New isotopes of cobalt; Activation cross-sections of nickel, cobalt, and zinc for 14.8 MeV neutrons

Nuclear Physics 15 (1960) 326--336 ; ~ ) North-Holland Pubhsh,ng Co., Amsterdam Not to be reproduced by photoprint or microfilm wathout written permission from tlJe pubhsher

N E W I S O T O P E S OF COBALT; A C T I V A T I O N C R O S S - S E C T I O N S OF NICKEL, COBALT, A N D ZINC F O R 14.8 MeV N E U T R O N S I L. P R E I S S t a n d R

W. F I N K t t

Department o/ Chemistry, Umvevsttv o/ Arkansan, Fayettemlle, Arkansan t t t Recetved 17 S e p t e m b e r 1959 A b s t r a c t : Absolute n e u t r o n activation cross-sectaons a t 14 8 MeV h a v e been m e a s u r e d for t h e five

stable mckel isotopes, for cobalt, and for zinc, based on comparison w i t h t h e CueS(n, 2n)Cu It reaction (556 mb), t h e CueS(n, 2n)Cu e~ cross-sectaon (1000 mb), a n d the AltT(n, ~)Na s~ reaction (115 mb). During the course of tins investigation, three new cobalt activities h a v e been identified as Coe8 1 404-0 05 h, a n d Co~ roomers C o u = 2 0 ~ 0 2 ram, a n d Co64j, 7 8+ 0 2 nun T h e reactions studied, measured half-byes, a n d a c t i v a t i o n cross-sections are listed in table 1 Comparison is m a d e between t h e experimental cross-sections a n d the c o n t i n u u m model of the c o m p o u n d nucleus described b y B l a t t a n d Weisskopf A discussion is presented concermng t h e possible reasons for t h e devtation of t h e experimental values from those calculated from theoretical considerations.

1. I n t r o d u c t i o n

Several groups have reported nuclear reactions on nickel isotopes such as (n, np) 1, 9, 8), (~, ~np) and (at, x2p) 4, s), (p, np) 6), (p, 2p) v), and (~, pn) and (7, P) s). The unusual feature of these is that those reactions involving proton emission predominate markedly over those involving neutron emission, e.g. a(p, 2p)/a(p, pn) ---- 2.8 '), a(7, p)/a(?, n) = 2.35 s), and a(n, p)/a(n, 2n) = ft.5 for NI ss from values reported in this paper (table I). Allan z) and March and Morton 2), using emulsion techniques, and Colli and Co-workers 9), utilizing a proportional coincidence scintillation counter method, found two groups of low energy protons in excess of that predicted by the continuum model of compound nucleus reactions. These observatlons lead to the conclusion that direct interaction processes contribute to proton emission to an appreciable extent. Levkovskli Io), in summarizing available yield data on (n, p) and (n, at) reactions, has noted sharply decreasing cross-sections with increasing neutron number at constant Z. The five stable nickel isotopes, ranging in N from 30 to * Philhps Petroleum C o m p a n y predoctoral fellow, 1957--1958. Present address H e a v y Ion Accelerator Laboratory, Yale Umverslty, New H a v e n 11, Conn. *t P r e s e n t address Gustaf W e r n e r Instztute for Nuclear Chemistry, U m v of Uppsala, Sweden t i t Supported m p a r t b y t h e U S Atomic E n e r g y Commission A p r e h m m a r y report of this work appeared m Bull. Am. P h y s Soc. 3 (1958) 322, H2 Thin work constitutes p a r t o1 the P h D thesis of I L Pretss a t the U m v e r s l t y of A r k a n s a s 326

I) b) 0) a) e)

Z n 68

Zn'*(n, 2n) Zn** (n, p)

C u 68

-4- 5 sec

2 254-O 02 h

36

36 -4- 2 m m 52 +03mm

13.0 + 0 2 h

90 4-02h 72 d 45 d 2 57-4- 0.02 h

Stable 9 0 -+-0 2 h 72 d 37 ± l h

1.7 4-0 05 h 105 + 0 2 m m 105 -}-0 2 m m

ZlIIL, I [ ~ x u u ~

~,vL~ ~=

2

25 51

±10 ~10

±2o ±1o

tt f) e) n) i)

a

r--

22 22

11

20

19

15 15 15

15

16

16

17

18

(MeV-~)

- - 4 42

--11 8 --22

- - 0 96

--105 --06 +10

1 61 --12.1

1 06

--97

- - 3 14 - - 9 70 - - 0 63

-- 10 86

--5 08

Q-value (MeV)

~ A..~ . . . . . . . . . . . . . . . . . . .

