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Short-lived activities produced in 70Zn with 14 MeV neutrons
Nuclear Physics A249 (1975) 166-- 172, (~) North-Holland Pubhshin# Co, Amsterdam Not to be r e p r o d u c e d by p h o t o p r m t or m,crofilm without written permzsslon from the pubhsher
S H O R T - L I V E D A C T I V I T I E S P R O D U C E D I N 7°Zn W I T H 14 M e V N E U T R O N S W . L. R E I T E R , W . H B R E U N L I C H
and P HILLE
blstltut fur Radtumforschung und Kernphystk, A-1090 Vienna, Austrta
Received 16 Aprd 1975 Abstract: Beta spectra have been studied after 14 MeV neutron lrradiatmn of 7°Zn Beta-ray singles spectra and fl-y coincidences using plasttc and NaI(T1) scmtdlatmn counters were measured to determine total decay energies for (1 +) 7°"Cu (T~ = 4-4-1 s) and for (5-) 7°bCu (T½ = 52±4s) The decay energies found are 6 17±0 11 MeV and 6 31-t-0 11 MeV for 7°aCu and 7°bCu respectwely. Five fl-branches and four y-rays could be placed m the decay scheme A 3 83±0 09 MeV fl-branch belongmg to the decay of 67N1 (Z4= = 16±4s) has also been found
E
RADIOACTIVITY 7°Cu [from 7°Zn(n, P)I, 67N1 [from V°Zn(n, ct)], measured T+, E#, I#, fly-corn, deduced logft, E # m a x , Q# 7°Zn deduced levels, J, ~r
1. Introduction After the irradiation o f an enriched 7°Zn sample by 14 MeV neutrons three shortlived activities have been observed According to the w o r k of T a f t et al. ~) these activities have been assigned to two isomers o f 7°Cu(7°aCU, T~ = 4_+ 1 s, 7°bCu, T~ = 52_+ 5 s) and to 67NI(T~ = 16_+ 4 s). The main purpose o f the present study was the investigation o f fl-rays f r o m 7°"Cu and 7°bCu in order to determine decay energies and to find out which one is the g r o u n d state o f 7°Cu The results o f M e a s o n and K u r o d a 2) disagree with those o f T a f f e t al. l) M e a s o n and K u r o d a z) were not equipped to observe half-hves shorter than 30 s and assigned the 50-t-3 s actwlty to 67N1. Because o f th~s not a fully satisfactory situation, further investigation o f the short-lived actlwties induced by 14 M e V neutrons m 70Zn seemed to be interesting and rewarding. (A more detaded presentation o f this w o r k is gwen m the thesis by Relter 3) )
2. Experimental procedures The actlwt~es were p r o d u c e d m a 22 m g Z n metal bead enriched with V°Zn (78.3 °o V°Zn, 9 4 % 68Zn, 0 7 ~ 67Zn, 3 9 % 66Zn and 7 7 % 64Zn) made by O a k Ridge N a t i o n a l L a b o r a t o r y The n e u t r o n source was a 200 kV C o c k c r o f t - W a l t o n generator using the 3H(d, n)4He reaction for neutron p r o d u c t i o n The neutron yield was 5 × 10 t o n/s in 4n The useful flux was o f the order o f 10 ° n/s. A fast pneumatic transport system was used to move the sample m o u n t e d in a polyethylene "rabb~t" f r o m the irradiation 166
7°Zn
167
position to the counting locatmn (in the accelerator control room). The transfer time was 0 4 s The y-ray spectra were measured by a Ge(L 0 spectrometer with a resolution of 2.2 keV ( F W H M for 6°Co) and a 19 1 peak/Compton ratio. Because of the high energetic 7-ray background, producing Compton electrons in a large plastic sclntdlator, a ]?-spectrometer was used, consisting of a 12 7 x 12 7 cm 2 plastic sclnttllator and a proportional counter in front. The signals from both detectors were counted in coincidence A drasttc reduction of the y-background and thus a considerable improvement of the shape of the//-spectra was achieved To keep //-absorption low, thin AI fods (4 lng/cm 2) were used as proportional counter windows; the counting gas was Ar (95 % ) + COz(5 %) at normal pressure A schematic drawing of the//-spectrometer is given in fig. 1. Because of the low efficiency of the p r o p o m o n a l counter for 7-rays the background counting rate m the experimental area was reduced by an average factor of 250 It
GAS
//
G5cm
•
/ /
,
/
.// / /" /" z/
.
/
/ /
,
/
// //
/ "//// / / /
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/ // ~"
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1
~1 .,-- 3 5 --,,, cm cm
127 ¢rn
,,.
