therefore deduce that this peak is due to two gamma-rays of energies 1.25 and 1.35 MeV, and of approximately equal

8O

6o

!

{ti

)

:

o 2p

t

t

I

~

t

I

,

t#ff

3p Q5

0.75

10 125 Gamma ray energy(MeV)

1.5

. 'I/ 1.'75

Fig. l. The region 0.3 to 1.7 M e V o f the gamma-ray spectrum, taken in coincidence with protons scattered at 25 ° from a lithium fluoride target.

120

100

80

60

8

i

t

40

20 Kicksorter channel 3.0 4,0 5.o 2b

6,o 7,0 , 3'.0 4.'0 Gamma ray energy (MeV)

510

610

Fig. 2. The region 1.7 to 6.0 M e V o f the gamma-ray spectrum, taken in coincidence with protons scattered at 25 ° from a lithium fluoride target.

intensity, as is shown in fig. 1. Also produced are gamma-rays of energies 2.64 + 0.07 MeV and 4.45 + 0.05 MeV. To calculate the corresponding proton differential crosssection we assumed that these gamma-rays were produced isotropically. This is a

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good approximation and any possible small departures from isotropy would not change the differential cross-sections enough to affect the interpretation presented in sect. 3. The resulting proton differential cross-sections are: Gamma-ray energy (MeV)

Differential cross-section (rob • sr -1)

1.25-+-0.05 1.354-0.05 2.64 4- 0.07 4.45-4-0.05

0.724-0.10 / 1.45-t-0.06 0.72-4-0.10 J 0.49 -4-0.06 0.354-0.05

Any other gamma-rays produced must have distinctly smaller intensities. Interpretation of these results would have been simpler if we could also have measured the cross-section for the excitation of the J = ~+ state at 0.20 MeV. Unfortunately its lifetime 2) (100 nsec) was longer than our coincidence resolving time (14 nsec) and so precluded such a measurement. A previous experiment 12) studied the total gamma-radiation produced in the bombardment of t9F by protons of a mean energy of 143 MeV. Production of a 4.43 MeV gamma-ray was observed with a cross-section of 4.9 + 1.0 mb so we might think of identifying this with the 4.45 + 0.05 MeV gamma-ray seen in the present work. However, the differential cross-section seen in the present measurements is so small that this 4.45 + 0.05 MeV gamma-ray can only be a minor contributor to the 4.43 MeV gamma-ray seen in the previous work. Therefore, the bulk of the production of that 4.43 MeV gamma-ray could not be due to the 19F(p, p ' ) reaction and probably came from production of the first excited state of x2C. The possibility of the present 4.45 +0.05 MeV gamma-ray coming from the same origin is excluded by the thickness of 4.6 cm of aluminium between the two proton counters, since such a reaction would be equivalent energetically to an excitation of at least 19 MeV in 19F. We conclude therefore that the 4.45___ 0.05 MeV gamma-ray seen in the present experiments comes from the 19F(p, p') reaction.

3. Discussion We must now identify these gamma-rays with known, or possible, gamma-rays in 19F. First consider the 4.45-1-0.05 MeV gamma-ray. This could be a transition

from either of the states at 4.57, 4.76 MeV to one of the lowest three states (0, 0.11, 0.20 MeV) or from a state near 6 MeV to one of the states near 1.5 MeV. However, states of 19F at energies above 3.99 MeV can break up into 15N+~. Thus this state we are exciting must have a very slow rate for break-up into 15N+~ if the observed gamma-ray decay is to be faster. We will show that for the states at 4.57, 4.76 MeV the Coulomb and angular m o m e n t u m barriers for a suitable spin assignment are high enough to inhibit heavy particle break-up, so that gamma-ray decay can compete