Ratm

10

5

17

0 91

1 57

180

280 45

0412 0 57 79

0 61 016

O 65

81 0 27 380

0.31

0 68 388

~e~pWc~le

365 145 37

320

462

120

17 0 008 30 0.01

67 0 0024

(rob)

Calculated cross-section

Upper hmzt for cross-section of lower state Report to AEC, NCSAG Wash-191 (1956) L Rose and A H Armstrong, Bull Am Phys Soc 1 (1956)224 B L Cohen, Phys Rev 81 (1951) 184 M G Blosser, C D Goodman, T H Handley and M L Randolph, Phys Rev. 100 (1955) 429

7 j)

110 h)

~) g) s) s) t)

254 77

386 590 224 100 80

216 t)

-4-20

284

42 *) 38 a)

560 d)

310 c)

68 b)

80 total b)

39 s) 25 e)

-4,6 4-8

+

-4-5

,4,-15 =h20

-4-2

181 s)

-4-3

82 30

145

4

40 237 52

9

22 -4- 2 38±1

043-4- 0 0 2 t 4 1 -4-0 0 5 t t O 9 3 ± 0 04 2 0 :h 0 s t 3 3 -4- O 0 2 t t 0 6 5 + 0 15

Measured cross-section (rob) Present work [ Other work

C O D a . l ~ a,n u

t Lower h m l t for cross-section of upper state See ref is) See ref 2) See ref 1) K M. Purser and E. W Tltterton, ANU/p-200 (1959) I Kumabe, J Phys Soc J a p a n 13 (1958)325

ZneS (n, p) Zn 6s (n, ~)

CuS*

Zn*°(n, p)

Cue6

Co~g FeS! Mn"

Co~,sm

~167

Co u m Co0Sg

CoeOg COso

CoeOm

Co{Sore

1 7 0 ~ 0 05 h

1 9 4-0 3 m m 138 4-0 2 m m

1 40-+- 0 05 h

2 0 4-0 2 m m 7 8 :J:O 2 mln

Co64m

Coe4g Coe8 Co68m CoeZg CoO* Coe*

Measured half-hfe

nickel,

Product

C o 5' in, 2n) Co" (n, 2n) Co5' (n, p) Co"(n, a)

N#O(n, np) N, 0'(n, p) N*~(n, p) N,Is (n, 2n)

Nl'a(n, p) NlSa(n, p) N,Sa(n, np) NlSS(n, p) Nz" (n, p) N*S~(n, np) Nl't(n, p) N* 6. (n, np) N, eO(n, p) Nx'° (n, p)

Reactmn

Absolute neutron act,vatlon cross-sect*ons f o r

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36, afford an excellent opportunity to study the effect of increasing neutron number on (n, p) and (n, np) cross-sections. Parmley and coworkers 11) have reported a highly uncertain half-life of four to six minutes for the product of neutron irradiations of NI e4 enriched to about 83 %. Using the Arkansas 400 kV Cockcroft-Walton Accelerator, which provides total neutron fluxes ranging from 109 to over 1011 neutrons]sec, it was possible in the present study to resolve the uncertmn ~ 6 mm half-life into two isomeric states of Co64. Cross-sections for (n, 2n), (n, p), and (n, ~) reactions of Co s9 and natural zlnc also were investigated for comparison with those found for the nickel isotopes.

2. Experimental 2 1 SAMPLE IRRADIATION Cross-sections in all cases were measured with 14.8+0.9 MeV neutrons (109 to 1011 neutrons]sec) from the T(D, n)He 4 reaction at 0 ° to the incident deuteron beam on the Arkansas 400 kV Cockcroff-Walton accelerator for periods ranging from 30 sec to 8.9 h. TABLE 2 D a t a for irradiated nmkel motopes Major

tmpurltaes %

Isotope

Form

I

N 1 e4

NI 5.,

95 9 ]

!

Nl**

97 8

2 12 Nl 6s, 0 77

N10 p o w d e r

N15s,

N10 p o w d e r

.NlS0,

N 1 eo, N161 ,

NI st,

07 11 02 02

NI*I

83 1

N I ss, 2 78 NI 6o, 10 4 N1.2, 2 4 N16`, 1 4

NIO p o w d e r

N16°

99 1

N15s, 0 8 %

Ni metal p o w d e r

NI 6s

99 6

N1so,

N l m e t a l foil

!