Fag 1 Beta-spectrometer wlth (1) pneumatic tube, (2) propomonal counter, (3) plastic scmtdlator, (4) photomultlpher 58 AVP and (5) rabbit
The fl-? coincidences were measured between a 12.7 x 12.7 cm e plastic scintillator and a 7.6 x 7 6 cm z NaI(T1) detector To determine the fl-ray absorption (both for fl-singles and fl-7 coincidence measurements), fl-radlators of different thicknesses. equivalent to the thickness of the Zn sample, wlth well-known end-point energies were compared with the fl-end-polnts of 70aCu, 70bCu and 67N1. The samples used for the cahbratmg procedure were
168
W L. REITER et al
(a) 6ZCu (2.923_+0007 MeV fl-end-pomt energy4)), produced by the reaction 63Cu(n, 2n)62Cu, (b) ~SRb (with fl-end-polnts at 5 17___0.12 MeV, 3 33_+0.12 MeV and 2.44_+0 12 MeV, ref. 5)), after Irradiating Sr(NO3)2 wa the reaction 88Sr(n, p) 88Rb, (c) 16N(with fl-end-points at 10.44 MeV and 4.27 MeV, ref. 6)), after irradmtmg (C5H802) n, via the reaction 160(n, p)16N. Thus the fl-spectrometer could be cahbrated in a wide energy range, from 2 5 MeV up to 10.4 MeV for thick samples The fl-end-point energies were determined from F e r m l - K u n e plots. A Fortran leastsquare fit program was used for the analysis of fl-spectra*. The region for the best fit to the data of each fl-component was determined by a g2-mmimum procedure. The evaluation of fl-spectra with Fermi-Kurle plots was tested by fl-spectra of the calibration sources (see above) The data given in literature could be reproduced
3. Results 3.1. GAMMA-RAY SPECTRA To keep activation of long-living nuchdes low, the 7°Zn sample was irradiated for 10 s. The counting periods were 15 s and 100 s in succession. To Improve statistics, 120 irradiahon cycles were recorded for one spectrum. Excellent agreement with the results of Taft et al 1) was achieved. Thus our results are not reproduced here and furthermore we will refer to the 7-data obtained bk Taft et aL because of their better statistics. The 7-ray at an energy of 884 8 keV was used for the determination of half-lives; two components with 4_+ 1 s and 52 + 5 s half-lives could be determined 3 2 SINGLES fl-RAY SPECTRA After an irradiation time of 10 s, the fl-actlVltles were stored successively in four sections of a multlchannel analyzer ( N D 2200) for 15 s, 45 s, 100 s and 100 s to separate the different half-life components. The procedure was repeated 143 times to give suitable statistical accuracy. In the first two counting periods fl-end-points of 6.09_+0.16 MeV and 3 83_+0.09 MeV were found In the last two sections fl-endpoints with 4 5 5 _ 0.12 MeV and 2.57_+ 0.11 MeV were determined. Besides the two above-mentioned half-byes, multlscahng of the fl-achvltles showed a third half-life component with 16_+4 s. The correction for background fl-achvitles due to oxygen contamination in the polyethylene rabbit was determined experimentally. The spectra are shown in fig 2 3 3 BETA-GAMMA COINCIDENCE MEASUREMENT The spectra were observed in coincidence with a 440 keV broad region around the ),-peak formed by the two hnes at 884.8 keV and 901 7 keV The measurement was t The calculatJons were done at Interfakultares Rechenzentrum d Umv Wlen
7°Zn
o
x
x
.-1
169
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13.
