- Email: [email protected]
Studies of 95Ru and 94Ru with the 96Ru(p, d) and (p, t) reactions
Received 14 September
1930
Ab&ract: The lever structure of psR~ has been investigated with the *6R~(p, d) reaction at a proton energy of 31.5 MeV. Experimental anguular distributions are compared with ~sto~ed-wave Born approximation cakufations to extract spin, parity and spectroscopic-factor assignments for the levels observed up to about 3.5 MeV of excitation. The ground state of p5Ru is observed to be reached by I = 2 transfer which is consistent with the 4” assignment expected from shellmodel considerations. The measured Q-values for the (p, d) and (p, t) reactions rest& in a new value far the mass of g”R~ and a deterrn~~a~~on of the previously unmeasured mass of 94Ru.
E
I
NUCLEAR REACTIONS 96Ru(p, d), 96R~(p, t), E, = 31.5 MeV; measured Q, o(E, #). q5R~ deduced levels, &, J, X, spectroscopic strengths, ground state mass excess. BQRu deduced around state mass excess. Enriched taraet.
The shell-model systematics for nuclei near the major neutron shell closure at N = 50 seem to be well established, The nearby odd-neutron nuclei with N > 50 all exhibit a ground state with $’ spin parity which is attributed to the unpaired neutron occupying the 2d, shell-model orbital Previous studies I-‘) of neutron stripping reactions on even N = 50 targets have shown strong I = 2 transitions to the ground state of the N = 51 final nuclei, Such results indicate a predominantly singleparticle nature for these $* states. In addition, these same reactions populate the first excited states of the final nuclei with strong t = 0 transitions. These first excited fevefs are thus assigned bs +* levels and deseribed in terms of promoting the odd neutron from the 2d+ orbital, which it occupies in the ground state configuration, into the 3s, orbital. The moly~enum isotope seqnence is the highest proton number, 2 = 42, for which the N = 50 member is stable. Thus ““MO is the heaviest N = 51 nucleus that can be studied by the neutrc_m stripping reaction and it follows the above systematics very nicely, The next heavier odd-neutron nucleus is 95Ru. Although little data exists on the level structure of “5Ru, much of the rather fragmentary data that is available seems to disagree with these systematics, t Research sponsored Carbide Corporation.
by the US Atomic Energy Commission 225
under tontract
with the Union
226
J. B. BALL
On the basis of their /3y coincidence studies, Riehs and Warhenek “) proposed a p’ branch from the ground state of 95Ru feeding the e+ ground state of g5T~. Their measured /?’ energy led to an allowed log ft value and, hence, an assignment of SC, Bf, or 9’ to the ground state of 95R~. From a study of internal conversion electron spectra, Chikhladze “) assigned a 256 keV y-transition to ’ 5R~ with c(k and K/L ratio indicating a probable M2 transition. Since an M2 transition requires a change of parity, and since the lh, is the only odd-parity orbital in this region of the shell-model neutron potential, it seems likely that one of the levels connected by the 256 ke\, transition is an -I$- level. An M2 transition could then connect to a $* level. The above observations have led to a tentative level scheme for “Ru with a I’ ground state and an y- excited level at 256 keV [ref. ‘)I. More recently, Pinston et al. *), repeated j?y coincidence studies of the 9 5Ru decay and concluded that the highest energy /I’ branch feeds an excited level at 336 keV and not the ground state of 9 5T~. They suggest that the ground state of ’ 5Ru could indeed be 3* and the level at 336 keV in g5Tc could be 5’. The observation by Heuer “) that the 336 keV level in 95T~ decays to the 4’ g round state by an Ml transition is consistent with such a picture, but neither experiment established ConclLlsively the spin of the 336 keV level in 95Tc or of the ground state of g5R~. (These two experiments would, however, seem to establish both of these states as having even parity.) To try and resolve this possible anomaly in the systematics of the N = 51 nucIei, and to provide more detailed information on the structure of 95Ru, we have studied the g6R~(p, d)q5Ru reaction. We have also used our measured Q-value to obtain a new mass measurement for the “Ru isotope. The ground state transition from the g6Ru(p, t)94Ru reaction was also observed and the Q-value measured to obtain the previously unknown mass of the N = 50 isotope, g4Ru. 2. Experimental details The reactions were studied by bombarding a thin foil of isotopically enriched g6Ru with 31.5 MeV protons from the Oak Ridge Isochronous CycIotron. The deuterons and tritons were momentum analyzed with the broad-range spectrograph facility and spectra were recorded on 50pm thick Kodak NTB emulsions. Aluminum absorbers were used to eliminate low-energy triton contamination of the deuteron spectra. A spectrograph entrance angle of -t_1.5” and a scanning zone of 3 cm were used which yielded a solid angle of order 6 x 10e4 sr. Typical beam currents on target were 250-350 nA and overall resolution for the system was 20 keV FWHM. The q6R~ foil target was prepared by electrodeposition. Because of possible interest in the general problem of target preparation, the technique employed will be described briefly. Two circumstances prevented our preparing the target by conventional methods. The high cost of the separated isotope precluded obtaining enough
96R~(P, d), %ufp,
material nium
to prepare
the target by rolling.
makes it impossible
to prepare
t)
The extremely
227
high melting
point
the target by the usual evaporation
It should be possible to evaporate ruthenium was not available to us at the time.
with an electron
of ruthe-
techniques.
gun but such a device
The plating cell employed was a small glass cylinder. The cylinder was placed on a thin copper foil (Z 3 mg/cm2) which served as the cathode. A platinum spiral served as both anode and mechanical stirrer. The plating solution consisted of about 10 ml of saturated ammonium sulfate solution containing 10 mg of enriched g6R~. A current of 100 mA at 3 V produced a plating thickness of about 0.1 mg/cm’ in 10 minutes. Actual thickness of the plated foils was determined by weighing the copper foils before and after plating. After plating, the copper foil was removed with an etching solution of tri-chloroacetic acid and ammonium hydroxide. Unlike chromium, cobalt, and iron foils prepared in a similar manner, the ruthenium foils could not be made self-supporting. Although the plated ruthenium had a shiny metallic appearance, it proved to be very brittle and could not withstand the etching process. For the targets used in this work, a thin layer of polystyrene was applied to the plated side of the copper foil before etching. Films of order 0.25 mg/cm’ were found to provide adequate support for the ruthenium foil. The obvious advantages to this method of preparing targets are the rather simple equipment required and the high yield of targets for the amount of isotope used. In our case, from the single plating solution using 10 mg of g6R~ isotope, we plated four targets ranging in thickness from about 0.1 to 0.3 mg/cm2. A major disadvantage to this plating technique is the problem of copper contamination of the target. This is particularly apparent in the spectra from the g6R~ target used for this experiment where a significant background from copper is observed. Although this caused no problem in analyzing the strong peaks due to reactions on g6R~, it makes analysis of weak peaks very difficult. It seems likely that the target used for this work was not completely etched. Calibration exposures on natural ruthenium foils prepared in a similar manner did not seem to show this problem. The use of served The riched
the polystyrene foil for support is also somewhat less than satisfactory but adequately for the present work. target used for this experiment had a thickness of 0.18 mg/cm’ and was ento 94.9 % in g6R~. The uncertainty in target thickness is estimated at 10 %.
