Nuclear Physics A142 (1970) 35--48; (~) North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

ENERGY LEVELS OF 91Nb, 93M0, 9STc A N D 97Tc VIA (p, n)

R E A C T I O N S A N D T H E R E A C T I O N Q-VALUES

H. J. KIM, R. L. ROBINSON, C. I-L JOHNSON and S. RAMAN Oak Ridye National Laboratory,

Oak Ridye, Tennessee t

Received 23 October 1969

Abstract: The (p, n) reactions on isotopically enriched targets of 91Zr, 9SMo and 97Mo and mono- isotopic 93Nb were investigated for a bombarding energy range 4 to 5.4 MeV. The neutron groups populating the low-lying excited states of 91Nb, 93Mo, 9STc and 97Tc were studied by the use of a time-of-flight system. The reaction Q-values for 91Zr, 95Mo and 97Mo target nuclei were determined to be --2.045±0.006, --2.490~0.006 and --1.128-t-0.008 MeV, respectively. Many low-lying states were populated by these reactions and their excitation energies determined with a high precision.

E/NUCLEAR REACTIONS 91Zr, 93Nb, 9s' 97Mo(p, n), E : 4--5.4 MeV; measured a(E; En, 0). I

t 91Zr ' 95, 9 7 M o ( p ' n ) , measured Q. 91Nb, 93M0, 95.97Tc deduced levels. I

1. Introduction

The structure of low-lying states in the mass region near A = 90 is expected to be relatively simple because of the closing of one of the major shells with 50 neutrons.

Although there are numerous theoretical calculations of level structures, such as those in refs. ~ -3) and in references cited therein, the experimental knowledge of the levels is rather meager 4,5). This report concerns the use of the (p, n) reaction to study four nuclei in this region. The targets were the spin-~ nucleus 93Nb and the spin-~ nuclei 9~Zr, 95M0 and 97M0, the corresponding product nuclei, whose levels we studied, are 91Nb, 93M0, 95Tc and 97Tc.

For medium or heavy nuclei, a (p, n) reaction induced by protons incident with energies below the Coulomb barrier is an efficient tool for finding most, rather than a select few, of the low-lying final states because the dominant 6)statistical compound

nuclear mechanism, unlike a one-step direct process, populates final states without regard for their detailed structures. Mild selection rules depend on spins and parities

but not on the collective or single-particle or other nature of the states. For example, for a spin-z ~ target, a (p, n) reaction involving only s-, p- and d-waves can easily populate residual states with spins ranging from ½ to ~- . Finckh and Jahnke 7) found a good example of this indiscriminate behavior in their study of the 93Nb(p, n)

t Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation.

35

36 H. ft. KIM et al.

93MO reaction; they found not only the low-spin single-particle states that had been reported s) from 92Mo(d, p)93Mo stripping and most of the high-spin states ob- served 9) from the decay of 93Tc but also many additional levels. We find similar results for all four nuclei that we studied.

A high-resolution neutron spectrometer is required for this type of study. A few years ago such was not available; but, with the advent of sophisticated time-of-flight methods, neutron spectroscopy with energy resolutions comparable to the best for charged particle spectrometers has become a reality 1 o)~ One can view a time-of-flight system in combination with nuclear (p, n) reactions as a composite tool for deter- mining energy levels in which the better resolution for the slower neutrons is com- plementary to the properties of the reactions. Since the incident proton energies must be high enough to penetrate the Coulomb barrier to give a usable yield, the lowest final states are usually populated by relatively high-energy neutrons. The resolution is not the best for these neutrons, but it need not be because the lowest states are usually widely spaced. The higher levels are more closely spaced, but also the resolution is better for the slower neutrons populating those states.

We have at our disposal the necessary detector and associated electronics and a 6 MV Van de Graaff accelerator capable of delivering subnano second beam pulses. The (p, n) thresholds for the four targets are all below 2.5 MeV, so that with this accelerator we can populate levels up to at least 3 MeV excitation in the four final nuclei.

