States of F16 excited in the reaction O16(p, n) F16

1.E.1 :

Nuclear Physics 68 (1965) 36--48; ~ ) North-Holland Publishing Co., Amsterdam

2.A.1

Not to

be reproduced by

photoprint or microfilm without written permission from-the

publisher

STATES OF F 16 EXCITED IN THE REACTION Ot6(p, n)F t6 J. A. R. G R I F F I T H

Physics Department, University of Birmingham, England and C. J. BA'IWY, R. S. GILMORE and G. H. S T A F F O R D

Rutherford High Energy Laboratory, N.I.R.N.S., Chilton, Berks., England Received 9 November 1964 Abstract: The neutron spectra produced at 0 ° from the reaction Ole(p, n ) ~ 6 have been measured, using time-of-flight techniques, for 30 and 50 MeV incident protons. The neutron groups found are attributed to reactions proceeding to the ground state and excited states of b"a6 at 1.30-4-0.05, 4.204-0.05, 6.164-0.05, 7.3=[=0.3, 9.26=[=0.05, ll.l-t-0.1, 11.7±0.1 and 19.54-0.3 MeV. A comparison is made with the excited states of N TM and the known T ----- 1 levels of O x6. El

N U C L E A R REACI'ION OI'(P' n)lm" E = 30"50 MeV; measured a(En), b"ae deduced levels.

I

1. Introduction

The structure of the "giant resonance" in 016 has been the subject of much theoretical and experimental work because of the doubly closed shell structure of this nucleus. The shell model calculations of Elliott and Flowers 1) and the treatment within the particle-hole formalism of Brown 2) reproduce the main features of the giant dipole resonance quite well. Several different reactions have been used in experimental studies of the giant dipole resonance. The reaction NlS(p, ~,)O~6 has been used by Cohen a) and Tanner 4). Neutron time-of-flight techniques have been applied to the reaction O16(y, n)O is by Firk s) and resonances in the total crosssection for this reaction have been examined by Caldwell 6). Inelastic electron scattering has been used by Isabelle and others 7) and the reaction O16(e, e'p)O ts by Dodge a). The total gamma-ray absorption cross-section has been determined as a function of energy by Burgov 9) and the photo-neutron resonance work has been extended above 30 MeV by Anderson lO). All these experiments agree reasonably well as to the positions of the individual states contributing to the resonance between about 15 MeV and 30 MeV, although none of them covers the entire energy range and there are some detailed differences where the ranges overlap. Comparatively little information has been obtained, however, on the properties of the individual states. Generally it is to be expected that not only the well-known electric dipole excitations of spin and parity 1-, but also other multipoles, both magnetic and electric, should be present. In the case of O ~6, which has T3 = 0 for its ground 36

STATES OF FTM

37

state, T = 0 and T = 1 excited states are possible. The T = 1 states should be reproduced in F x6 (which has T 3 = 1), but the T = 0 states will not. A study of the reaction O16(p,n)F 16 will therefore assist in making isobaric spin assignments for the levels already found in 016 . Little information is available about the nucleus F x6, as the only reactions leading to its formation are Ot6(p, n)F 16, o t 6 ( H e 3, t)F 16 and NX4(He a, n)F 16 of which only the last has been studied 11). Two excited states at 0.88 and 1.26 MeV were observed and these have not yet been confirmed by any other measurements. 2. Experimental Method The 50 and 30 MeV beams from the Rutherford Laboratory proton linear accelerator were used to bombard a thin water target and the energy spectra o f neutrons emitted at zero degrees were measured using nsec time-of-flight techniques. 2.1. T I M E - O F - F L I G H T S Y S T E M