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successfully. For states around 6 MeV the barrier penetrability will be so much greater, that it is inconceivable that the reduced width for heavy particle break-up could be so small that gamma-ray decay would be observed. Thus the 4.45 +0.05 MeV gamma-ray must be due to direct production of either of the states at 4.57, 4.76 MeV. The decay of such a state cannot, from our pulse-height spectra, be producing any states above 1 MeV with any appreciable intensity, so that none of the other observed gamma-rays can be due to such a cascade. The 1.24 MeV gamma-ray must be due to the known decay of the 1.35 MeV state to the 0.11 MeV state. If no further information were available, the identification of the other two gamma-rays would be beset with considerable ambiguity. The 1.35 MeV gamma-ray could be due to the known cascade decays 1.46 ~ 0.11 MeV (1.35 MeV) or 1.56 -* 0.20 MeV (1.36 MeV), while the 2.64 MeV gamma-ray could be due to the known cascade decay 2.78 ~ 0.20 MeV (2.58 MeV) or to the possible cascade decays: 3.91 ~ 1.35 MeV (2.56 MeV), 4.00--* 1.46 MeV (2.54 MeV), 4.00 --¢ 1.35 MeV (2.65 MeV), 4.04 ~ 1.46 MeV (2.58 MeV) or 4.04 ~ 1.35 MeV (2.69 MeV). The gamma-ray decay modes of these states around 4 MeV are not known. We thus see that there must be important direct production of states around 1.5 MeV in the inelastic scattering. It is also possible that there is some production of these states by a cascade decay from a state around 4 MeV produced directly in the inelastic scattering; alternatively the 2.78 MeV state is produced directly in the inelastic scattering. We now estimate expected cross-sections for excitation of the states around 1.5 MeV from their known radiative decay rates 3). We assume that the direct interaction inelastic scattering matrix element is proportional to the radiative matrix element 13), and that the proportionality constant, for a given multipole, is much the same as in neighbouring nuclei. The 1.46 MeV state can be produced directly only by electric dipole excitation, for which the cross-section should be small at our scattering angle. It has been shown that the electric dipole decay of this state is slow 3), so that even at more forward scattering angles its excitation cross-section should be very small. We therefore ignore the possibility of direct excitation of this state. The decays to the ground state of the other two states are fast: the 1.35 MeV state has an electric octupole decay with IMI2 = 12 + 4 while the 1.56 MeV state has an electric quadrupole decay with [MI 2 = 9 + 3 . We now compare these decay rates with those of the 6.14 and 6.92 MeV states 1,) of 160 (we choose these as the octupole and quadrupole transitions in the nearest nucleus to 19F for which both inelastic scattering crosssections and radiative decay rates are known) and find expected differential crosssections for inelastic scattering at a scattering angle of 25°: 1.35 MeV state: 0.80+0.48 mb • sr -1, 1.56 MeV state: 2.3 +0.9 mb • sr -1 Thus the expected cross-section for direct production of the 1.35 MeV state by electric octupole excitation is close to the observed cross-section for producing the

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state, although the latter is due both to direct production of the state in inelastic scattering and to indirect production by cascade decays from higher states. The expected production of the 1.56 MeV state by electric quadrupole excitation is three times the observed production of the 1.35 MeV gamma-ray, so it seems probable that the observed production of the 1.35 MeV gamma-ray is due to direct production of the 1.56 MeV state. It is not so evident that the observed production of the 1.24 MeV gamma-ray is due to direct production of the 1.35 MeV state, as it could be argued that the direct production of the 1.35 MeV state might be only one third of the observed cross-section computed above, with the remaining two-thirds due to production by a cascade decay from one of the 4 MeV states. We shall show that production of the 1.35 MeV state is almost certainly direct, and not by cascade, in spite of the fact that such production of a state around 4 MeV would be expected in the model of Paul 1). He showed that a fit to the low-lying positive parity levels with a simple K = ½ band, based on an intrinsic state formed by full ls and lp shells, and three nucleons on level 6 of Nilsson's energy levels in a spheroidal potential 15), would require a larger moment o f inertia than would fit the lowest rotational bands in the neighbouring even nuclei 1sO and 2°Ne. He therefore introduced another rotational band with K = ½, based on an intrinsic state formed by promoting one of the three nucleons to Nilsson's level 7, with an implicit assumption that the two nucleons left on level 6 coupled to K = 0. The mixing of states of the same J due to the rotation-particle coupling le) was then taken into account to produce an energy level scheme close to that found experimentally. In particular, the mixing of the two J = ~ states was very strong, producing one J = ~ state at 1.5 MeV which was 54 % (in intensity) K = ½, and another at about 4 MeV which was 46 ~o K = ½. If this were so, one would then expect that both these states would be excited in the 19F(p, p') reaction with approximately equal crosssections. One might therefore think that we are here observing the excitation of such a J = ½+ state around 4 MeV, which then decays by electric dipole emission to the J - ~ - state at 1.35 MeV. However, we will now present arguments which make this interpretation very improbable. Firstly, purely electric dipole decay of this state is improbable. The J = ½+ state at 1.56 MeV decays 94 % of the time 17) by magnetic dipole and electric quadrupole emission, leaving only 6 % for an electric dipole transition of slightly higher energy. For such a J = ½+ state at 4 MeV, magnetic dipole and electric quadrupole transitions to the 0.20 MeV state would have higher energies than the electric dipole transition to the 1.35 MeV state, and so should compete even more successfully. In addition, the electric dipole transition to the 0.11 MeV state would have a higher energy than the transition to the 1.35 MeV state and so would probably also be faster. No such gamma-rays of 3.8 or 3.9 MeV are present in our pulse-height spectra with any suitable intensity. The measured radiative transition rates between states of the positive parity band argue against such strong mixing. The observed rates (upwards) from the J = ½+