03

Enriched nickel isotopes (table 2) either as metallic foils or powder or as NiO powder were irradiated Flux momtormg was accomphshed b y utilizing the CuSS(n, 2n) Cu e~ (556 mb) cross-section reported b y Yasumi12), the

NEW

ISOTOPES OF COBALT

329

CueS(n, 2n)Cu e4 (1000 rob) and Al~7(n, at)Na ~4 (115 mb) cross-section reported by Poularikas and Fink 13). Sample and monlfor tlucknesses, in all cases, were approximately the same (3 to 35 mg/cm2). Foils of natural cobalt and zinc metals t were irradiated under similar conditions. In the case of long-lived activities produced in Ni ~ and Co5~irradiations, the cross-section reported in this paper (52 mb) for the N16S(n, 2n)N15~ (37-4-1 h) reaction was employed as a flux monitor. Cross-section determinations were based on at least three runs, and the reported experimental errors are estimated from the maximum deviation from average. 2.2

COUNTING TECHNIQUES

All beta counting was done with 2--~ methane-flow proportional counters having 0.9 mg/crn ~ alununized-mylar end-windows and/or 2 - - n windowless proportional counters. All samples were counted on saturation backscattermg thicknesses of iron and corrected for geometry, self-adsorption, self-scattering, and back-scattering 14,15). Corrections for air absorption and scattering, window absorption, scattering from housing, and counting efficiency are assumed to be identical for samples and monitors. In the case of electron capturing nuclei (Co58 and Ni 57) and for the highly converted isomeric transltion in Coe°m, gross counting rates in end-window and windowless proportional counters were compared to the disintegration rates found by integrating under specific gamma peaks. For the case of Zne4(n, p)Cu e4, efficiency corrections were computed sirmlar to those estimated b y Poulankas and Fink 13). Gamma counting was performed with a 1 × l½ inch NaI (T1) scintillation spectrometer with single-channel analyzer and manual scaler. The total disintegration rate found by integrating under a given gamma-ray peak, and corrected for the branching ratio were then corrected for geometry and for crystal efficiency le). 3. R e s u l t s 3 1 B O M B A R D M E N T S O F E N R I C H E D N1el A N D N1et S A M P L E S

Enriched samples of NIe40 weighing about l0 mg were irradiated with 14.8 MeV neutrons, and gross beta decay was followed and is shown m fig. 1. Half-lives of 2.0-~-0.2 rain, 7.84-0.2 rnin, and 1.40-4-0.05 h were observed togehther with long-lived activities (attributed to an adrnixutre of 9 h Co 5era, 72 d Co 5sg, and 36 h N157, which are products of (n, p) and (n, 2n) reactions on N15s, respectively, (N15s being present in the enriched sample to the extent of t Foils were o b t a i n e d f r o m A D. Mackay, Inc., N e w York. N a t u r a l cobalt, 35 m g / c m e, 100 ~/o, n a t u r a l zinc, 5 m g / c m I, 100 %

330

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2 percent, table 2). The Ni~(n, ~) reaction, which produces 5.5 rain Fe 61, can be ruled out because the threshold lies about 20 MeV iv). Samples of enriched Ni s 2 0 (96 ~o), were bombarded under conditions similar to those in the Ni ~ experiments. Half-lives of 1.94-0.3 min, 13.84-0.2 rain, belonging to Co6~m and Co e~ isomers, respectively, and long-lived activities (9 h Co 5sin, 72 d Co6sg, and 36 h Ni 5v arising from Ni ~) were observed, b u t no 1.70 h Coel was detected. However, under conditions of extremely high flux (4--6) × l0 s neutron/cm~- • sec, the 1.70 h activity was observed. IOOC TIM| IN MiNUT|$ &•T|R END OP •'OMBAOOM|NT [ | I|N| l |N| T| ||T l ~ ~ ~

iOC

40( HISBTI

J#OIT--Li¥|O PORTION OF ||GIY 011 ||PAliO|D ? | i i | ~IIOAL| | AFTER liUBTllAOTIOll • • | NOU• >IViTY }

400

DO0

!,o,\K ~0

-- --X----____ -----

l I 41 keelrs, ¢0(1~11~

TIM|

' *"Of NI ~NIocllo

IN HOUR| AFT|R lIND O• IOilARDM|NT

Fig I Typical gross beta decay from a 30-ram Irradiation of enrxched (96 ~o) NI|4 with 14.8 MeV neutrons.