The proton linear accelerator is a pulsed machine operating at 50 pulses per second, each pulse being approximately 200 #see long. As the accelerating R.F. power is at a frequency of 202.5 M H z each beam pulse has a fine structure of proton bunches with a period of 4.94 nsec. The intrinsic time spread of the bunch is 0.3 nsec. By radio-frequency deflection (at a sub-harmonic of 202.5 MHz) of the proton beam before injection into the first accelerating tank, bunches of protons spaced 177.8 nsec apart are obtained at the end of the accelerator 12). The radio-frequency deflection equipment also provides a series of timing pulses which are phased with the time of arrival of the protons at the target. By measuring the time difference between the arrival of a neutron at a detector placed at the end of a known flight path from the target, and the next timing pulse, the time-of-flight of the neutron and hence its energy can be determined. As the neutron counter also detects ~-rays originating in the target which have a known flight time a convenient reference peak in the observed time spectrum is provided. Since the proton bunches occur at 177.8 nsec intervals slow neutrons from a previous bunch could be recorded in the position of a faster neutron produced by the following bunch. This ambiguity is eliminated by biassing the neutron counter at a sufficiently high level so that the lower energy neutrons will not be recorded. With a burst spacing of 177.8 nsec and a flight path of 9.58 m, which was normally used to give a good energy resolution for the higher energies, the required bias enabled neutron energies down to 14 MeV to be measured. By decreasing the flight path to 5.51 m, 7 MeV neutrons could be measured, but then the resolution in the high energy portion of the neutron spectrum was worse. 2.2. E X P E R I M E N T A L A R R A N G E M E N T

The layout of the equipment is shown in fig. 1. Protons from the accelerator first passed through a 1.4 mg • cm -2 aluminium foil viewed by two scintillation counters

]% A. R. GRIFFITR e t a [ .

38

set at 30 ° to the right and left of the beam direction. These counters were used both to monitor the time structure of the proton beam and the beam intensity. The beam then entered the target chamber through a 2.7 mg- cm -2 Melinex foil. The target consisted of a continuously flowing, freely falling sheet o f water 10 cm 2 and of thickness 350 #m (35 m g . cm-2). The sheet was produced by water at a pressure head of 2 m passing through a slot 10 cm by 320 #m formed between two polished stainless steel jaws. A rectangular sheet was ensured by guiding the water along two glass rods, placed parallel to the direction o f flow at each end of the slot. The pressure in the target chamber surrounding the sheet was the vapour pressure of water. Full details o f this type of liquid target are to be published elsewhere 1a).

~L

PROTON

~/MON,TOR ~"F~F/~f/I/ ~

~

/

.

..~

CLEARING MAGNET

/

--'CONCRETE

PROTON BEAM STOPPER

SHIELDING

|

lm

i

Fig. 1. Experimental arrangement. Immediately following the target chamber a magnet was used to sweep the main proton beam into a "stopper" lined with polythene. The target area was surrounded by a shielding wall approximately 4 m high and 1 m thick containing 40 tons of concrete. Lead bricks were placed around the proton monitors to reduce background. Neutrons produced at zero degrees passed through a steel collimator 1 m long, to the neutron counter at the end of the flight path. The counter was a 10 cm long by 12.7 cm diameter cylinder of plastic scintillator viewed by a 56 AVP photomultiplier. During the experiments, flight paths of 9.58 m (long) and 5.51 m (short) were used. 2.3. ELECTRONICS A block diagram of the electronics is shown in fig. 2. Pulses from the neutron counter were fed into a "synchronizing" discriminator 14) which used the zero-

STATES OF F16

39

crossing principle to reduce the time jitter between input pulses of varying amplitude and the standardized output pulse. The time delay between the output pulse and the next timing pulse from the beam deflection equipment was measured by a timeto-amplitude converter and 512-channel pulse-height analyser. In order to have a constant known efficiency for the neutron detector it was essential to know accurately the level at which pulses from recoil protons would be detected. This bias level was fixed by discrimination o f a slow linear pulse from the photomultiplier dynode which gated the output of the time to amplitude converter. Neutron

•

. .

Deflector

signal

nm~a;fi..