19r(p, p') RUCTION

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g r o u n d state, expressed as ratios to the Weisskopf single-particle value (so as to correct for the different energies involved), are a): to the J = i + state at 0.20 MeV: 21.6, to the J = ½+ state at 1.56 MeV: 18+6. The calculated rate for a transition to a state of spin J of a K -- 5 rotational band, from the J -- 5 state is: B(5 ~ J ) = [ ( 5 2 5 0 1 J S ) Q o + ( 5 2

- ½ llJ5)Q1] 2,

where Qo, QI are matrix elements depending on the wave-functions of the intrinsic state. By an argument similar to that presented for the similar case in 7Li°~), Q~ should be much smaller than Qo, so we will neglect it. Then

B(5 ~ ½)/B(5 ~ 3) = [(5 2 5 01½ 5)/(5 2 5 013 5)] 2 = ~. Thus the expected total transition rate to the two J = 3 + states combined is twothirds the observed rate to the J = 3 + state: 14.4. (We assume that an electric quadrupole transition from a state of the K = 5 band to a state of the K = ] band would not be strong). As the observed rate to the lower J = 3 + state is 184-6, this implies that the proportion of K = 5 in the upper J = ½ state must be distinctly smaller than one-half. This analysis of the experimental data suggests a weaker band mixing than was used in the calculation of Paul. This may be achieved even if one uses the large rotation-particle coupling matrix elements which one calculates by assuming the intrinsic wave-functions to be just single nucleons on Nilsson's levels 6 and 7 respectively, and ignoring the presence of the other two nucleons which are on level 6 in each case. (Taking these two nucleons into account reduces the matrix elements and so reduces the degree of mixing substantially). The strong mixing occurred because, before mixing, the two J = ½ states were very close in energy. We find that the initial positions of these states before mixing can be considerably further apart and a good fit to the experimental energy level scheme obtained with much weaker mixing of the J = ½ states. Such a fit assumes a larger m o m e n t of inertia (h2[2~¢ 0.25 MeV) than was used by Paul. However, more states of the rotational bands of 2°Ne are now known an) and imply h2/2..,¢ = 0.27 MeV for the ground state rotational band, and smaller values for higher bands (only the first two states of the ground state rotational band of 2°Ne were identified as such at the time of Paul's calculation). In addition, the smaller rotation-particle coupling matrix element mentioned earlier would reduce the mixing further. It should, however, be noted that XgF must really be somewhat more complicated than this. The two nucleons on level 6 couple to both K = 0 and K = 1, with the K = 0 intrinsic state forming two rotational bands; T = 0 with I = 1, 3, 5 and T = 1 with I = 0, 2, 4. All these three rotational bands are found to lie close together in XaF. These bands will couple with a K = ½ nucleon on Nilsson's level 7 to form a larger number of states than we have considered in the previous paragraphs. Thus

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a unified model of the states of 19F above 3 MeV is probably complicated (though not as complicated as a full intermediate coupling calculation) and much more information will be necessary to work out the details of the observed energy level scheme. We note that the intermediate coupling calculation of the positive parity states of 19F by Elliott and Flowers 19) shows that the first states to appear above the ground state rotational band have J = 5 and ~ in agreement with these qualitative suggestions. These states are high enough in energy for the admixtures of them into the lower states of the ground state rotational band to be small, in agreement with our deductions from the experimental data. We note that the changes we suggest in Paul's model do not affect the fit he found with several properties of the J = 5 + ground state, and the J -- ~+ state at 0.20 MeV, as these states were largely K = ½ in his model and remain so. We thus see that there are strong grounds for believing that the production we observe of the 1.35 MeV state is not due to production by a cascade decay from a high energy state, but is due to direct production by strong electric octupole inelastic scattering. We now show that the excitation of the state near 4.6 MeV can be well understood as electric octupole excitation of the J = 7 - state of the odd parity rotational band. This interpretation only works if the observed production of the 1.35 MeV state is largely direct production, so supporting our previous arguments. I f the odd parity states are members of a rotational band, we would expect electric octupole excitation of the J = ~r- and 7 - members from the J = 5 + ground state to be of similar strengths. The cross-section for exciting a state of spin J of the negative parity band would be: da (5 + --' J ) = [(½ 3 5 01J 5)00 + (½ 3 - 5 11J½)01 ] 2, dO where 0 o, 0 x are matrix elements relating the intrinsic states of the two bands. We have no knowledge of the relative magnitudes of 0o, 01 but it seems improbable that they would be such that the excitation of the state with J = 7 would be much less than that for the state with J = ~:. I f we fit:

hz E ( J ) =A + 2 J [ J ( J + 1 ) + a ( - 1)J+½(J+5)], to the J = 5, $, ~ - states at 0.11, 1.46 and 1.35 MeV we find h 2 [ 2 j = 0.213 MeV and a = 1.11, so that the J = 7 - state would be expected to be at 4.49 MeV, which is very close to the energy of the state we believe we excite. (Note that this value of h2[2J is even smaller than the values we considered for the positive parity rotational band; however, the negative parity bands of a°Ne have h2[2~¢ ~ 0.15 MeV, which is even smaller). Such a J = 7 - state would decay to the J = ~+ state at 0.20 MeV by electric dipole emission, which even with IMI 2 = 10 -a, (as is found for other electric dipole transitions between the two bands), should compete favourably,

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because of the higher energy release, with magnetic dipole and electric quadrupole decay to the J = } - state at 1.35 MeV. For such a J = 3 - state, break-up into 15N + ct would be by an I = 4 wave. Then the particle break-up would be sufficiently inhibited for gamma-ray decay to compete successfully. As this state probably has a comparatively simple structure we might expect its reduced width to be quite large. To give us some idea of magnitudes we have therefore taken it to be the singleparticle value, h2/2Ma. Using the graphs of Coulomb wave-functions of Sharp et al. 20) we estimate the following lifetimes, for various values of/, for the 4.76 MeV state to break up into 15N+ct, (corresponding lifetimes for the 4.57 MeV state would be even slower): l sec

0 5 . 1 0 -15

1 2 - 10 -14

2 2- 10 -13

3 5" 10 -12

4 5" 10 -1°.

If we take IMI 2 = 10 -3 for the electric dipole decay we find a radiative lifetime of 2- 10 -14 see, so that the gamma-ray decay should compete successfully with the heavy particle decay, particularly if the state has a high spin. Taking this information with the facts that the expected energy is so close to the observed gamma-ray energy, of 4.45 +_0.05 MeV, and that the excitation cross-section is so close to that for exciting the 1.35 MeV state, we suggest that we are probably observing the electric octupole excitation of the J = 3 - member of the odd parity rotational band. It must be remarked that a further possible explanation of this state is that it is the higher J = ½+ state we have discussed previously. However, it seems impossible that this state could both be at an energy as high as this, and be so strongly mixed with the J = ~+ state at 1.56 MeV as to make its production cross-section as large as is observed. We now wish to comment on the result that for the electric quadrupole excitation the observed cross-section is only one-third of that expected by comparison with 160, while for electric octupole excitation the observed and expected cross-sections are equal. Agreement between such expected cross-sections and those observed should not be perfect as the radiative transition rate is determined by the matrix element of rlY~(O, q~) while, in a plane-wave theory for example, the inelastic scattering crosssection is determined by the matrix elements ofjt(qr)Y~'(O, q~). It has been shown 21) how, for harmonic oscillator wave-functions, the ratio of the square of an inelastic scattering matrix element to the square of the corresponding radiative matrix element depends on the wave-functions involved, varying over a range of a factor of three. As the electric quadrupole excitation in 160 is probably due largely to the excitation of a lp nucleon into the 2p and If shells, while that in 19F is probably due largely to the rearrangement of nucleons in the ld and 2s shells, it is not surprising that the ratio is somewhat different in the two cases. The first significant remark about the electric octupole excitation in 19F is that it is very strong. Harvey 5) has discussed a model of the negative parity rotational

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band based on an intrinsic state where there is one hole on level 4 of Nilsson's scheme and level 6 is filled. It was found that electric octupole transitions from the J = Jr+ ground state to the states of this band had only one tenth the strength of those observed. This model is therefore inadequate, a conclusion supported by the fact that in this model we would expect 01 >> 0o, so that d__~a(½+ d12