3. Results and analysis Energy spectra were obtained at 5” intervals from 10” to 45”. A typical deuteron spectrum is shown in fig. 1. Most of the unlabelled peaks are ascribable to the previously mentioned copper contaminant in the target. The peaks due to (p, d) reactions from the copper could be readily identified by their characteristic kinematic shift over the angular range studied. From comparison of the spectra at all angles, the distinction
228
J.
B. BALL
IOc)O
e75
750
E
625
2 $
500
z : 8
375
250
125
0 54
56
59
60
62
64
66
68
70
72
74
DISTANCE ALONG FOCAL PLANE
76
78
80
82
84
86
88
90
km)
Fig. 1. Spectrum for 96R~(p, d)95Ru at 10”. Peaks corresponding to levels in 95R~ are labelled with their appropriate excitation energies in MeV. Two light element impurity peaks are labelled. The remaining unlabelled peaks are associated with copper contamination of the target as discussed in the text.
Summary of the optical-model
TABLE1 parameters Proton
V
(MeV) (fm) (fm) W (MeV) WD (MeV)
r0 a
1'0
Vm)
a'
(fm) (fm) (MeV) (fm) (fm)
ra V, rs as
48.9 1.16 0.15 4.1 6.0 1.37 0.42 1.25 6.04 1.064 0.738
used in the DWBA calculations Deuteron 93.2 1.15 0.81 0.0 18.0 1.34 0.68 1.15 0.0 0.0 0.0
The notation for the parameters is the same as that of Satchler “). “) G. R. Satchler, Nucl. Phys. A92 (1967) 273.
peaks due to ruthenium and those due to any target contaminants was quite apparent. It is possible that a,few weakly populated levels may have been obscured by this contaminant background and missed in our analysis. Below 3.4 MeV of excitation, however, it is unlikely that we have missed any level in 95R~ carrying significant single-particle transfer strength. between
96Ru(~, d), "Ru(P,
229
t)
0.05 I
0.02
1
0
I
10
20
I
I
30
40
I
50
60
ec.m.(W
Fig. 2. Angular distributions for two levels excited by I = 2 transfer in the 96R~(p, d) reaction. The solid curves are results of DWBA calculations. 5,
I
/
I
I I g6Ru(p,djg5Ru
I
I
0.05
/ 0.02
1 0
I
40
/
I 20
I I
I
30
4!l
i i
50
60
8C.rn. (-'Jr
Fig. 3. Angular distributions for two levels excited by I = 4 transfer in the 96R~(p, d) reaction. The solid curves are results of the DWBA calculations.
230
J.B. BALL
f0
5
I
I
/
, 1
I
I
i
I
I
I
I
I
‘%u
(~7, dlg5Ru
I
I
2
0.2 0.1 I
I
0.05 0
I
I
I
I
I
10
20
30
40
50
60
acm. kW
Fig. 4. Angular distribution
for a typical f = 1 transfer observed in the Q6R~(p, d) reaction. The solid curve is the DWBA calculation.
4
0.5
z
0.2
-z G e
0.1
-z 0.05
0.02
0.04 0
t0
20
30
40
50
60
Rc.m.ideg)
Fig. 5. Angular distribution of the deuteron group leading to the first excited state in the reaction g6R~(p, d)95Ru. The solid curve is the DWBA calculation for an I = 0 transfer.
dh 96Ru(~,t)
--(P,
Assignment by comparison
of I-transfer
values and extraction
of the experimental
angular
231
of spectroscopic
distributions
factors
with DWBA
was made
calculations.
These calculations were performed with the computer code JULIE lo) and used the parameters shown in table 1. The proton parameters are based on preliminary results of Menet et al. ‘I) for a parameter-free proton optical-model potential applicable to a wide range of A-values and proton energies. The deuteron parameters are set B from Perey and Perey “). The bound state wave functions were modified to include the effects of finite range and non-locality in the local energy approximation. The computation of the form factors was performed with the FANLFR 2 program 13) and used a Woods-Saxon potential with radius parameter r. = 1.16 fm, diffusivity a = 0.75 fm, and depth adjusted according to the separation energy prescription. A Thomas-type term was TABLE
Levels observed Excitation energy
2
in 95R~ with the 96R~(p,
Al
d) reaction
Assumed Jr
c2.s
f’
1.30 0.07
(MeV) 0.000 0.779 (0.89) (0.94) 1.141 1.342 2.108 2.294 2.713 3.178 3.214 3.383 3.410 All energies
2 0
1+ -5
cl+, f+) (3, a+ ct.;z)+
(4) 2 4 4 4 1 1 1 4
ho.005
8’ 9+ 2 I-
(;,
4)
-
‘;;a f
0.32,0.20 0.26, 0.22 0.5, 0.8 5.0 1.0 1.2 1.2, 1.4 0.5, 0.4 1.5
MeV
used for the spin-orbit potential with a strength 1 = 25. Non-locality range parameters were 0.85 Em for the neutron bound state and incident proton channel 14) and 0.54 fm for the deuteron channel ’ “). The range parameter for the finite range correction was 1.54 fm. The uncertainties introduced by employing lower cut-offs of the radial integrals for (p, d) reaction calculations have been discussed previously 16). No cut-offs were used in the present work. Angular distributions for two levels exhibiting a shape characteristic of pickup of an I = 2 neutron are shown in fig. 2. Note that one of these is the transition to the ground state of “Ru. Two angular distributions corresponding to pickup of an I = 4 neutron are shown in fig. 3 and an example of an I = 1 pickup is shown in fig. 4. All of these angular distributions are seen to agree quite well with the shapes predicted by DWBA