Cathode

[-~

Stow

~" '~'"'~' F-] I--~ ~'c£~v~r~'e~r[

"

' ,

] Scaler ~ - d

]

'

I

'

]

]

'

'

'

pulse

,

t P u t . height I

I

I

Fig. 2. Block diagram of the electronics for the neutron detector. One of the two counters used to measure the beam intensity also monitored the time structure of the proton beam. The arrangement of electronics for this was identical with that described above except that no slow linear gating system was used (see fig. 2); 10 M H z discriminators and sealers were used to count the number of protons detected in the two monitor counters. 2.4. T I M E C A L I B R A T I O N

The time-of-flight equipment was calibrated by moving the neutron detector to a position normally occupied by one of the proton monitor counters, so that a time spectrum of the protons scattered from the A1 foil was obtained. The deflection signal normally applied to the proton beam was then removed so that all the fine structure bunches of the accelerator were allowed through. Because the timing pulses were still at intervals of 177.8 nsec the resulting spectrum contained many peaks, each separated from its neighbours by the fine structure spacing of the accelerator (i.e. 4.938 nsec). The centroid of each peak was determined to within +0.04 channels of the 512-channel analyser. Only a very slight deviation from Uneadty was measured. 2.5. D E T E R M I N A T I O N O F N E U T R O N C O U N T E R E F F I C I E N C Y

In order to have a known efficiency for the neutron detector it is necessary to know the bias level for the slow side in terms of the energy o f the recoil protons in the counter. The value of this bias is fixed by the requirement that slow neutrons which could produce overlap of the spectra (subsect. 2.1) would not be detected. This gives

40

J. A. R. GRIFFITH e t al.

a bias equivalent to 6 MeV for the short flight path and 12 MeV for the long flight path. The slow side was calibrated using 30 MeV protons from the accelerator and degraders to give calibration points at 15 and 8 MeV. In this way the bias was set at 10.2_ 0.5 MeV for the long path and 6.3 4- 0.5 MeV for the short path. The stability of the slow side electronics was checked frequently and the quoted error for the bias includes the effect of gain changes in the photomultiplier due to temperature variations. The efficiency of the neutron counter was calculated using the method described by Kurz 16). This computer programme calculates the efficiency from the interaction of the incident neutron with both protons and carbon in the scintillator and includes re-scattering contributions. The effects due to the non-linear response of the scintillator to charged particles and the finite resolution of the detector threshold are also included. The values calculated by this method agree to within 4-10 ~o with the measurements of Wiegand et al. 17) and Bowen et al. 15). Including the uncertainty in the bias level it is estimated that the calculated efficiency and hence the cross-section values have an absolute error of 4-25 ~o. However, the shape of the curve of efficiency as a function of energy is not strongly dependent on small changes in the bias and it is estimated, therefore, that the relative error between cross-section values at different energies measured using the same neutron detector bias is 4-10 ~o.

3. Procedure 3.1. GENERAL It was intended to look for analogues in F 16 of the excited states of 016 with excitation energies of as much as 30 MeV. Since the first T = 1 state of 016 lies at 12.78 MeV, this is equivalent to searching for levels in F 16 up to 17 MeV excitation. The ground state reaction Q value for F 16 is known to be about - 16.4 MeV so that a minimum proton energy of 34 MeV is required. With the long flight path and 50 MeV protons, neutron energies down to 14 MeV (Q = - 3 6 MeV) could be measured and so the desired range was easily covered. To look for structure at even higher excitation energies, the 50 MeV proton beam was used in conjunction with the short flight path, allowing Q values of the order of - 4 3 McV to be observed. The 30 MeV proton beam was also employed with the short neutron flight path and enabled excitations of F 16 below 6 MeV to be studied. All measurements were made at zero degrees. 3.2. SPECTRA Time spectra were measured, both with and without the water target which allowed corrections to be made for background. The time spectrum from one of the

STATES

OF

41

1~16

proton monitor counters was also recorded to ensure that the R.F. beam deflection equipment was operating correctly. Frequent checks were made of the stability of the neutron slow side amplifier and discriminator. The stability of the overall time calibration was also checked several times during the course of the experiment. The observed spectra were corrected for background and dead-time losses.