Z
O
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1
(d)
5
.~
I0
20
30
0
\,
10
20
30 40 50 CHANNEl_ NUMBERx 6
7b
s 60 CHANNEL NUMBERx
Fig 2 Smg|es E-ray spectra after irradmtlon o f ~°Zn, irradmtlon time I0 s: (a) first section, accumu|atlon tlme IO s, (b) fourth section, accumulation time 100 s, (c) second sectmn, accumu]atmn tlme 45 s, (d) third sechon, accumu|atlon time lO0 s
carried out as a two-parameter experiment The ]?-spectra were digitized into 128 channels, the y-spectra into 8 channels, four of these groups were counted m succession to get information of the time dependence of the coincidence events. To give sufficient statistics, 120 repetitions were accumulated cychcly. The Irradlatmn time was 10 s, the achvlties were counted for 10 s, 20 s, 40 s and 300 s. Beta endpoint energies with 5.42___0.25 MeV for the short-lived achvlty and 4 62_+0.22 MeV, 3.37_+0.17 MeV and 2 . 3 3 + 0 15 MeV for the long-lived activity were found
4. Discussion
4 1 THE DECAY OF 7°=Cu AND 7°bCu The assignment of the 4 s and the 52 s activity to two isomers of 7°Cu is supported by the present results. Using the measured ]?-slngle~ and ]?-? coincidence end-point energies in addition to the associated y-rays, best Q-values and best values for the energy differ-
170
W L REITER et al
TABLE 1 Experimental fl-end-pomt energtes and best Q-values, end-point energies and energy d~fference 70bCu.70aCu
fl-transmon 7OCu(1+) -:~ 7OZn(O+) 7OCu(1+) --~ 7OZn(2+) 7OCu(5-) -~ 7OZn(4+) 7°Cu(5-) ~ 7°Zn((4, 5, 6)-) 7OCu(5-) --~ 7OZn(5-)
Measured end-points (MeV) singles comc
Best values (MeV)
6 0 9 ± 0 16 4 55±0.12 2 574-0 11
5.42±0.25 4.62±0.22 3.374-0.17 2 3 3 ± 0 15
5 29:]:0 11 4 53±0.11 3.27±0.11 2 5 2 ± 0 11
Q-value 7°~Cu(l +) ~ 7°Zrl(O+) 7ObCu(5- ) -+ 70Zn(O+ ) 7°bCu'7°aCu
6.17~0.11 6.31 ±0.11 0 24±0.16
0 1 0 ± 0 09
0 1 4 ± 0 08
ence of both isomers have been determined. Since the asstgnment of the lowest energetic fl-btanch is somewhat tentative, it has not been used. Here, a mean value of 2.52+0.11 MeV for singles and coincidence end-points was found. All other best value fl-end-point energies have been determined flom the Q-values using the 7-energies. A compilation of the measured fl-smgles and coincidence end-point energies and their best values, the Q-values and the energy difference for the 7°Cu isomers is given m table 1. The errors gwen are standard deviations including systematic errors. The Q-values for 7°aCu and 7°bCu are in fair agreement with the calculated decay energy of 6.54 MeV for 7°Cu from the mass table of Garvey et al. 7). The 6.09+0.16 MeV//-branch (measured m the first time section of the single fl-measurement) with a branching ratio of (46+ I 0 ) ~ is assigned to the ground state transition of 7°~Cu. The transition seems to be allowed (log J? = 5.2), hke the branch to the first excited 2 + state (log f t = 4.5) with 5.29+0.11 MeV end-point energy and a transition rate of (54+ 10) ~ . Thus from the possible spin-panty values, (1 +) has been assigned to the 7°Cu state with a 4 s half-life. The 4.53+0.11 MeV branch (measured in the last t~me section of the fl-smgles and fl-~ coincidence measurement) from 70bCu,with a branching ratio of (30_+ 3) %, is very likely to feed the level at 1786.5 keV. The comodence spectrum showed fl-end-pomts at 3 37+0.17 MeV and 2.33-+0 15 MeV for the 52 s 7°bCu asomer. The branching ratios of these two fl-rays are (60-+ 5) }o and (10-+ 5) %, the logft values 4.9 and 4 3, respectively. With regard to the energy balance, the 3.37 fl-transltlon feeds the level at 3038 2 keV. [According to Taft et al ~) one can assume that the levels i n 7 ° Z n up to an excitation energy of 3.5 MeV should be populated by fl-branches.] Cooper et al 8) found a level at 3048 keV m their (e, e') scattering work (with no spin-panty assignment), which can be Jdentxfied with the level at 3038 2 keV given by ref. a), but no level of this energy has
7°Zn
171