232
J. B.
BALL
calculations. The angular distribution for the first excited state is shown in fig. 5. Although the fit is not as good as for the other I-transfer values, it is still satisfactory and establishes the transition as AZ = 0. The experimental angular distribution is essentially identical to that observed in the 92Zr(p, d) reaction to the well-known 3’ first excited level in 91Zr, previously studied at this same energy 16). The energies of the observed levels, their Z-transfer assignments and spectroscopic factors are summarized in table 2. Spectroscopic factors have been extracted by applying the relationship c+),X, = 3~;(C2~w),“LIE~ without any arbitrary normalization. The value of the overlap integral 0; was taken as 1.6 to be consistent with the results of calculations which specifically include the effect of the D-state admixture in the wave function for the deuteron 17). 4. Discussion The primary purpose of this experiment was to establish the ground state properties of 95R~. From fig. 2 it is seen that the ground state is reached by an I = 2 transition. This requires that the parity be even and restricts the possible spin values to 3 or 3. This result, then, eliminates 5 as a possible ground state spin for 95Ru. On the basis of the rather well established shell-model systematics, we expect the 2d, orbital to be filling at N = 51 and also expect the 2d, to be empty. It seems quite conclusive, therefore, that the observed 1 = 2 transitions is due to pickup of a 2d, neutron from 96R~ and that the p ro pe r assignment for the ground state of 95R~ is 3’. From the various sources of uncertainty in the calibration of the spectrograph facility, absolute Q-values can be obtained only to an accuracy of order 100 keV. In the present experiment, however, the presence of copper and light element impurities with well-known Q-values provided an absolute calibration of the system. Additional check points were obtained from the (p, d) and (p, t) ground state transitions from exposures on a natural ruthenium target. In all, twelve reactions with well-known Q-values were observed. All agreed with the tabulated values ‘*) to an accuracy of 7 keV or better. (The uncertainties for the listed values are typically 5 to 7 keV.) The Q-value determined in this study for the 96R~(p, d) reaction is - 8.470+0.010 MeV. This is in marked disagreement with the listed value ‘*) of -7.90+0.08 MeV and helps to explain the inconsistent results of the /I’ decay experiments. From the above Q-value for the 96Ru(p, d) reaction, the binding energy of the last neutron in 96Ru is 10.695 +O.OlO MeV. Assuming that the value listed I*) for the binding energy of the last proton in 96Ru is correct (7.310+0.022), the computed value of QEc for the decay of “Ru to g5T~ is 2.602kO.024 MeV. From this, the maximum possible /I’ energy is 1.58 MeV. Therefore, the highest energy p’ group reported by Riehs and Warhanek, with their quoted endpoint of 1.33 MeV, could not feed directly the ground state of “Tc but must feed an excited state. The only level in 9 5T~ in the required region of excitation is the previously mentioned 0.336 MeV level. Assuming
d), g6Ru(p,
g6Ru(p,
233
t)
that the /I’ decay of 95R~ populates this state, the maximum B’ energy becomes 1.25f0.02 MeV. This value falls between the two reported experimental values of 1.33kO.03 [ref. “)I and 1.20+0.03 [ref. *)I. From the above, it seems reasonable to conclude that the +’ ground state of 95R~ decays by p’ emission to the 0.336 MeV level of 95T~. The allowed logft of this transition then requires the 0.336 MeV state to be either $+, $+, or 3’. Heuer’s observation “) of the Ml character of the decay of this level to the 4’ ground state of 95T~ now allows only the _5’ assignment.
(9/2,7/2)+ 9,2+
7/2+ (g/2,7/2)+
3/2+ \ \
\
\\ (5/2+)
\\
\
e
3/z+-
7/2+
9/z+
‘\\ (9/E,
7/2 1’
‘\\
4/2+ ‘.
‘\
C3/2,5/2,+
‘. -\\
,
f/2+ _.
0
L
5/2+
------
?Zr(p,dPZr BALL
AND
5/2+ q4Mo
FULMER
_.
‘-.-i/2+
------_5/2+ (cY,,~)‘~M~
DIEHL , e/ a/.
q6Ru
(p,dlg5Ru
THIS
WORK
Fig. 6. Energy systematics of those low-lying levels in the odd-neutron N = 51 nuclei excited by pickup reactions on the neighboring N = 52 isotope. The 92Zr(p, d) results are from ref. 16), the 94Mo(d, t) results from ref. 20), and the 96R~ results from the present study. More complete information on the levels of 95Ru, including higher excited states, is given in table 2.
Thus the results of the present work, in establishing the ground state spin and parity for 95R~ and its decay energy, when combined with the previous decay data also uniquely determine the assignment for the 0.336 MeVlevel of 95T~.A more complete discussion of the levels in 95Tc is given in a report of the recent restudy of the decay of 9‘Ru by Tucker and Hein ’ ‘). The low-lying levels in 9sR~ observed in the present study are compared in fig. 6 with the results of similar studies populating levels in the other N = 51 nuclei 91Zr [ref. ‘“)I and 93Mo [ref. “‘)I. Although a few other levels are known to lie in the
234
J.