Counts r c~nnct

Neutrons Main gamma

-200

peak .o ; " j~.¢.

-I00

• 7/ • .% • °o'.: r

• ..~'.-:.•o .

. • :

":. -4.o

! ]

1011 [

Channel

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number I

ol I

Fig. 3. A typical time spectrum at 50 MeV for long flight path. A typical time spectrum is shown in fig. 3. The isolated peak to the right is due to gamma-rays originating from the target, and all neutron energies were calculated with reference to this peak. The accuracy of the time calibration was such that the uncertainty in the calculated energy of a neutron group was defined almost entirely by the errors in deciding the position of the group. 4. R e s u l t s

The neutron energy spectra are shown in figs. 4-6. All the spectra were characterized by a prominent peak at the high-energy end, with others appearing at lower energies superimposed on a continuum, which probably results chiefly from the reaction O16(p, pn)O 15. The high energy group corresponds to a reaction Q value o f - 16.4___0.2 MeV, the error on this being mainly due to uncertainty in the proton beam energy. The F 16 nucleus should have a closely spaced group of four levels (one being the ground state) which will be analogues of the first four levels of N 16 and the first four T = 1 levels of 0 16. The positions, spins and parities of these levels in N 16 and O 16 are well-established (see for example ref. is)) so it is possible to make an estimate (see subsect. 5.1) of the positions of their analogues in F 16. T h e results are shown in table 1, which gives the expected level order, the estimated excitation energy of e a c h one and the Q value for its excitation in the present ex-

J.A.R.

-42

GRIFFITH e t al.

°

!

0.~

~ Neutron

Energy,

MeV

21o

Fig.

4.

3o

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Neutron energy spectrum at 50 MeV, for long flight path. States were seen at excitation energies of 4.2, 6.16, 7.3, 9.26, 11.1 and 11.7 MeV.

t')

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Energy,MeV

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I

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, 30

i

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l

40

~

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F i g . 5. N e u t r o n energy spectrum at 5 0 M c V for short flight path.

pedment. It has recently been brought to our attention that these four states have been experimentally observed using time-of-flight techniques and the reaction Nt~(He a, n)F 16. These experimental values of Zafiratos et al. 19) are included in the table for comparison. Spin and parity assignments arc not yet available. It can

STATES OF Fle

43

be seen that their observed g r o u n d state Q value is somewhat lower t h a n the prediction, but that the spacing o f the levels is approximately right. The shift in energy is scarcely surprising considering that all states o f F 16 are u n b o u n d . We do n o t expect to resolve the F 16 quartet in the present work. The measured Q value o f the main neutron group is in g o o d agreement with the predicted value

ta

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4¢

i

¢

¢

.0.5 ¢ 0¢¢(~¢~¢#

¢*¢ ¢ ¢ ¢ #

,,,

Neutron Energy, 8 MeV

¢ ¢ ¢

¢ ¢

I ,~#

,o

Fig. 6. Neutron energy spectrum at 30 MeV for short flight path. TABLE 1

Predicted excitation energies Ex of the Fle quartet of levels, with the corresponding Q values for their excitation in the Old(p, n)F x6 reaction Spin and Parity j=

O123-

Predicted values Ex Q (MeV) (MeV) g.s. 0.35 0.52 0.81

--16.37 -- 16.72 -- 16.89 --17.18

Experimental values 19) Ex Q (MeV) (MeV) g.s. 0.20 0.4-36 0.736

--16.22 -- 16.42 -- 16.66 --16.96

The experimental energies, derived from the Nt4(He 3, n)F 18reaction xg) are included for comparison. Spin and parity were not determined. for a 0 - g r o u n d state o f F 16, but it corresponds with the experimentally observed first excited state o f ref. 19). The difference is within the probable error in the p r o t o n beam energy, however. The width o f the neutron g r o u p can be fully explained by k n o w n instrumental widths and it has no detectable asymmetry. It would appear that one o f the quartet o f levels in preferentially excited in this experiment. It has been suggested by G. E. B r o w n 22) that direct reaction theory would lead one to expect preferential excitation o f the lowest angular m o m e n t u m states at zero degrees in this reaction. It seems reasonable, therefore, to assume that the main neutron g r o u p