2. Experimental procedure

Since the method of neutron time-of-flight spectroscopy is well known lo,11) and the characteristics of our accelerator have been given by Stelson lo), we will present only a few details here. We used proton pulses with 0.5 or 1 MHz repetition rates, flight-paths ranging from 4 to 16 m, an effective combined time resolution of 1.5 ns and proton bombarding energies ranging from 4 to 5.4 MeV. For each nucleus we bombarded both a thick and a thin target. The thick targets were self-supporting foils of about 1 mg/cm 2, and the thin ones were films of about 0.25 mg/cm 2 evapo- rated on clean Pt blanks. The 93Nb target was monoisotopic and the other three were enriched to better than 92 ~o. The energy loss for 5 MeV protons in the thin targets was about 10 keV. (An additional 93Nb target of intermediate thickness, 0.8 mg/cm 2, was also used.)

The energy spread or resolution 6E for neutrons of energy E is proportional to E ~- 6t /d where 6t and d are the time resolution and flight path for the time-of-flight system. Although 6t is fixed in our system, f E can still be varied to some degree by varying d or E, the latter by changing the incident proton energy. Variations of d, E or the observation angle 0 also help to identify neutron groups. We measured spectra with various combinations of d, E and 0 for both targets of each nucleus in order to identify the neutron groups, to determine their energies precisely, and to resolve those groups whose spacings were comparable to the target thicknesses.

911N19 93MO~ 95, 97Tc LEVELS 3 7

We calibrated the equipment at intervals throughout the experiment to ensure that the Q-values and excitation energies could be measured precisely. The linearity and dispersion, i.e., ns/channel, of the time-of-flight system were found by the method of Langsford and Dolley 12) by reference to the periods of an r.f. signal (20-50 MHz). At each flight path a fiducial time was found from the observed channel for neutrons of known energy produced by the 7Li(p, n) reaction in a thin lithium target. The incident protons were analyzed in a 90 ° magnet previously calibrated in terms of the known (p, n) thresholds for 7Li, 19 F and z 7A1" During the experiment we remeasured the 7Li(p, n) threshold in order to renormalize the magnet's calibration curve. In regard to this calibration, we note that an error in proton energy propagates a mini- mum error to a Q-value if the observed neutron channel is near the fiducial channel. Thus, in our measurement of each ground state Q-value, we minimized such uncer- tainties and other systematic errors by comparing the flight-time for the ground state neutrons directly with that for neutrons from a lithium target at the same in- cident proton energy. A small correction was required for the difference in the average proton energies in the targets.

3. Results and discussion

3.1. TH.E 9:Zr(p, n)91Nb REACTION

We found the ground state Q = -2.045-+0.006 MeV. Maples e t a l . 13) give -1.93_+0.06 MeV, which is two standard deviations less negative than our value.

Figs. 1A and 1B show representative time-of-flight spectra for the thick and thin targets. For each figure the flight times increase linearly to the left from an off-scale

l o o o , I I I I I I I I . ~ ' ' ' ' ~ ' I I I I I ~ - 500 -- ~ o o V, = i !

- ~ h - I . - - - ~ /- . IF : . - . -

200- ~ I . . . . :. • o~ • • (13

o 1 : (kl . - - " . . . "

~00 ___ a -."• . .:

50 -- I • ,: .., : . -" . :

- .? •.: ... . . . . . . : . . . . , " : 9 , :~" ." : . _

- " . -'::'• .::" .• ,• •••.• "•-:-7 -

.¼. • ,

• : . : 7 . : . - ' : . . • . : . . . : : . . •• . . . . . 1o

:5 ~• [ ' " [ . 91Zr ( p , n ) 91Nb - --

. . . . . E p = 5 , 1 2 0 MeV, 0 = 1 2 0 % . .~

d = 8 , 5 4 0 rne le rs

2 - - " THICK T A R G E T ' - -

1 I I I I 1 1 I I I 1 I [ [ I 1 I I I I [ I C H A N N E L NUM E~ER

Fig. 1A. Neutron time-of-flight spectrum for the 91Zr(p, n)91Nb reaction• The flight time increases from the right to the left.