B. BALL
energy region shown, for both ‘iZr and g3Mo, the states indicated are those populated in the neutron pickup reaction from the appropriate N = 52 isotope. The 4’ first excited state of g5R~ is seen to extend the systematics in a reasonable fashion. The preference for the 3’ assignment to the 1.141 MeV level is based on the observed trend from 91Zr to 93Mo and the fact that the s3+ level in 93Mo is much stronger in the pickup reaction than a possible 3’ level nearby ‘I). A similar argument yields the e+ preference for the 1.342 MeV level. The higher-lying I = 4 transitions are assigned z9 + because of their large spectroscopic factors. Two possible low-lying levels are listed in table 2 but not shown in fig. 6. Both of these levels are obscured at several angles by target contaminants but appear to show the proper kinematic behaviour for a (p, d) reaction in ruthenium. In neither case are data obtained at enough angles to establish an angular distribution. The possible level at 0.89 MeV is very weak and could arise from a ruthenium isotope impurity. The possible level at 0.94 MeV is somewhat more intense and is not inconsistent with an I = 4 transition. This transition seems a distinct possibility since we expect to see some population of the lg, orbital by the least-bound neutron pair in 96R~ and the systematics of fig. 6 also suggest the possibility of a 3’ in this region of excitation. This assumption also brings the sum of the spectroscopic factors for 2d,, 2d,, 3s+ and lg, pickup to 1.95 compared with the expected sum of 2.0. A letter by Alen and Kapteyn 22) reports on the production of 95Rh by the 96R~(p, 2n) reaction and attributes two y-transitions, 0.942 and 1.36 MeV, to 95R~. The observation of the 0.942 MeV transition lends additional support to our possible 3’ level at about the same energy. The same two y-transitions have also been observed in recent (a, xny) work of Lederer et al., and they report the higher energy as 1.346 MeV [ref. ““)I. This higher-energy transition seems in good agreement with the present observation of a level at 1.342 MeV. No evidence is seen in the present work for a level corresponding to the possible yinferred previously from the results of Chikhladze “). A 0.256 MeV transition could connect to our lowest 3’ (th e p ossible level at 0.94 MeV) and still place an qbelow any level which could produce a transition of lower multipolarity than M2. The appropriate excitation energy falls in a clean portion of the spectrum and no peak is observed. Since the other results support the general shell-model systematics for the nuclei in this region, an -l+- level would not be expected at such a low excitation energy. Lederer et al. 23) a 1so report observing the transition seen by Chikhladze but measure a lifetime that implies an E2 transition. The level systematics shown in fig. 6 are reflected in the spectroscopic factors observed for these reactions and in the implications these have on structure of the ground state wave functions of the target nuclei. As the proton number increases, a general compression of the excited levels is observed accompanied by increased transition strength to these levels. The spectroscopic factor for the d, pickup in the present work is about 1.3 compared with a value of about 1.7 for the 92Zr(p, d) reaction 16). While the least-bound neutron pair in 92Zr was found to occupy the
96R~(p,
2d, orbital
with a probability
d),
96Ru(p,
335
t)
of 85 %, the present
results indicate
an occupation
of
only 65 % in the g6R~ ground state. Thus the addition of protons causes an increased mixing in the ground-state wave functions of the even N = 52 nuclei. The indicated pair in the g6R~ ground
distribution of the neutron 2d, and about 17 % lg+.
state is 65 % 2d,, 4 % 3s,, 14 %
2
0
20
i0
30
40
50
60
8 c. m. (deg’
Fig. 7. Angular distribution of the triton group leading to the ground state in the reaction 9GR~(p, t)94Ru. The solid curve is the DWBA calculation for L = 0 transfer of two neutrons.
Summary of Q-values measured
TABLE 3 in the present study with binding energy and nuclear mass data derived therefrom
96R~
Q(P, t) B(2n) Q(P, d) B(n) Mass excess
-11.165~0.010 19.647f0.010 --8.470+0.010 10.695~0.010 (-86.071 jzO.005) “)
=Ru
9’Ru
-6.727*0.014 8.952+0.014 -83.44710.012
-82.567+0.012
All quantities listed are in units of MeV. “) From ref. Is).
5. The 96R~(p, t) reaction Triton spectra were recorded at the same time as the deuteron spectra. Only the ground state (p, t) transition was observed with sufficient strengthtoobtain an angular distribution and this is shown in fig. 7. The Q-value determined for this transition is
236
J.
B. BALL
-11.165~0.010 MeV. From the listed value 18) of the mass excess for 96R~ of -86.071 f0.005 MeV the present result yields a mass excess for 94R~ of - 82.567 kO.012 MeV. This quantity has not been measured previously. A summary of the nuclear mass data obtained from the present experiment is given in table 3. The DWBA calculation shown in fig. 7 is essentially identical to previously reported two-neutron transfer calculations on 90Zr at this same energy 24). Unfortunately, the 92Zr(p, t) reaction has not been studied at 31.5 MeV so that no direct comparison can be made to assess the effect of the change in the distribution of the neutron pair on the ground state transition intensity. The L = 0 calculation is shown only to illustrate that the observed level has the shape expected for the ground state to ground state transition. From the exposures taken in the present experiment, only one other triton group was observed with sufficient intensity to allow assignment to the 96R~(p, t) reaction. This corresponds to a level in 94R~ at 1.431 MeV. A level at 1.428 MeV has been reported previously as the 2+ first excited state of 94R~ [ref. ““)I. The considerable help of D. M. Scheef with the preparation of the target and A. E. Pugh with the data collection is gratefully acknowledged. The form of the electroplating cell was suggested by R. J. Silva. The scanning of the emulsions was performed by V. Jones and R. Shelton. The author would like to thank A. B. Tucker and C. M. Lederer for communication of experimental results prior to publication. References 1) K. Haravu, C. L. Hollas, P. J. Riley and W. R. Coker, Phys. Rev. Cl (1970) 938 2) E. R. Cosman, H. A. Enge and A. Sperduto, Phys. Rev. 165 (1968) 1175 3) B. L. Cohen and 0. V. Chubinsky, Phys. Rev. 131 (1963) 2184 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)
20) 21) 22) 23) 24) 25)
J. B. Moorhead and R. A. Moyer, Phys. Rev. 184 (1969) 1205 P. Riehs and H. Warhanek, Nucl. Phys. 44 (1963) 164 V. L. Chikhladze, Sov. J. Nucl. Phys. 1 (1965) 520 C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes (Wiley, New York, 1967) p. 232 J. A. Pinston, E. Monnand and A. Moussa, J. de Phys. 29 (1968) 257 D. Heuer, Z. Phys. 201 (1967) 142 R. M. Drisko, unpublished J. J. H. Menet, E. E. Gross, J. J. Malanify and A. Zucker, to be published C. M. Perey and F. G. Perey, Nucl. Phys. 132 (1963) 755 J. K. Dickens and F. G. Perey, unpublished, ORNL-3858 (1965) 49 F. G. Perey and B. Buck, Nucl. Phys. 32 (1963) 353 R. H. Bassal, Phys. Rev. 149 (1966) 791 J. B. Ball and C. B. Fulmer, Phys. Rev. 172 (1968) 1199 R. M. Drisko, private communication J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 1 A. B. Tucker and W. W. Hein, Nucl. Phys. A155 (1970) 129 R. C. Diehl, B. L. Cohen, R. A. Moyer and L. H. Goldman, Phys. Rev. Cl (1970) 2132 S. A. Hjorth and B. L. Cohen, Phys. Rev. 135 (1964) B921; K. H. Bhatt and J. B. Ball, Nucl. Phys. 63 (1965) 286 A. H. W. Aten, Jr. and J. C. Kapteyn, Physica 33 (1967) 705 C. M. Lederer, private communication J. B. Ball, R. L. Auble, R. M. Drisko and P. G. Roos, Phys. Rev. 177 (1969) 1699 J. M. Jaklevic, C. M. Lederer and J. M. Hollander, Phys. Lett. 29B (1969) 179
1930
Ab&ract: The lever structure of psR~ has been investigated with the *6R~(p, d) reaction at a proton energy of 31.5 MeV. Experimental anguular distributions are compared with ~sto~ed-wave Born approximation cakufations to extract spin, parity and spectroscopic-factor assignments for the levels observed up to about 3.5 MeV of excitation. The ground state of p5Ru is observed to be reached by I = 2 transfer which is consistent with the 4” assignment expected from shellmodel considerations. The measured Q-values for the (p, d) and (p, t) reactions rest& in a new value far the mass of g”R~ and a deterrn~~a~~on of the previously unmeasured mass of 94Ru.