J. A. R. GRIFFITH e t al.

44

observed represents reactions proceeding to the ground state of F t6 and this assumption has been made in calculating excitation energies for the other states. It then follows that the 50 MeV, long flight path, spectrum shows states in F 16 at excitation energies o f 4.20+0.05; 6.16+0.05; 7.3_+0.3; 9.26_+0.05; 11.1_+0.1 and 11.7 ± 0.1 MeV. The 30 MeV spectrum (fig. 6) provides evidence that the first o f these is not a single state, but the components cannot be resolved. Zafiratos 19) reports two states at 3.78 MeV and 4.4 MeV and N t6 exhibits at least four in the same region 2o). There is slight evidence in our 30 MeV spectrum for a weak group at an excitation of 1.3 MeV in the tail of the main peak, which may be the level reported by Bonner 11) at 1.26 MeV, but unconfirmed in the work o f Zafiratos. The 50 MeV, short flight path, spectrum (fig. 5) shows little structure at excitation energies in excess o f 12 MeV except possibly for a weak group at 19.5__+0.3 MeV. 5. Comparison of the Observed Levels with Those of 016 and N lb 5.1. M E T H O D

To compare the energy levels o f 016 and F 16, the spectrum of F 16 may be shifted so that the ground state coincides with the 1st T --- 1 state o f 016 (the 0 - state at 12.78 MeV) as shown in fig. 7. To a first approximation the levels of F 16 may then be expected to coincide with higher T = 1 states of 016. However, this assumes that the causes of the displacement of F 16 levels from the equivalent 016 levels, namely the n - p mass difference and the Coulomb energy difference, are independent of the excitation. This is not true for the Coulomb effect, which will depend on the configuration. It is possible to display the results in the form of a nomogram where some allowance is made for the dependence of the Coulomb energy Ec on the configuration o f the particular state. It is assumed that Ec may be written in the form Ec = G(A, J'~)F(Z), where F(Z) is a function of the nuclear charge only and G(A, J") is a function o f the atomic weight A, spin J a n d parity 7r o f the state. I f the members o f the triad have charges Z, Z + 1 and Z + 2 then the ratio o f the changes in Coulomb energy in going from a particular state in one nucleus to the analogous state in the adjacent nucleus is

3Ec(Z+2, Z+ 1) = (F(Z+2)-F(Z+ 1))G(A, J'~) = f(Z), b E e ( Z + l, Z) (F(Z+ I ) - F ( Z ) ) G (A, J ' ) where f is a function o f Z only. If the nuclei with charges Z, Z + 1 and Z + 2 have ground state mass excesses of A t, A2 and A s and the states under consideration have excitations E l , E2 and Es, then E3-(E2-Eo) = 6 E c ( Z + 2 , Z + I ) , (E2-Eo)-E1

= cSEc(Z+ 1, Z),

STATES O F F i s

45

where E o, the energy of the first T = 1 state in the T3 = 0 nucleus, is given by

Eo- -

1 Aa q. f AI_A2_f--1 f+'---1 f+ 1 f-~

6~p,

and 6.p is the neutron-proton mass difference. 016

F is

25

11.7 *- 0.1 11.1 0.1 10

9.26 0.05

>

q

to

to

O _z

ii _z

.......... t

7.3

20 tu

z 0J ~.16

to

4.21

15 t 13

.30 T=I GROUND STATE II STATE !

I st

Fig. 7. Direct comparions of the levels of 0 ~s and F ls.

Using the equation f o r f given above E2

_

1 E3+~ f+ 1

EI+Eo.