1 o o o ~ _ I I 9 1 Z r ( p , n ) 91Nb I I I I I I I I I I t I I I _ - - - - E p = 5 . 0 2 5 i e V , e = 3 3 ~ r~ 0 ~ ~ _

5 0 0 --" d = t Z J I l m e t e r s ~ 0 co ~ r~ - - - - - - ~ THIN T A R G E T ~ ~ ¢' ~" - - I

I +% 200 I o~ = "

, ÷

§ 5 0 - - " - ~: - - : . . ÷ ~. +÷ +÷

"++. . ~, , -~,%'~ . . . . %

* " % + • + * +

10 __

5 - - I l I I f I I I T I I I I I f I I I T f C H A N N E L NUMBER

Fig. lB. Neutron time-of-flight spectrum for the 91Zr(p, n)gtNb reaction. The flight time increases from the right to the left.

TABLE 1

Reaction Q-values and level energies

Reaction 91Zr(p, n)9tNb 95Mo(p, n)9STc 97Mo(p, n)97Tc 9aNb(p, n)93Mo Q (keV) - -2045±6 --24904-6 - -1128±8 Not measured

Level energy Level energy Level energy Level energy (keV) (keV) (keY) (keV)

106±6 46 ± 8 101 q-10 944 4-4 11884-5 340 -k6 213 4-10 1362 b) 1313±4 631 ± 6 328 ±10 1477 4-4 15804-4 651 4-6 575 ±10 1519 4-4-4 1610±4 670 ± 6 662 4- 9 1693 4-4 16334-4 932 4-4 785 ± 9 2150 4-4 17874-4 960 ± 4 857 ± 9 2164 4-4 18424-4 1087 4-4 941 4. 9 (2186)4-4 1962±8 1178 ± 4 962 4- 9 2253 4-4 2120±8 1213 ± 4 (987)4- 9 2310 :k4

1281 4-4 1050 ± 8 2365 ± 4 (1332)±6 1134 5 : 8 2416 4-4 1415 4-5 1202 ± 8 2448 4-4 a) 1435 4-5 1234 ± 8 2486 4-4 1618 4-5 1268 4- 8 2539 4-4 1636 4-5 1376 ~ 8 2580 ± 4

(1407)4- 8 2650 ± 4 1517 4. 8 a) 2679 ± 4 1580 ± 8 2738 5:4

2769 ± 4 2829 5:4 2845 4-4 2875 5:4 2914 4-4 2989 4-4 3037 ± 4 3062 ± 4 3080 5 : 4

a) Peaks too wide to be single peak. b) Reference level. ( ) Uncertain levels.

91Nb ' 93Mo ' 95.97Tc LEVELS 39

zero channel, and the neutron energies increase nonlinearly in the opposite direction. The energy resolution, which varies as E ~, is less to the left; or, stated simply, the energy resolution is better for the slower neutrons. Thus the widths of the broad peaks to the left come from the spread in neutron energies associated with the proton energy loss in the target; whereas the widths of the narrow peaks at the right are greater than this neutron energy spread and about equal to the overall time resolution of the system. It follows that the widths would increase rather than decrease with energy if the energy scale were linear.

2120 1962 1965

1842 1845 1787

1633 -1610 1637 1624 t580 1581

1313 1313 1314

1188 1187

106 105 104.3 / g.s. g.s. ~ g.s.

PRESENT RESULTS 90Zr(P,7)91Nb RESULTS DECAY RESULTS

Fig. 2. The levels o f 91Nb from the present (p, n) study, from ref. 14) and the radioactive decay results from refs. 4, 15).

The excitation energies for levels in 91Nb derived from these spectra are indicated at the peaks in the figures. The impurity peak for the thin target showed an angular dependence characteristic of some light element, which was probably on the surface of the target. The absence of this peak in the thick-target spectrum demonstrates one way in which the use of two targets helps to identify authentic peaks. The essen-

40 H.J. K]M et aL

tial role of the thin target is illustrated by the fact that the triplet near 1600 keV is fairly well resolved for the thin target but not for the thick one.

Table 1 lists the excitation energies with the uncertainties. The uncertainties in the two highest energies, 1962 and 2120 keV, are rather large because these were found from the thick-target spectrum. The other energies and the ground state Q-value were derived from the thin-target spectrum, which included neutron groups to the ground and first excited states as well as the groups shown in fig. lB.