E
I
NUCLEAR REACTIONS 96Ru(p, d), 96R~(p, t), E, = 31.5 MeV; measured Q, o(E, #). q5R~ deduced levels, &, J, X, spectroscopic strengths, ground state mass excess. BQRu deduced around state mass excess. Enriched taraet.
The shell-model systematics for nuclei near the major neutron shell closure at N = 50 seem to be well established, The nearby odd-neutron nuclei with N > 50 all exhibit a ground state with $’ spin parity which is attributed to the unpaired neutron occupying the 2d, shell-model orbital Previous studies I-‘) of neutron stripping reactions on even N = 50 targets have shown strong I = 2 transitions to the ground state of the N = 51 final nuclei, Such results indicate a predominantly singleparticle nature for these $* states. In addition, these same reactions populate the first excited states of the final nuclei with strong t = 0 transitions. These first excited fevefs are thus assigned bs +* levels and deseribed in terms of promoting the odd neutron from the 2d+ orbital, which it occupies in the ground state configuration, into the 3s, orbital. The moly~enum isotope seqnence is the highest proton number, 2 = 42, for which the N = 50 member is stable. Thus ““MO is the heaviest N = 51 nucleus that can be studied by the neutrc_m stripping reaction and it follows the above systematics very nicely, The next heavier odd-neutron nucleus is 95Ru. Although little data exists on the level structure of “5Ru, much of the rather fragmentary data that is available seems to disagree with these systematics, t Research sponsored Carbide Corporation.
by the US Atomic Energy Commission 225
under tontract
with the Union
226
J. B. BALL
On the basis of their /3y coincidence studies, Riehs and Warhenek “) proposed a p’ branch from the ground state of 95Ru feeding the e+ ground state of g5T~. Their measured /?’ energy led to an allowed log ft value and, hence, an assignment of SC, Bf, or 9’ to the ground state of 95R~. From a study of internal conversion electron spectra, Chikhladze “) assigned a 256 keV y-transition to ’ 5R~ with c(k and K/L ratio indicating a probable M2 transition. Since an M2 transition requires a change of parity, and since the lh, is the only odd-parity orbital in this region of the shell-model neutron potential, it seems likely that one of the levels connected by the 256 ke\, transition is an -I$- level. An M2 transition could then connect to a $* level. The above observations have led to a tentative level scheme for “Ru with a I’ ground state and an y- excited level at 256 keV [ref. ‘)I. More recently, Pinston et al. *), repeated j?y coincidence studies of the 9 5Ru decay and concluded that the highest energy /I’ branch feeds an excited level at 336 keV and not the ground state of 9 5T~. They suggest that the ground state of ’ 5Ru could indeed be 3* and the level at 336 keV in g5Tc could be 5’. The observation by Heuer “) that the 336 keV level in 95T~ decays to the 4’ g round state by an Ml transition is consistent with such a picture, but neither experiment established ConclLlsively the spin of the 336 keV level in 95Tc or of the ground state of g5R~. (These two experiments would, however, seem to establish both of these states as having even parity.) To try and resolve this possible anomaly in the systematics of the N = 51 nucIei, and to provide more detailed information on the structure of 95Ru, we have studied the g6R~(p, d)q5Ru reaction. We have also used our measured Q-value to obtain a new mass measurement for the “Ru isotope. The ground state transition from the g6Ru(p, t)94Ru reaction was also observed and the Q-value measured to obtain the previously unknown mass of the N = 50 isotope, g4Ru. 2. Experimental details The reactions were studied by bombarding a thin foil of isotopically enriched g6Ru with 31.5 MeV protons from the Oak Ridge Isochronous CycIotron. The deuterons and tritons were momentum analyzed with the broad-range spectrograph facility and spectra were recorded on 50pm thick Kodak NTB emulsions. Aluminum absorbers were used to eliminate low-energy triton contamination of the deuteron spectra. A spectrograph entrance angle of -t_1.5” and a scanning zone of 3 cm were used which yielded a solid angle of order 6 x 10e4 sr. Typical beam currents on target were 250-350 nA and overall resolution for the system was 20 keV FWHM. The q6R~ foil target was prepared by electrodeposition. Because of possible interest in the general problem of target preparation, the technique employed will be described briefly. Two circumstances prevented our preparing the target by conventional methods. The high cost of the separated isotope precluded obtaining enough
96R~(P, d), %ufp,
material nium
to prepare
the target by rolling.
makes it impossible
to prepare
t)
The extremely
227
high melting
point
the target by the usual evaporation
It should be possible to evaporate ruthenium was not available to us at the time.
with an electron
of ruthe-
techniques.
gun but such a device
The plating cell employed was a small glass cylinder. The cylinder was placed on a thin copper foil (Z 3 mg/cm2) which served as the cathode. A platinum spiral served as both anode and mechanical stirrer. The plating solution consisted of about 10 ml of saturated ammonium sulfate solution containing 10 mg of enriched g6R~. A current of 100 mA at 3 V produced a plating thickness of about 0.1 mg/cm’ in 10 minutes. Actual thickness of the plated foils was determined by weighing the copper foils before and after plating. After plating, the copper foil was removed with an etching solution of tri-chloroacetic acid and ammonium hydroxide. Unlike chromium, cobalt, and iron foils prepared in a similar manner, the ruthenium foils could not be made self-supporting. Although the plated ruthenium had a shiny metallic appearance, it proved to be very brittle and could not withstand the etching process. For the targets used in this work, a thin layer of polystyrene was applied to the plated side of the copper foil before etching. Films of order 0.25 mg/cm’ were found to provide adequate support for the ruthenium foil. The obvious advantages to this method of preparing targets are the rather simple equipment required and the high yield of targets for the amount of isotope used. In our case, from the single plating solution using 10 mg of g6R~ isotope, we plated four targets ranging in thickness from about 0.1 to 0.3 mg/cm2. A major disadvantage to this plating technique is the problem of copper contamination of the target. This is particularly apparent in the spectra from the g6R~ target used for this experiment where a significant background from copper is observed. Although this caused no problem in analyzing the strong peaks due to reactions on g6R~, it makes analysis of weak peaks very difficult. It seems likely that the target used for this work was not completely etched. Calibration exposures on natural ruthenium foils prepared in a similar manner did not seem to show this problem. The use of served The riched
the polystyrene foil for support is also somewhat less than satisfactory but adequately for the present work. target used for this experiment had a thickness of 0.18 mg/cm’ and was ento 94.9 % in g6R~. The uncertainty in target thickness is estimated at 10 %.