In fig. 8 the energies of excitation (which in the case of the/'3 = 0 nucleus are

46

J.A.R.

GRIFFITH el'

aL

taken as (E2-Eo)) are represented by points on the ordinates corresponding to the three nuclei. These ordinates are separated by distances proportional to 1 and to f so that any straight line intersects them in points which satisfy the above equation. The results are not particularly sensitive to the precise form used for F(Z). The relation given by Peaslee 21) for F(Z) was used to calculate f and gives values of Eo for a range of nuclei in good agreement with those experimentally observed. For the 016 triadf = 1.163.

27-

14-

-14

-12

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to

8-

23-

-10

21-

.-8

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6-

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o- ~

N

F

-o

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F

Fig. 8. Nomogram comparing the levels of N le, Oxe and Fxe. (For explanation see subseet. 5.1). In fig. 8 the N 16 energy levels and those of 016 below 16 MeV are obtained from the compilation of Ajzenberg-Selove and Lauritsen is). Known T - - 0 states have been omitted from the 016 scheme. The levels of 016 above 16 MeV are those obtained in the experiments listed in sect. 1 and are subject to energy uncertainties of about _ 0 . 1 MeV. The levels of F 16 are those obtained from the 50 MeV spectrum using the long flight path. Both the F 16 and N 16 schemes incorporate the levels reported by Bonner 11), at 0.88 and 1.26 MeV in F 16 and at 1.27 and 1.71 MeV in N 16. Reasonable agreement is obtained between the three level schemes within the limits of the experimental accuracy. However, the comparison is not entirely conelusive in the absence of reliable spin and parity assignments for many of the states. A comparison of the data for N 16 and 016 with the main levels which were observed in the present experiment is discussed below.

STATES OF Fia

47

5.2. THE 4.2 MeV LEVEL This state corresponds well with one at 4.8 MeV in N 16 and the level seen at 17.3 MeV in O 16. The calculations o f Elliott and Flowers 1) and of Brown 2) predict a 1 - , T = 1 state at this energy in O ~6 and angular distributions obtained by Tanner 4) from N15(p, 7)016 indicate an assignment of 1 - to the 17.3 MeV level O u r result confirms its T = 1 nature. The experiments of T a n n e r show evidence for some structure in the peak. This is consistent with our observation that the neutron group contains unresolved structure. 5.3. THE 6.16 MeV LEVEL N o corresponding level has yet been observed in N 16 in the appropriate region. F r o m the width of the observed neutron group and its shape, this almost certainly represents a single level, which is probably the analogue o f either the 19 MeV or the 19.5 MeV state in 016. One of these states is therefore probably T = 1 and the other T = 0. Electron scattering experiments have observed one group in this region which presumably can be explained by the presence of two s t a t e s , o n e 2 + and t h e other 2 - . This is consistent with the fact that this neutron group is strongly excited in the present work, since one expects collective vibrations of this type to be prominent in inelastic particle scattering. It is possible that one of the two states in O ~6 is the T = 1, 2 - state predicted to occur in this region. 5.4. THE 7.3 MeV LEVEL This state is only weakly excited and its energy is not well defined. It appears t o correspond to that state observed in O 16 at about 20.7 MeV which is also weakly excited. The predictions for O 16 suggest that there should be a 1- state at about 20.4 MeV carrying a small percentage of the photonuclear sum, so that this may be it. 5.5. THE 9.26 MeV LEVEL The energy tallies well with the state in 016 at 22.2 MeV, which is excited strongly in all the p h o t o n u d e a r experiments, referred to in sect. 1. Electron scattering has shown this level to be excited via the E1 mode and this experiment confirms a T = 1 assignment. A state seen as a large sharp peak by Burgov 9)at 23.05 M e ¥ in 016 and by most of the other listed experiments at about the same energy does not appear in the present work and is therefore probably a T = 0 state. 5.6. THE 11.1 AND 11.7 MeV LEVELS A reasonable correspondence exists between the 11.1 MeV level, one in N 16 at 12.2 MeV and one in O 16 at 24.3 MeV. The electron scattering experiments assign E2 to the excitation mode of the latter state, so that if both this and the level a n a l o g y are correct, the state would be T = 1, 2 +. The 11.7 MeV level compares well with that in O 16 at 25 MeV and N ~6 at 12.8 MeV. The fifth of the dipole states predicted theoretically is expected at about this energy and the electron scattering work reveals

48

J.A.R.