In fig. 2 these levels, which we found populated by the (p, n) reaction, are com- pared with levels observed previously from the 90Zr(p ' 7)91Nb reaction 14) and from decay measurements 4,a 5). As might be expected, the indiscriminate nature of the (p, n) reaction allows it to populate some new levels, three of them, in addition to all of those reached by the (p, 7) reaction. We find excellent agreement with the energies which were determined to + 1 keV from the (p, 7) reaction•

3.2. THE 95Mo(p, n)OSTc REACTION

We found the ground state Q --- -2 .490+0.006 MeV. Here again Maples e t a l • 13) give a less negative value: Q = -2.441 ___0.021 MeV. The discrepancy is nearly three standard deviations.

The spectra in figs. 3A-3C illustrate roles that the flight path, bombarding energy and target thickness can play in time-of-flight work• The gross structure of the low- lying states are revealed by the thick-target spectrum, which we obtained with little expenditure of accelerator time. For the thin-target spectrum in fig. 3B, we reduced the proton energy so that the low-lying states would be populated by slower, more easily resolved neutrons. The resulting low yield required a shorter flight path; but, even so, the ground state doublet is resolved and the triplet near 650 keV is nearly resolved. For fig. 3C, we increased the proton energy in order to populate the higher

zooo I I I I I I I I I I I I I I I ~ 1 I I I I 95Mo (p ,n ) 95Tc O~ too_

-- tO-- O ~ , 0 0 0 - - E , = 5 . , 2 O M e V . 0 = 1 3 0 % - ~ ,~ [ ~ . ~ ~" ~ ---_

- h r ' 500 d = 8 .860 meters m~'t°~ eaco ~ ~ . ' '~. I 7r : __ - - THICK TARGET "* .- I • . " "

"-" "' ' 5 ,~. .""- 200 -- to ,"

~ ~ .: : . : •

______T_:___ T o l o o : . , : : . . . . : ? . : . . _

- :;"" " : : :',:?" ' ; ' " t.;:':': .:.::" . . n 5 0 - ." ":" -'::.:- ...:..- ~.:( "" " : ' . 7 . . : ' ." ", •1 "11.: . . ' :y . -" . : , - : : - , . . . . . . :.." . . . . . -.:.

2 0 "-"- " " • ' '

lo I I I I I I I I I I I I I I I I I I I I CHANNEL NUMBER

Fig. 3A. Neutron time-of-flight spectrum for the 95Mo(.p, n)95Tc reaction. The flight time increases from the right to the left.

9 1 N b ' 9 3 M 0 ' 9 5 . 9 7 T c L E V E L S 41

1000 E_gS'Mo(;,.)~JSTe' I I I I I I I I I I I I I I I I__----

500 : Ep=4 .045MeV , 8=66 ° , - - -

- d = 5 .987 me te r s 0 0 ~; - - ~ r t o

- TH IN TARGET ~ ~ ~ ~ " - -

_ 2 0 0 - - -'~ . ~ ~ .÷ , .

. ! , , , , . I . . 100__ ~ ~ " , ~ ÷ __

g - - , . . : _ _

8 ":

* ÷ * ÷÷ .+** • ÷ , ÷ ÷*

1 0 * * -.'~-'÷ * . ÷ . : 7 % 2 . . . . . * * . . * : . " : . ° . . ÷ * . . . J "." "d . - -

* * ~ • ÷ + ~÷ ÷ ,÷ * * ~÷ ,~ . , ÷÷ ÷÷÷ ,÷ , , ÷ ,÷~ • ÷ - - ~ , , ~ ,÷ ÷ ÷÷ ÷ . , , , ÷~ , ~ , ÷ , ÷÷÷ ÷ . . ~ ÷ • ÷ •

5 --÷ ÷* *÷ ~ ~ ÷÷ ÷ *÷÷ ÷ ÷ *÷ * * ÷ * *÷÷ ÷ ~ ÷ ÷ ,÷ , * *÷÷ ÷ ÷ ÷ ÷ ~ • • . ÷~

- - ÷ . - -

I ,I I I I I 1 I I I ] I I I I I I [ CHANNEL NUMBER

Fig. 3B. Neutron time-of-flight spectrum for the 95Mo(p, n)95Tc reaction. The flight time increases from the right to the left.