3. Results and analysis Energy spectra were obtained at 5” intervals from 10” to 45”. A typical deuteron spectrum is shown in fig. 1. Most of the unlabelled peaks are ascribable to the previously mentioned copper contaminant in the target. The peaks due to (p, d) reactions from the copper could be readily identified by their characteristic kinematic shift over the angular range studied. From comparison of the spectra at all angles, the distinction
228
J.
B. BALL
IOc)O
e75
750
E
625
2 $
500
z : 8
375
250
125
0 54
56
59
60
62
64
66
68
70
72
74
DISTANCE ALONG FOCAL PLANE
76
78
80
82
84
86
88
90
km)
Fig. 1. Spectrum for 96R~(p, d)95Ru at 10”. Peaks corresponding to levels in 95R~ are labelled with their appropriate excitation energies in MeV. Two light element impurity peaks are labelled. The remaining unlabelled peaks are associated with copper contamination of the target as discussed in the text.
Summary of the optical-model
TABLE1 parameters Proton
V
(MeV) (fm) (fm) W (MeV) WD (MeV)
r0 a
1'0
Vm)
a'
(fm) (fm) (MeV) (fm) (fm)
ra V, rs as
48.9 1.16 0.15 4.1 6.0 1.37 0.42 1.25 6.04 1.064 0.738
used in the DWBA calculations Deuteron 93.2 1.15 0.81 0.0 18.0 1.34 0.68 1.15 0.0 0.0 0.0
The notation for the parameters is the same as that of Satchler “). “) G. R. Satchler, Nucl. Phys. A92 (1967) 273.
peaks due to ruthenium and those due to any target contaminants was quite apparent. It is possible that a,few weakly populated levels may have been obscured by this contaminant background and missed in our analysis. Below 3.4 MeV of excitation, however, it is unlikely that we have missed any level in 95R~ carrying significant single-particle transfer strength. between
96Ru(~, d), "Ru(P,
229
t)
0.05 I
0.02
1
0
I
10
20
I
I
30
40
I
50
60
ec.m.(W
Fig. 2. Angular distributions for two levels excited by I = 2 transfer in the 96R~(p, d) reaction. The solid curves are results of DWBA calculations. 5,
I
/
I
I I g6Ru(p,djg5Ru
I
I
0.05
/ 0.02
1 0
I
40
/
I 20
I I
I
30
4!l
i i
50
60
8C.rn. (-'Jr
Fig. 3. Angular distributions for two levels excited by I = 4 transfer in the 96R~(p, d) reaction. The solid curves are results of the DWBA calculations.
230
J.B. BALL
f0
5
I
I
/
, 1
I
I
i
I
I
I
I
I
‘%u
(~7, dlg5Ru
I
I
2
0.2 0.1 I
I
0.05 0
I
I
I
I
I
10
20
30
40
50
60
acm. kW
Fig. 4. Angular distribution
for a typical f = 1 transfer observed in the Q6R~(p, d) reaction. The solid curve is the DWBA calculation.
4
0.5
z
0.2
-z G e
0.1
-z 0.05
0.02
0.04 0
t0
20
30
40
50
60
Rc.m.ideg)
Fig. 5. Angular distribution of the deuteron group leading to the first excited state in the reaction g6R~(p, d)95Ru. The solid curve is the DWBA calculation for an I = 0 transfer.
dh 96Ru(~,t)
--(P,
Assignment by comparison
of I-transfer
values and extraction
of the experimental
angular
231
of spectroscopic
distributions
factors
with DWBA
was made
calculations.
These calculations were performed with the computer code JULIE lo) and used the parameters shown in table 1. The proton parameters are based on preliminary results of Menet et al. ‘I) for a parameter-free proton optical-model potential applicable to a wide range of A-values and proton energies. The deuteron parameters are set B from Perey and Perey “). The bound state wave functions were modified to include the effects of finite range and non-locality in the local energy approximation. The computation of the form factors was performed with the FANLFR 2 program 13) and used a Woods-Saxon potential with radius parameter r. = 1.16 fm, diffusivity a = 0.75 fm, and depth adjusted according to the separation energy prescription. A Thomas-type term was TABLE
Levels observed Excitation energy
2
in 95R~ with the 96R~(p,
Al
d) reaction
Assumed Jr
c2.s
f’
1.30 0.07
(MeV) 0.000 0.779 (0.89) (0.94) 1.141 1.342 2.108 2.294 2.713 3.178 3.214 3.383 3.410 All energies
2 0
1+ -5
cl+, f+) (3, a+ ct.;z)+
(4) 2 4 4 4 1 1 1 4
ho.005
8’ 9+ 2 I-
(;,
4)
-
‘;;a f
0.32,0.20 0.26, 0.22 0.5, 0.8 5.0 1.0 1.2 1.2, 1.4 0.5, 0.4 1.5
MeV
used for the spin-orbit potential with a strength 1 = 25. Non-locality range parameters were 0.85 Em for the neutron bound state and incident proton channel 14) and 0.54 fm for the deuteron channel ’ “). The range parameter for the finite range correction was 1.54 fm. The uncertainties introduced by employing lower cut-offs of the radial integrals for (p, d) reaction calculations have been discussed previously 16). No cut-offs were used in the present work. Angular distributions for two levels exhibiting a shape characteristic of pickup of an I = 2 neutron are shown in fig. 2. Note that one of these is the transition to the ground state of “Ru. Two angular distributions corresponding to pickup of an I = 4 neutron are shown in fig. 3 and an example of an I = 1 pickup is shown in fig. 4. All of these angular distributions are seen to agree quite well with the shapes predicted by DWBA