been found by Alpert et al. 9) m their s-particle scattering work and with the (p, p') reaction studied by Calderbank et al. lO). A comparison of the transition rate of the 2.33___0.15 MeV fl-branch (measured m the coincidence experiment) to that of the 7-transitmn of 1108.7 keV (9_+2) ~o given mref. ~) suggests a 7°Zn level at 4146.9 keV, which decays to the level at 3038 2 keV. An in&cation of a level m 7°Zn around 4200 keV is gwen by the (~, ~') scattering experiment of Alpert et aL 9) with spin-parity 5-. This weak transltmn has not been analyzed m the fl-V coincidence measurement of Taft et al. 1). (5") (1 +)
52.+5s
Eo
7°bc u
10 9 f t
52-+011
10±5
43
27-+011 53+-0.11
60+-5 30+-3
4.9 5 5
2 9 -+ 011
64+-10
4 5
17+-0.11
46-_10
5.2
5") --
617-+011
~'o
-r- - - 4146.9
338.2
631-'011
4+)
8848
L=---L--__
0
7OZn F]g. 3 Decay scheme of 7°Cu isomers, thin work. For 7'-ray energies see ref 1)
The Q-values for 7°aCu and 7°bCu as mean values of all fl-transltlons plus v-ray energies (weighted with the reclprocal square of the statistlcal errors) are 6.17+0 11 MeV and 6.31 _0.11 MeV, respectwely. In the same way, one gets 0 14___0.08 MeV for the difference between the decay energies of the two isomers Thus the longhvlng isomer has been assigned to be the energetically higher one. This is supported by the spin-panty assignment of (5-) for 7°bCu given by Taft et al. 1), because it is unhkely that the 70Cu ground state is not (1 +) (strong Nordhelm rule) Furthermore, in accordance with the (5-) assignment of the 7°bCu and (4 +) for the level at 1786 5 keV m 7°Zn [for the &scusslon of the (4 +) assignment of this level see Taft et al 1)] the log f t value of the 4.53+0.11 MeV fl-branch has been determined to be 5.5, suggesting a first-forbidden transition. The level at 3038.2 keV may be ass]gned neg-
172
w . L . REITER et al
atlve p a n t y with spin 4, 5 o r 6, since 70bCu decays to this level with a n allowed log f t value o f 4.9. O u r p a r t m l decay scheme for the 7°Cu isomers IS shown in fig. 3. 42 THE DECAY OF 67N1 T h e mass &fference for 67N1 a n d 67Cu, given by G a r v e y et al. 7) is 4 17 M e V T a f t et al 1) c o n s i d e r e d their 3.7 _ 0.4 M e V fl-branch to be the g r o u n d state transition. This w o r k gives a value o f 3 . 8 3 + 0 . 0 9 M e V ( l o g f t = 4.5). W a h one n e u t r o n in the p~ subshell one c a n expect ( ½ - ) f o r the g r o u n d state o f 67N1, in a n a l o g y to the spins o f 6aCu a n d 65Cu [ref. 11)], (½)- IS a p o s s i b l e a s s i g n m e n t for 67Cu. This w o u l d be in a c c o r d a n c e with a n a l l o w e d t r a n s i t i o n to the g r o u n d state. T h e first excited state r e p o r t e d f r o m the (~, p ) r e a c t i o n 12) is 0.71 M e V with spin ½. T h e difference between the fl-energy observed in this w o r k a n d the Q-values r e p o r t e d by G a r v e y et al. 7) is 430 keV, which p r o v i d e s n o s t r o n g a r g u m e n t a g a i n s t a fl-branch to the first excited level. T a f t et al. l ) find a t r a n s i t i o n with a n energy o f 708.5 keV. T h e d e c a y scheme o f 67Ni p r o p o s e d b y M e a s o n a n d K u r o d a 2) c o u l d n o t be affirmed. O n the o t h e r h a n d , their o b s e r v a t i o n o f the long-lived 67Cu activity (T~r = 62 h ) is a p r o o f t h a t 67N! is f o r m e d d u r i n g fast n e u t r o n i r r a d i a t i o n o f 7°Zn. T h e a u t h o r s w a n t to t h a n k Prof. B. K a r h k for h e r k i n d interest a n d s u p p o r t o f this w o r k W e w a n t to t h a n k Dipl. Ing. G. Stengl for help d u r i n g the experiments. W e t h a n k P r o f H V o n a c h for r e a d i n g the m a n u s c r i p t a n d f o r useful discussions.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