l O O O -- [ 95Mo(p ,n ) 95T c I I 1 I I I I I I i I I I I I -

500 - - Ep= 5 .025 MeV , 0=33 ° ,

d = 12 .111 me te r s

TH IN TARGET

200 co - -

1 0 0 _ _

_ T I , - I -

_ _ * ÷ ÷ • • *

. * ' * , ÷ *% t ' %

. . . . . . . . "-" . - " . . . . " .7 : '~ . . . . . . ". -

. . . . . ' . . . . " . . . . : . : . . . . . : . . : : • . = . ÷ * *

5 ÷ * ÷ * * ÷ *

2 - -

, I I 1 I I I I I I I I I I I I I I [ I I CHANNEL NUMBER

Fig. 3C. Neutron time-of-flight spectrum for the 95Mo(p, n)95Tc reaction. The flight time increases from the right to the left.

s t a t e s a n d u s e d a long flight-path to resolve them. Of the peaks in this spectrum, t h e o n e a t 1 3 3 2 k e V s e e m s t o b e rea l b u t i t s o r i g i n i s u n c e r t a i n .

42 H.J. KIM et al.

Fig. 4 compares our levels with those previously seen in radioactive decay experi- ments 4,1 ~). Again, the (p, n) reaction populates many more states than does radio- active decay. A level-by-level comparison in the region of high level density is not feasible because the energies from the decay work are not precise enough.

1636 '1618

1435 __ - - 1 4 5 0 1415

1332 1281

I213 1178

1087

9 6 0 . . . . 965 932

670 - - . - - 6 8 0 651 631 - - - - 6 2 5

340 ~ - - 340

L 46 38.9 g.s. ~ g.s.

PRESENT RESULTS DECAY RESULTS

Fig. 4. The levels of 95Tc from the present (p, n) study and the decay results from refs. 4, 1~).

3.3. THE 97Mo(p, n)97Tc REACTION

We found Q = -1.128__+0.009 MeV. 7he uncertainty is larger than for the pre- vious two cases mostly because, for the chosen bombarding energy, the neutrons to the ground state were faster and could not be as well resolved. Only an estimated Q-value, - 1.1 ___ 1.0 MeV, was known 13) previously.

Fig. 5 shows a thin-target spectrum. The level density is seen to be more than for the previous two nuclei. This and the fact that the low Q allows many higher states to be populated result in small yields to the individual states. The counting rates were so low that we could not study this reaction as thoroughly as the others. The resulting energy uncertainties in table 1 are relatively large, the two peaks that are indicated by

I000

--I I I I I ~: I I I I I I I I I I I- __ W

._j 500 - - m

- - ~ C,J

200 - - ~ to t~,n Qo ~ o ~-I ~ ~ ^, -, - - -- o~_ w ~ o '~" LO~Ol~CO t-- ~ ' ~ ~, ~,

E • - " ~ T - - - . L - I ' l ' ~ I S 100

,. 50 .~.....~. ÷i ~

20 -- ** % • , +÷ ,~÷÷ ~ ÷÷÷ ÷ ÷ ÷ **

• " ~ ' 5 ÷ " / 10 .÷,~ 9 7 M o ( p, ,9) 97Tc

5 -- Ep =4.045Mev, 8= 550,

d = 5.987 meters

THIN TARGET 2 - -

1 I I I I I I I I I I I I I 1 I I I CHANNEL NUMBER

Fig. 5. Neutron time-of-flight spectrum for the 971V[o(p, n)97Tc reaction. The flight time increases from the right to the left.

1580 1517 (DOUBLET)

1405 1576

1268 1234 1202

1154

1050

962 - - 969 941

857

785 - - ~ -- 784

I I 662

575 - - - - ] - ' 1 " -T- ' - -570

328 - ~ ~ - - 324

_ L I : 215 ~ - - 2 1 5

PRESENT RESULTS DECAY RESULTS

Fig. 6. The levels of 97Tc from the present (p, n) study and the decay results from refs. 4' ~s, 16).

44 H.S. KIM et al.

energies in parentheses could have come from a contaminant, and the peak at 1517 keV is at least a doublet.

Fig. 6 compares these levels with those observed in radioactive decay experi- ments 4,t5,16). Again, the (p, n) reaction populates more levels than does radio- active decay.