232
J. B.
BALL
calculations. The angular distribution for the first excited state is shown in fig. 5. Although the fit is not as good as for the other I-transfer values, it is still satisfactory and establishes the transition as AZ = 0. The experimental angular distribution is essentially identical to that observed in the 92Zr(p, d) reaction to the well-known 3’ first excited level in 91Zr, previously studied at this same energy 16). The energies of the observed levels, their Z-transfer assignments and spectroscopic factors are summarized in table 2. Spectroscopic factors have been extracted by applying the relationship c+),X, = 3~;(C2~w),“LIE~ without any arbitrary normalization. The value of the overlap integral 0; was taken as 1.6 to be consistent with the results of calculations which specifically include the effect of the D-state admixture in the wave function for the deuteron 17). 4. Discussion The primary purpose of this experiment was to establish the ground state properties of 95R~. From fig. 2 it is seen that the ground state is reached by an I = 2 transition. This requires that the parity be even and restricts the possible spin values to 3 or 3. This result, then, eliminates 5 as a possible ground state spin for 95Ru. On the basis of the rather well established shell-model systematics, we expect the 2d, orbital to be filling at N = 51 and also expect the 2d, to be empty. It seems quite conclusive, therefore, that the observed 1 = 2 transitions is due to pickup of a 2d, neutron from 96R~ and that the p ro pe r assignment for the ground state of 95R~ is 3’. From the various sources of uncertainty in the calibration of the spectrograph facility, absolute Q-values can be obtained only to an accuracy of order 100 keV. In the present experiment, however, the presence of copper and light element impurities with well-known Q-values provided an absolute calibration of the system. Additional check points were obtained from the (p, d) and (p, t) ground state transitions from exposures on a natural ruthenium target. In all, twelve reactions with well-known Q-values were observed. All agreed with the tabulated values ‘*) to an accuracy of 7 keV or better. (The uncertainties for the listed values are typically 5 to 7 keV.) The Q-value determined in this study for the 96R~(p, d) reaction is - 8.470+0.010 MeV. This is in marked disagreement with the listed value ‘*) of -7.90+0.08 MeV and helps to explain the inconsistent results of the /I’ decay experiments. From the above Q-value for the 96Ru(p, d) reaction, the binding energy of the last neutron in 96Ru is 10.695 +O.OlO MeV. Assuming that the value listed I*) for the binding energy of the last proton in 96Ru is correct (7.310+0.022), the computed value of QEc for the decay of “Ru to g5T~ is 2.602kO.024 MeV. From this, the maximum possible /I’ energy is 1.58 MeV. Therefore, the highest energy p’ group reported by Riehs and Warhanek, with their quoted endpoint of 1.33 MeV, could not feed directly the ground state of “Tc but must feed an excited state. The only level in 9 5T~ in the required region of excitation is the previously mentioned 0.336 MeV level. Assuming
d), g6Ru(p,
g6Ru(p,
233
t)
that the /I’ decay of 95R~ populates this state, the maximum B’ energy becomes 1.25f0.02 MeV. This value falls between the two reported experimental values of 1.33kO.03 [ref. “)I and 1.20+0.03 [ref. *)I. From the above, it seems reasonable to conclude that the +’ ground state of 95R~ decays by p’ emission to the 0.336 MeV level of 95T~. The allowed logft of this transition then requires the 0.336 MeV state to be either $+, $+, or 3’. Heuer’s observation “) of the Ml character of the decay of this level to the 4’ ground state of 95T~ now allows only the _5’ assignment.
(9/2,7/2)+ 9,2+
7/2+ (g/2,7/2)+
3/2+ \ \
\
\\ (5/2+)
\\
\
e
3/z+-
7/2+
9/z+
‘\\ (9/E,
7/2 1’
‘\\
4/2+ ‘.
‘\
C3/2,5/2,+
‘. -\\
,
f/2+ _.
0
L
5/2+
------
?Zr(p,dPZr BALL
AND
5/2+ q4Mo
FULMER
_.
‘-.-i/2+
------_5/2+ (cY,,~)‘~M~
DIEHL , e/ a/.
q6Ru
(p,dlg5Ru
THIS
WORK
Fig. 6. Energy systematics of those low-lying levels in the odd-neutron N = 51 nuclei excited by pickup reactions on the neighboring N = 52 isotope. The 92Zr(p, d) results are from ref. 16), the 94Mo(d, t) results from ref. 20), and the 96R~ results from the present study. More complete information on the levels of 95Ru, including higher excited states, is given in table 2.
Thus the results of the present work, in establishing the ground state spin and parity for 95R~ and its decay energy, when combined with the previous decay data also uniquely determine the assignment for the 0.336 MeVlevel of 95T~.A more complete discussion of the levels in 95Tc is given in a report of the recent restudy of the decay of 9‘Ru by Tucker and Hein ’ ‘). The low-lying levels in 9sR~ observed in the present study are compared in fig. 6 with the results of similar studies populating levels in the other N = 51 nuclei 91Zr [ref. ‘“)I and 93Mo [ref. “‘)I. Although a few other levels are known to lie in the
234
J.