Samples of enriched NielO (83 %) were bombarded in an attempt to confirm the 187 mb cross-section value reported b y Paul and Clarke IS) for the Ni el (n, p) Coel reaction. The cross-section obtained in the present work was found to be only 2 2 ± 2 mb (see section 3.2). However, even if the value of Paul and Clarke were correct, the amount of Ni el present (0.17 °/o) in the enriched NI ~ sample would account for only 0.04 percent of the amount of 1.40 h activity observed from Ni ~, so that the 1.70 h activity observed in the high flux bombardment of Ni e2 must be attributed to a Nie2(n, np)Co 61reaction (see table I). The Ni62(n, p) Coe2m cross-section was found to be 2.0~0.5 mb, again not large enough to be a contributing factor in the Ni ~ bombardments. Low energy neutrons (2.95 MeV) from the D(d, n)He 3 reaction at 400 kV

NEW

ISOTOPES

Ot t

331

COBALT

also were used to irradiate enriched Ni *t and Ni e2. With Ni e* a half-hfe of 3--6 minutes was found, together long-lived products ( ~ 6 h), but no other activity was observed, which establishes the fact t h a t the 2.0 min activity from Ni** is not identical with the 1.9 min half-life found from Ni e2. Very short bombardments of enriched Ni ~ also were carned out at 14.8 MeV I000

500

|00

Long livedwo~lucts of Na511 rtochOns.

T SO zo.~.c# 4=.

t,t

IZ

flrom Ni64(rt ,p} ,.,^64~1

8 met , . ~

fromN~4(h,p)

tO

\ |i,,, 0

|

3O

,,,,,

|

6O

,,,,,,,, ,

|

90

TiME iN MINUTES AFTER END OF BOMBARDMENT"'----'~ Fxg 2 T y p i c a l gross b e t a decay f r o m a 1.5 m m xrradlatlon of enrxched N1** w i t h 13 8 MeV n e u t r o n s

for periods of 1.5 mm and gross-decay curve shown in fig. 2. The 7.8 mm activity was observed to grow in, b u t no 1.40 h activity was found even though enough of the 2.0 min and 7.8 min acltvlties were formed so t h a t an isomeric state with a half-life either of 2.0 m m or 7.8 rain which decays to a 1.40 h daughter would have been detected. In other experiments, natural nickel foils were bombarded with 14.8 MeV

~

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PREISS AND R W

I~INK

neutrons for durations of less than 1 minute, and no activity between about 1 second and 2 minutes was detected, which rules out the possibility of shorterhved isomers. The gamma-ray spectra of the new cobalt activities produced in the Ni ~ irradiations were investigated with a 3 x 3-inch NaI (T1) scintillation spectrometer with both a 50-channel analyzer and a greywedge analyzer. Because of the initial low intensity of the activity, it was not possible to follow the decay of the individual gamma-rays. However, all gamma-rays were found to exhibit half-lives of several minutes. No gamma-rays of energy above 100 keV were observed to be associated with 1.40 h Co sa also produced in these irradiations. The observed gamma-ray energies from the Co64 isomers are as follows: 0.394-0.1 MeV, 0.664-0.1 MeV, 0.70+0.18 MeV, 0.974-0.14 MeV, 1.37-]-0.27 MeV and 2.344-0.6 MeV. The most intense gamma-rays appeared at energies of 0.66, 0.97 and 1.34 MeV. The 1.34 MeV gamma Is in agreement with a known level in N1~ found in the positon decay of Cu ~ 19), and so it serves as additional evidence for the mass assignment of the Co ~ isomers. 3 2

BOMBARDMENTS

OF

ENRICHED

N I a° A N D

N161 S A M P L E S

The cross-section for the Ni61(n, p)Co el (1.70 h) reaction was measured as described above. At relatively high neutron fluxes (6--9 × 10s neutrons/cm 2. sec). a 10.5 rain activity was observed, possibly arising from the (n, np) reaction on NieL To obtain an accurate value for this cross-section, it was necessary to obtain a value for the Nie1(n, p)CoSlm(10.5 rain) reaction. Samples of enriched Ni 6° metal were irradiated under similar conditions, and the decay of the 10.5 mm Coe° isomer was followed by: (I) following the total decay with a beta proportional counter and (2) b y following the decay of the 1.33 MeV gamma populated from fl--deeay of the isomeric state. Since 0.3 ~/o of all decay of the isomeric state proceed b y fl- transitions, I00 ~/o of which populate the 1.33 MeV level of Ni ~° 20), it was possible to determine the counting efficiency of the beta-proportional counter for gamma-rays and conversion electrons. This correction of approximately 3 ~/o was applied to the gross-decay observed from Ni el and the contribution of the Ni6°(n, p)Co e°m (10.5 min) {94-2 mb) reaction subtracted. After making the counting efficiency and Ni e° contributmn corrections, the N161(n, np)Co e°m, (10.5 min), cross-section was found to be 44-1 mb (see table I). 3.3