, r - , , b . . . . moo ~ I I I I I I l I 93Nb{p,n)93Mo I I I I l I I I I I

L--- 5 0 0 } - ' - EP = 4 " 6 5 MeV' 8 = 4 0 °

[--- d = 9 . 5 0 meters ~

~'-- ~ THIN TARGET ohZ ~ . " -~

F 4 200 o to ~ * T I -- I I~ ~ ~ ~ ~ I~ I

l o oh CO- - - - 0 4 - - - - - 0 4 ¢x. tt3 >- . . . . . . . . . . I00 ~-

50 - I I n / A

1 o _ - " " ' * " " ' . ' " "" . ' " " ' ) ' " . ' " : L . ' . ' " " " : - -

5 . . . . . . . . . . . . . . . . .

1 I I I I i I I I I I I I I I I 1 1 I , I I I CHANNEL NUMBER

93 93 Fig. 7A. Neutron time-of-flight spectrum for the Nb(p, n) Mo reaction. The flight time increases from the right to the left.

1 0 0 0 1 1 I t I I I 1 I I ' I I I I I I I I I I I I - - - - go _ _

500 go ~ o-, ~ --~ ~- ed _ l-- co ~

~ ,q- ~ 04 goto ~ to ~ u.~ 0 I 2 0 0 L o o-i ~ c o l . . . . . . / ]

0 4 N >. I t . o~ l I O to go t',,I oa 04 04t~ ~0 ~ ro to - - >. to r ~ --

E. I " ~ I : - =1

~ o - : . . . . " ' /.::~..::,i~'i--;:-

I0 ---- e 3 N b ( p , n ) 93Mo ~ * ~ " " " " " " " ~

5 -- Ep'= 5 . 2 6 0 MeV, 8 = 4 0 °, . --

-- d = 9 . 5 0 mete rs * " --

THIN TARGET __ o - -

] I I I I I i ] I I I i I I I I I I ] I I I C H A N N E L N U M B E R

Fig. 7B. N e u t r o n time-of-flight spectrum for the 9SNb(p, n)gSMo reaction. The flight time increases from the right to the left.

91Nb ' 93M0 ' 95.97T¢ LEVELS 45

2989

2914 2875 2895 2845 2850 2829 2768 2753 2758 2725 2754 2715 2679 2664 2650 2639 2645

2580 2570 2548 2552

2486 2474 2448 2456 2425 2450 2416 2401 2408 2555 2550 2510 2502 2315

2255 2242 (2186) 2191 2164 2157 2162 2150

1695 1695 1697

I519 1519 1521 1477 1465 t477 1495

1562 1362 1562 1365

944 944 942

g.s. g.s, g.s. g,s.

PRESENT RESULTS (p , r,,) RESULTS DECAY RESULTS (d, p) RESULTS

Fig. 8. The levels of 93M0 from the present (p, n) study, previous (p, n) results from ref. ~), the decay results from ref. 9) and the (d, p) results from ref. a).

46 H.J. K1M e t aL

3.4. THE 93Nb(p, n)93Mo REACTION

Essentially, we have repeated some of the work of Finckh and Jahnke 7) in an effort to achieve better correlations between the levels from the (p, n) reaction and those found from the decay of 93Tc and from the 92Mo(d, p)93Mo reaction. Only a few levels were seen in the decay work 9), but these were located to within 1.5 keV. More levels were found from the (d, p) reaction s), but these have 20 keV uncer- tainties. Many of the levels observed 7) in the (p, n) reaction could be identified with those populated by these other two processes, but in some cases the correlations were not clear.

Figs. 7A and 7B show two of the several spectra that we obtained. For this reaction, rather than determining the energy scale from the flight time for 7Li(p, n) neutrons, we used the precisely known 9) energy difference between the ground and 1362 keV excited states. We also used the ground state Q-value reported by Finckh and Jahnke 7), but this entered only in a secondary role related to the conversion to the c.m. system. We estimate a -t- 4 keV uncertainty for all of the levels.

Fig. 8 summarizes the 93Mo levels from the various experiments. With the excep- tion of the 2425 keV isomeric state t all of the levels that are populated by radioactive decay appear also to be populated by the (p, n) reaction. Our energies are in good agreement with the energies determined from the decay studies; of course this agree- ment is exact for the 1362 keV level. Apart from the few states that Finckh and Jahnke 7) did not resolve, their results agree with ours. A little better agreement of these experiments with each other and with the decay work would be achieved if their higher levels were shifted up about 5 keV and ours down by the same amount. It seems probable that each of the levels seen in the (d, p) work was also seen here, but a one- to-one correlation cannot be made because the energy uncertainties in the (d, p) work are of the same order as the level spacings.