ORIFFITH e t

aL

an E1 state in this region. However, the situation in 016 is not very well established since various experiments do not agree to better than 0.5 MeV. Firk and Lokan 5) have reported the 25 MeV state as a doublet, in which ease the two states in F 16 could correspond with the two halves of the doublet. The fact that three states are seen in N 16 is evidence that information is still lacking for the other two members of the triad. 5.7. THE 19.5 MeV LEVEL

The corresponding excitation energy in 016 would be about 32.3-t-0.5 MeV. Recent work on photoneutron resonances in 016 by Anderson lo) has revealed a number of resonances above 30 MeV, the first of which occurs at 33.0-t-0.5 MeV. Our result suggests a T = 1 assignment to this state. We are indebted to Dr. R. J. Kurz for the use of his computer programme and to J. B. Hunt who assisted in the efficiency calculations. C. A. Baker and P. Ford assisted in the construction of the apparatus and running of the experiment. We also wish to acknowledge the help given to us by the P.L.A. Engineering, Workshop and Operations Staff and to thank them for their efficient operations of the accelerator. References 1) J. P. Elliott and B. H. Flowers, Prec. Roy. Soc. A242 (1957) 57 2) G. E. Brown, L. Castillejo and L A. Evans, Nuclear Physics 22 (1961) 1; V. Gillet and N. Vinh Mau, Nuclear Physics 54 (1964) 321 3) S. G. Cohen, P. S. Fisher and E. K. Warburton, Phys. Rev. 121 (1961) 858 4) N. W. Tanner, G. C. Thomas and E. D. Earle, Prec. Rutherford Conf., Manchester (1961) p. 295; Nuclear Physics 52 (1964) 45 5) F. W. K. Firk and K. H. Lokan, Phys. Rev. Lott. 8 (1962) 321 6) J. T. Caldwell et aL, Phys. Lett. 6 (1963) 213; R. L. Bramblett, e t aL, Phys. Rev. 133 (1964) B869 7) D. B. Isabolle and G. R. Bishop, Nuclear Physics 45 (1963) 209; L Goldombcrg and W. C. Barber, Phys. Rev. 134 (1964) B963 8) W. R. Dodge and W. C. Barber, Phys. Rev. 127 (1962) 1746 9) N. A. Burger et aL, JETP (Soviet Physics) 16 (1963) 50 10) D. W. Anderson e t aL, Phys. Rev. l.,¢tt. 10 (1963) 250 11) T. W. Bonner e t aL, Prec. Kingston Conf. on Nuclear Structure (1960) p. 470 H. C. Bryant e t al., Nuclear Physics 53 (1964) 97 12) D. J. Warner, R. H. E. L. Report NIRL/R/15 (1961) 13) J. A. R. Griflith and E. J. Burg¢, to be published 14) P. R. Orman, Nucl. Instr. 21 (1963) 121 15) P. H. Bowen e t aL, Nucl. Instr. 17 (1962) 117 16) R. J. Kurz, U C R L Report 11339 (1964) 17) C. E. Wiegand e t aL, Rev. Sci. Instr. 33 (1962) 526 18) F. Ajzenberg-Solove and T. Lauritsen, Nuclear Physics 11 (1959) 1 19) C. D. Zafiratos, F. Ajzenberg-Solove and F. S. Dietrich, preprint 20) D. B. Fossan et al., Phys. Rev. 135 (1964) B1347 21) D. C. Peaslee, Phys. Roy. 95 (1954) 717 22) G. E. Brown, private communication