BOMBARDMENTS

OF

ENRICHED

N# a

Foils of enriched Ni bs metal, 13 mg]cm ~ thick, were ]ITadiated, the gross beta-decay was followed, and the cross-section for the Ni 58(n, 2n)Ni s~ (37 4-1 h) reaction was measured. Having determined an accurate value for the Ni68(n, 2n)Ni ~7 cross-section (524-5 mb, see table 1), it was feasible to attempt a measurement of the NiSS(n, p) cross-section.

NEW

I S O T O P B S OF COBALT

333

Twenty mg of enriched Ni 68 foil were irradiated for 8.9 hours with a mean flux of 5 × 10l° neutrons/sec, and a 10 mg portion of the foil was counted for gross-decay with 2 - - ~ end-window and windowless proportional counters. The remaining 10 mg was treated chemically to separate Co 58 and N1~7 reaction products. The 10 mg sample selected for radio-chemical separatlon was dissolved in 10 ml of 7 N HCI, evaporated to 2 ml, and dropped onto a Dowex-1 anion exchange column, 10 cm × 1 cm dia which had been previously washed with 7 N HC1. The column was eluted with 3.5 ml of 7 N HC1 which removed the nickel fraction quantitatively. In the nickel fraction, decay of the 0.511 MeV annihilation gamma resulting from positon decay of Ni 57 was followed with a NaI (T1) scintillation spectrometer The cobalt fraction was collected b y further eluting the column with 8 ml of 3 N HC1, and its decay was studied with a manual single-channel scintillation spectrometer b y following the decay of its well known 0 811 MeV gamma-ray ,1). Positive identification was made from the gamma spectrum of a standard Co~8 sample obtained from Oak Ridge National Laboratory, and b y comparison with standard spectra ~2). Cross-section values obtained b y integrating under the 0 811 MeV gamma peak and from following the gross-decay agreed with one another to within 3 % and are shown in table 1. 3 4. B O M B A R D M E N T S

OF COBALT

In order to differentiate between Co59(n, p)Fe 59 (45 d) and Co59(n, 2n)Co 58s (72 d) reactions, it was necessary to separate the products of the neutron irradiation of Co 59 chemically. This was accomplished as follows: 40 m g of Co ~9 loll (35 m g / c m ~) was Irradiated for 8.9 hours. A portion (20 mg) was dissolved in 4 N HCI and dropped onto an anion exchange column I0 c m × I c m diameter packed with Dowex-I and previously made 4 M in HCI The column was eluted with 9 ml of 3 N HCI and the Co-Mn fraction collected, evaporated, and counted in a 2--~ windowless counter. Also the 0.81 M e V g a m m a of Fe 5s 21) was followed with a scintillationspectrometer. The decay of the remaining 20 m g of foil was followed on both windowless and end-window proportional counters. Cross-sections for 9 h Co 58m and 72 d Co58~(n, 2n) products were determlned as well as values for the CoSg(n, p)Fe 69 (45 d), and Co59(n, a)Mn as (2 56 h) and are tabulated in table I. The cross-sections found for CoS*(n, 2n)Co 5sin (150-[-5 rob), and Co59(n, a)Mn 5e (304-2 rob), in the separated samples agreed to 4-5 % with those found in counting Co 5~ neutron irradiated foils in 2--zt windowless counters. 3 5 BOMBARDMENTS

OF N A T U R A L ZINC

Foils of natural zinc (5 mg/cm 2) were irradiated and gross-beta decay followed The gross decay curves resolved into the following half-lives: 13.0+0 2 h,

334

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2.24-0.2 h, 364-2 min, 5.24-0.3 rain and 364-5 sec, corresponding to the Znei(n, p)Cu ~t, ZneS(n, ~)Ni u, ZnU(n, 2n)Zn ~, Znee(n, p)Cu ~ and ZnSS(n, p)Cu *8 respectively. The cross-sections for these reactions are tabulated in table 1.