4. Summary and discussion

By use of (p, n) reactions and a neutron time-of-flight spectrometer we have found levels up to about 2.1 MeV excitation in 91Nb, 1.6 MeV in 95Tc and 97Tc, and 3 MeV in 93Mo. Since (p, n) reactions have only weak selection rules at low energies and the resolution capabilities of our spectrometer are quite good, we believe that we found nearly all the levels except those with spins greater than about 12--~s. This belief is supported by the fact that we found levels in addition to all but the highest spin levels that others had discovered from more selective processes.

These nuclei, having 50, 52 or 54 neutrons, are situated in an interesting mass region just beyond the 50-neutron closed shell. The level structures of the odd-proton nuclei, the isotopes of Nb and Tc, have a systematic mass dependence as demonstrated

t Because of the high spin, _2j_, of this state, its excitation is very weak at this bombarding energy (see ref. 7)).

91Nb ' 93Mo ' 95, 97Tc LEVELS 47

in fig. 9. In this figure we have included the published 17) levels from another Nb isotope, 93Nb, because they were also studied with a non-selective compound nucleus reaction, namely 93Nb(n, n'?)93Nb. The figure shows that, whereas the energy of the first excited state does not have a systematic mass-dependence, that for the second

2200 --

2000

1800

1600

1400

*' t200

laJ z 1000 uJ

8OO

6OO

400

2OO

91 93 95 97, 41Nb50 41Nb52 43TC52 43Tc54

F i g . 9. C o m p a r i s o n o f four odd-mass nucle i wi th N = 5 0 - 5 4 .

state does; it decreases rapidly as neutrons or protons are added. Also the higher states seem to follow the second state down.

48 H. $. KIM et al.

References

1) S. Cohen, R. D. Lawson, M. H. Macfarlane and M. Soga, Phys. Lett. 10 (1964) 195 2) K. H. Bhatt and J. B. Ball, Nucl. Phys. 63 (1965) 286 3) N. Auerbach and I. Talmi, Nucl. Phys. 64 (1965) 458 4) C. M. Lederer et al., Tables of isotopes (John Wiley and Sons, Inc. New York, 1967) 5) K. Way et al., Nucl. data (Academic Press, New York 1966) 6) H. J. Kim and R. L. Robinson, Phys. Rev. 162 (1967) 1036;

H. J. Kim, R. L. Robinson and C. H. Johnson, Phys. Rev. 180 (1969) 1167 7) E. Finckh and U. Jahnke, Nucl. Phys. A l l l (1968) 338 8) A. Yen, F. Ajzenberg-Selove and B. Rosner, Nucl. Phys. A l l l (1968) 100;

S. A. Hjorth and B. L. Cohen, Phys. Rev. 135 (1964) B920 9) P. Alexander and G. Scharff-Goldhaber, Phys. Rev. 151 (1966) 964

10) P. H. Stelson, in Nuclear research with low-energy accelerators, eds. J. B. Marion and D. M.Van Patter (Academic Press, New York, 1967)

11) L. Cranberg, R. A. Fernald, F. S. Hahn and E. F. Shrader, Nucl. Instr. 59 (1961) 335 12) A. Langsford and P. E. Dolley, Nucl. Instr. 59 (1968) 120 13) C. Maples, G. W. Goth and J. Cerny, Nucl. data, 2 (1966) A429 14) F. Rauch, Phys. Lett. 27B (1968) 560 15) B. S. Dzelepow and L. K. Peker, Decay schemes of radioactive nuclei (Publishing House

Nauka, Moscow and Leningrad, 1966) 16) T. Cretzu, K. Hohmuth, G. Winter and J. Schintlmeister, Ann. der Phys. 17 (1966) 1 17) L. Beghian, F. Clickeman, F. Hofmann, F. Mahoney and S. Wilensky, in Proc. of Int. Conf. on

nuclear structure, Tokyo, 1967; Suppl. J. Phys. Soc. Jap. 24 (1968) 190