1[~ ½)] 2

/ ct~

4

= ~-'

which is in poor agreement with the observed ratio of 0.5. This model is an example of one electric octupole excitation formed by exciting a nucleon from the lp to the 2s and Id shells. To obtain the observed strength one presumably has to consider a coherent superposition of all such excitations, similar to that which has been used to describe 22) the strong octupole excitation of 160. One would therefore expect that a large part of these octupole excitations o f 19F would be formed by coupling three nucleons in the 2s and ld shells to the strong octupole excitation of 160, the J = 3 - state at 6.14 MeV. Such a similarity between these electric octupole excitations of 160 and 19F could explain why the ratio between radiative matrix element and inelastic scattering matrix element is the same for these two transitions. It would be desirable to improve the experimental accuracy with which these ratios are determined, to test this conclusion more definitely: at present the accuracy is such that the agreement can only be described as suggestive. It remains now to discuss the 2.6 MeV gamma-ray, which by a process of elimination we have concluded must be due to the decay of the 2.78 MeV state produced directly in the inelastic scattering. This state 23) has J = ~-, in agreement with the intermediate coupling shell model 19) and the unified model which both require a J = 3 + state in this neighbourhood, this being in the latter case a member of the ground state rotational band. Direct excitation of this state would be a strong E4 excitation, but there is no information to show whether it is such a one-stage process, or a two-stage process involving two fast electric quadrupole transitions, analogous to double Coulomb excitation. We note, however, that a strong one-stage E4 excitation of such a J = ~-+ state would not be surprising. Elliott 24) has remarked that the wave-functions calculated using the intermediate coupling shell-model for the states of the lowest rotational band are close to simple LS-coupling wave functions. The J = Jr+ states are approximately 80Yo (in intensity) 2S, the J = ½, i + states are approximately 80~o ZD, and the J = 3 + state is approximately 8 0 ~ 2G. Thus we should be surprised neither by strong electric quadrupole excitation from the S state to the D states nor by a strong E4 transition from the S state to the G state. This would suggest that such strong E4 transitions should be comparatively common in such rotational bands in light nuclei. At present there is no evidence for or against this suggestion: it would be interesting to search for further such strong E4 excitations.

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We have thus identified the observed inelastic scatterings as a strong electric quadrupole transition and a strong E4 transition exciting states of the ground state rotational band, and two strong electric octupole transitions exciting states of the negative parity rotational band. In this way we have been able to extend our knowledge of these rotational bands, in particular to suggest that the band mixing of the positive parity bands is weaker than given by Paul. We wish to thank D. J. Rowe and J. McL. Emmerson for help in the initial stages of the experiments, and B. Rose and all members of the Cyclotron Group at A.E.R.E. for their continuous cooperation. We are also indebted to D. A. Bromley for a helpful discussion. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24)

E. B. Paul, Phil. Mag. 2 (1957) 311 C. M. P. Johnson, Phil. Mag. 1 (1956) 573 A. E. Litherland, M. A. Clark and C. Broude Phys. Lett. 3 (1963) 204 J. D. Prentice, N. W. Gebbie and H. S. Caplan, Phys. Lett. 3 (1963) 201 M. Harvey, Phys. Lett. 3 (1963) 209 D. J. Rowe, G. L. Salmon, A. B. Clegg and D. Newton, Nuclear Physics 54 (1964) 193 W. F. Miller and W. J. Snow, Rev. Sci. Instr. 31 (1960) 39 G. L. Salmon, Nucl. Instr. 23 (1963) 338 J. C. Dickson and D. C. Salter, Nuovo Cim. 6 (1957) 235 M. A. Grace and B. Rose, unpublished D. Newton, A. B. Clegg, G. L. Salmon, P. S. Fisher and K. J. Foley, Proc. Phys. Soc. 79 (1962) 27 K. J. Foley, G. L. Salmon and A. B. Clegg, Nuclear Physics 31 (1962) 43 W. T. Pinkston and G. R. Satchler, Nuclear Physics 27 (1961) 270 D.J. Rowe, A. B. Clegg, G. L. Salmon and P. S. Fisher, Proc. Phys. Soc. 80, (1962) 1205 S. G. Nilsson, Mat. Fys. Medd. Dan. Vid. Selsk. 29, No. 16 (1955) A. K. Kerman, Mat. Fys. Medd. Dan. Vid. Selsk. 30, No. 15 (1956) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1 A. E. Litherland et aL, Proc. Rutherford Conference, (Heywood and Co., London, 1961) p. 811 J. P. Eiliott and B. H. Flowers, Proc. Roy. Soc. A229 (1955) 536 W. T. Sharp, H. E. Gove and E. B. Paul, Chalk River, A.E.C.L. 268 (1958) G. Schrank, E. K. Warburton and W. W. Daehnick, Phys. Rev. 127 (1962) 2170 G. E. Brown, J. A. Evans and D. J. Thouless, Nuclear Physics 24 (1961) 1 K. Huang et aL, J. Phys. Soc. Japan 18 (1963) 644 J. P. Elliott~ Proc. Roy. Soc. A245 (1958) 128, 562