4. D i s c u s s i o n

On the basis of the Ni ~t irradiations, the following mass assignments are proposed: Nie4(n, p)Co ~m (upper state), 2.04-0.2 min; Ni~t(n, p)Co ua (lower state), 7.84-0.2 min; Nie4(n, np) Co~, 1.404-0.05 h (table 1). A comparison of experimental with theoretical cross-sections based on the compound nucleus theory of Blatt and Weisskopf 28) are shown in table 1. The calculations were based on the level density formula 28) oJ(E) = C exp(2~/a-E), where C and a are empirical constants taken from NYO-636 24). Variation of the level densities among even, odd-mass and odd-nuclei is generally expressed by means of C, the relation taken from Brown and Muirhead 25) as 12 Ceven = 2.4 Codd--m.. = Codd.

Q-values were obtained from atomic mass tables based on experimental data zo) and tables computed from a semi-empirical mass formula s~). Theoretical estimates of the (n, 2n) cross-sections were computed from the formula given by Blatt and Weisskopf 28) with the threshold energies taken from Segr~ ,8). The (n, p) cross-sections calculated from compound nucleus theory 23) follow a trend similar to that predicted by Levkovskii 10). However, any theory having a large {)-value dependence will exhibit this property in the case of the nickel isotopes. The trend toward decreasing (n, p) cross-section with increasing neutron number is, evidently, an effect over and above the Q-value dependence as pointed out by Poularikas and Fink la). Experimental results based on charged particle detection by nuclear emulsions 1-4), indicate that direct interaction processes contribute to the observed in, p), (n, np) and (n, pT) reactions with Ni 6s and Ni e°. Brown and Muirhead **) have proposed a direct interaction model based on the Fermi gas approximation i.e. an equal probability of chrect interaction of the incident neutron with all nucleons. On the other hand, Wilkinson 30) and Butler 81) have pointed out that interactions of the incident neutron with nucleon "clusters" in the nuclear surface contribute significantly. Consequently, the anticipated shell structure effect might make the predictions of Brown and Mmrhead too large in most cases.

NEW

ISOTOPES OF COBALT

3~5

One possible explanation of the relatively large (n, np) cross-sections (as compared to the compound nucleus prediction) as well as the three proton energy groups (approximately 4, 8 and greater than 10 MeV) found in emulsion work 1-4) which exhibit a slightly anisotropic distribution is a direct interaction of the incident neutron with a proton, followed by a subsequent deexcitation b y the emission of a neutron or gamma-ray. This approach is analagous to the (n, 2n) process explanation offered by R e m y and Winter-*2). If it is assumed that nucleons in the filled f½ proton level of nickel lie near the surface, then these eight protons outside the 20 proton closed shell lend themselves to a direct interaction process resulting in (n, pn), (n, p) and (n, p?) reactions. With increasing neutron number in excess of the 28-neutron closed shell, direct interactions leading to (n, 2n) processes will become increasingly more important. This aspect offers itself to a qualitative explanation of the near integral decrease in (n, p) cross-sections. For the specific cases of Ni 5s and N16° another factor must be considered; specifically, Jensen a3) has predicted that the neutron coupling energy in the region of 28 neutrons amounts to some 3 MeV per neutron pair. If the (n, 2n) reaction is considered to proceed according to the above mechanism, it is clear that a direct interaction with the fi protons will predominate if only because they are more available for surface interactions, hence we can consider the (n, n'7) and proton emitting processes as being more energetically favorable than an (n, 2n) reaction. The Ievel densities of the product nuclei also have an effect, in addition to the shell structure considerations of the target nucleus mentioned above. The large (n, ~) cross-sections of Co59 and Zn 6s, as compared to the compound nucleus theory, lends support to the nuclear cluster idea. The possibihty of a pseudo-deuteron cluster model for the isotopes of Ni m a y explain the relatively large (n, np) cross-sections 34.ss). A comparison of a~/aeale for the nickel and zinc isotopes shows an apparent overemphasis on compound nucleus formation for nickel and theoretical values which are too small to explain the experimental cross-sections of the zinc isotopes. A conclusion as to the relative competition between compound nucleus formation and direct interaction for Ni and Zn must be based on angular distribution data which are not yet available for the latter and limited to only N1~ and Ni e°. The authors wish to thank Dr. R. Scalan and Dr. R. G. Wille for assistance m bombardment procedures and for help in counting. We wish to acknowledge the help of Mr. A. Poularikas on theoretical computations and the assistance of Mr. J. E. W r a y of the Accelerator Laboratory for operation of the accelerator during bombardments.

336

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References 1) 2) 3) 4) 5) 6) 7) 8) 9)

10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35)

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