B. BALL
energy region shown, for both ‘iZr and g3Mo, the states indicated are those populated in the neutron pickup reaction from the appropriate N = 52 isotope. The 4’ first excited state of g5R~ is seen to extend the systematics in a reasonable fashion. The preference for the 3’ assignment to the 1.141 MeV level is based on the observed trend from 91Zr to 93Mo and the fact that the s3+ level in 93Mo is much stronger in the pickup reaction than a possible 3’ level nearby ‘I). A similar argument yields the e+ preference for the 1.342 MeV level. The higher-lying I = 4 transitions are assigned z9 + because of their large spectroscopic factors. Two possible low-lying levels are listed in table 2 but not shown in fig. 6. Both of these levels are obscured at several angles by target contaminants but appear to show the proper kinematic behaviour for a (p, d) reaction in ruthenium. In neither case are data obtained at enough angles to establish an angular distribution. The possible level at 0.89 MeV is very weak and could arise from a ruthenium isotope impurity. The possible level at 0.94 MeV is somewhat more intense and is not inconsistent with an I = 4 transition. This transition seems a distinct possibility since we expect to see some population of the lg, orbital by the least-bound neutron pair in 96R~ and the systematics of fig. 6 also suggest the possibility of a 3’ in this region of excitation. This assumption also brings the sum of the spectroscopic factors for 2d,, 2d,, 3s+ and lg, pickup to 1.95 compared with the expected sum of 2.0. A letter by Alen and Kapteyn 22) reports on the production of 95Rh by the 96R~(p, 2n) reaction and attributes two y-transitions, 0.942 and 1.36 MeV, to 95R~. The observation of the 0.942 MeV transition lends additional support to our possible 3’ level at about the same energy. The same two y-transitions have also been observed in recent (a, xny) work of Lederer et al., and they report the higher energy as 1.346 MeV [ref. ““)I. This higher-energy transition seems in good agreement with the present observation of a level at 1.342 MeV. No evidence is seen in the present work for a level corresponding to the possible yinferred previously from the results of Chikhladze “). A 0.256 MeV transition could connect to our lowest 3’ (th e p ossible level at 0.94 MeV) and still place an qbelow any level which could produce a transition of lower multipolarity than M2. The appropriate excitation energy falls in a clean portion of the spectrum and no peak is observed. Since the other results support the general shell-model systematics for the nuclei in this region, an -l+- level would not be expected at such a low excitation energy. Lederer et al. 23) a 1so report observing the transition seen by Chikhladze but measure a lifetime that implies an E2 transition. The level systematics shown in fig. 6 are reflected in the spectroscopic factors observed for these reactions and in the implications these have on structure of the ground state wave functions of the target nuclei. As the proton number increases, a general compression of the excited levels is observed accompanied by increased transition strength to these levels. The spectroscopic factor for the d, pickup in the present work is about 1.3 compared with a value of about 1.7 for the 92Zr(p, d) reaction 16). While the least-bound neutron pair in 92Zr was found to occupy the
96R~(p,
2d, orbital
with a probability
d),
96Ru(p,
335
t)
of 85 %, the present
results indicate
an occupation
of
only 65 % in the g6R~ ground state. Thus the addition of protons causes an increased mixing in the ground-state wave functions of the even N = 52 nuclei. The indicated pair in the g6R~ ground
distribution of the neutron 2d, and about 17 % lg+.
state is 65 % 2d,, 4 % 3s,, 14 %
2
0
20
i0
30
40
50
60
8 c. m. (deg’
Fig. 7. Angular distribution of the triton group leading to the ground state in the reaction 9GR~(p, t)94Ru. The solid curve is the DWBA calculation for L = 0 transfer of two neutrons.
Summary of Q-values measured
TABLE 3 in the present study with binding energy and nuclear mass data derived therefrom
96R~
Q(P, t) B(2n) Q(P, d) B(n) Mass excess
-11.165~0.010 19.647f0.010 --8.470+0.010 10.695~0.010 (-86.071 jzO.005) “)
=Ru
9’Ru
-6.727*0.014 8.952+0.014 -83.44710.012
-82.567+0.012
All quantities listed are in units of MeV. “) From ref. Is).
5. The 96R~(p, t) reaction Triton spectra were recorded at the same time as the deuteron spectra. Only the ground state (p, t) transition was observed with sufficient strengthtoobtain an angular distribution and this is shown in fig. 7. The Q-value determined for this transition is
236
J.
B. BALL
-11.165~0.010 MeV. From the listed value 18) of the mass excess for 96R~ of -86.071 f0.005 MeV the present result yields a mass excess for 94R~ of - 82.567 kO.012 MeV. This quantity has not been measured previously. A summary of the nuclear mass data obtained from the present experiment is given in table 3. The DWBA calculation shown in fig. 7 is essentially identical to previously reported two-neutron transfer calculations on 90Zr at this same energy 24). Unfortunately, the 92Zr(p, t) reaction has not been studied at 31.5 MeV so that no direct comparison can be made to assess the effect of the change in the distribution of the neutron pair on the ground state transition intensity. The L = 0 calculation is shown only to illustrate that the observed level has the shape expected for the ground state to ground state transition. From the exposures taken in the present experiment, only one other triton group was observed with sufficient intensity to allow assignment to the 96R~(p, t) reaction. This corresponds to a level in 94R~ at 1.431 MeV. A level at 1.428 MeV has been reported previously as the 2+ first excited state of 94R~ [ref. ““)I. The considerable help of D. M. Scheef with the preparation of the target and A. E. Pugh with the data collection is gratefully acknowledged. The form of the electroplating cell was suggested by R. J. Silva. The scanning of the emulsions was performed by V. Jones and R. Shelton. The author would like to thank A. B. Tucker and C. M. Lederer for communication of experimental results prior to publication. References 1) K. Haravu, C. L. Hollas, P. J. Riley and W. R. Coker, Phys. Rev. Cl (1970) 938 2) E. R. Cosman, H. A. Enge and A. Sperduto, Phys. Rev. 165 (1968) 1175 3) B. L. Cohen and 0. V. Chubinsky, Phys. Rev. 131 (1963) 2184 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19)
20) 21) 22) 23) 24) 25)
J. B. Moorhead and R. A. Moyer, Phys. Rev. 184 (1969) 1205 P. Riehs and H. Warhanek, Nucl. Phys. 44 (1963) 164 V. L. Chikhladze, Sov. J. Nucl. Phys. 1 (1965) 520 C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes (Wiley, New York, 1967) p. 232 J. A. Pinston, E. Monnand and A. Moussa, J. de Phys. 29 (1968) 257 D. Heuer, Z. Phys. 201 (1967) 142 R. M. Drisko, unpublished J. J. H. Menet, E. E. Gross, J. J. Malanify and A. Zucker, to be published C. M. Perey and F. G. Perey, Nucl. Phys. 132 (1963) 755 J. K. Dickens and F. G. Perey, unpublished, ORNL-3858 (1965) 49 F. G. Perey and B. Buck, Nucl. Phys. 32 (1963) 353 R. H. Bassal, Phys. Rev. 149 (1966) 791 J. B. Ball and C. B. Fulmer, Phys. Rev. 172 (1968) 1199 R. M. Drisko, private communication J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nucl. Phys. 67 (1965) 1 A. B. Tucker and W. W. Hein, Nucl. Phys. A155 (1970) 129 R. C. Diehl, B. L. Cohen, R. A. Moyer and L. H. Goldman, Phys. Rev. Cl (1970) 2132 S. A. Hjorth and B. L. Cohen, Phys. Rev. 135 (1964) B921; K. H. Bhatt and J. B. Ball, Nucl. Phys. 63 (1965) 286 A. H. W. Aten, Jr. and J. C. Kapteyn, Physica 33 (1967) 705 C. M. Lederer, private communication J. B. Ball, R. L. Auble, R. M. Drisko and P. G. Roos, Phys. Rev. 177 (1969) 1699 J. M. Jaklevic, C. M. Lederer and J. M. Hollander, Phys. Lett. 29B (1969) 179