L M Taft, B K. S Koene and J. van Khnken, Nucl Phys. A164 (1971) 565 J L. Meason and P K Kuroda, Phys. Rev 138 (1965) B1390 W. L Relter, Thesis, Umv. Wlen, 1974 H Verheul, Nucl. Data Sheets for A = 62, Nucl Data B2-3 (1967) 11 C D Goodman et al, Nucl. Data Tables A8 (1970) 353 C. M. Lederer, J M. Hollander artd I. Perlman, Table of isotopes (Wdey, New York, 1968) G T Garvey, W J Gerace, R. L. Jaffe, I Talml and I. Kelson, Rev. Mod Phys. 41 (1969) SI L R. Cooper, H T Easterday and C D. Zafiratos, Nuovo Clm 69A (1970) 545 N. M. Alpert, E J Martens and W. Pickles, Bull. Am. Phys. Soc. 15 (1970) 62 M. Calderbank, E J Burge, V E. Lewis, D A Smith and N. K. Ganguly, Nucl Phys A105 (1967) 601 11) Nucl Data B2-6 (1968) 75 12) S. C Panchoh and W B. Ewbank, Nucl Data Sheets for A = 67, Nucl Data B2-6 (1968) 71
S H O R T - L I V E D A C T I V I T I E S P R O D U C E D I N 7°Zn W I T H 14 M e V N E U T R O N S W . L. R E I T E R , W . H B R E U N L I C H
and P HILLE
blstltut fur Radtumforschung und Kernphystk, A-1090 Vienna, Austrta
Received 16 Aprd 1975 Abstract: Beta spectra have been studied after 14 MeV neutron lrradiatmn of 7°Zn Beta-ray singles spectra and fl-y coincidences using plasttc and NaI(T1) scmtdlatmn counters were measured to determine total decay energies for (1 +) 7°"Cu (T~ = 4-4-1 s) and for (5-) 7°bCu (T½ = 52±4s) The decay energies found are 6 17±0 11 MeV and 6 31-t-0 11 MeV for 7°aCu and 7°bCu respectwely. Five fl-branches and four y-rays could be placed m the decay scheme A 3 83±0 09 MeV fl-branch belongmg to the decay of 67N1 (Z4= = 16±4s) has also been found
E
RADIOACTIVITY 7°Cu [from 7°Zn(n, P)I, 67N1 [from V°Zn(n, ct)], measured T+, E#, I#, fly-corn, deduced logft, E # m a x , Q# 7°Zn deduced levels, J, ~r
1. Introduction After the irradiation o f an enriched 7°Zn sample by 14 MeV neutrons three shortlived activities have been observed According to the w o r k of T a f t et al. ~) these activities have been assigned to two isomers o f 7°Cu(7°aCU, T~ = 4_+ 1 s, 7°bCu, T~ = 52_+ 5 s) and to 67NI(T~ = 16_+ 4 s). The main purpose o f the present study was the investigation o f fl-rays f r o m 7°"Cu and 7°bCu in order to determine decay energies and to find out which one is the g r o u n d state o f 7°Cu The results o f M e a s o n and K u r o d a 2) disagree with those o f T a f f e t al. l) M e a s o n and K u r o d a z) were not equipped to observe half-hves shorter than 30 s and assigned the 50-t-3 s actwlty to 67N1. Because o f th~s not a fully satisfactory situation, further investigation o f the short-lived actlwties induced by 14 M e V neutrons m 70Zn seemed to be interesting and rewarding. (A more detaded presentation o f this w o r k is gwen m the thesis by Relter 3) )
2. Experimental procedures The actlwt~es were p r o d u c e d m a 22 m g Z n metal bead enriched with V°Zn (78.3 °o V°Zn, 9 4 % 68Zn, 0 7 ~ 67Zn, 3 9 % 66Zn and 7 7 % 64Zn) made by O a k Ridge N a t i o n a l L a b o r a t o r y The n e u t r o n source was a 200 kV C o c k c r o f t - W a l t o n generator using the 3H(d, n)4He reaction for neutron p r o d u c t i o n The neutron yield was 5 × 10 t o n/s in 4n The useful flux was o f the order o f 10 ° n/s. A fast pneumatic transport system was used to move the sample m o u n t e d in a polyethylene "rabb~t" f r o m the irradiation 166
7°Zn
167
position to the counting locatmn (in the accelerator control room). The transfer time was 0 4 s The y-ray spectra were measured by a Ge(L 0 spectrometer with a resolution of 2.2 keV ( F W H M for 6°Co) and a 19 1 peak/Compton ratio. Because of the high energetic 7-ray background, producing Compton electrons in a large plastic sclntdlator, a ]?-spectrometer was used, consisting of a 12 7 x 12 7 cm 2 plastic sclnttllator and a proportional counter in front. The signals from both detectors were counted in coincidence A drasttc reduction of the y-background and thus a considerable improvement of the shape of the//-spectra was achieved To keep //-absorption low, thin AI fods (4 lng/cm 2) were used as proportional counter windows; the counting gas was Ar (95 % ) + COz(5 %) at normal pressure A schematic drawing of the//-spectrometer is given in fig. 1. Because of the low efficiency of the p r o p o m o n a l counter for 7-rays the background counting rate m the experimental area was reduced by an average factor of 250 It
GAS
//
G5cm
•
/ /
,
/
.// / /" /" z/
.
/
/ /
,
/
// //
/ "//// / / /
/
/ // ~"
/
1
~1 .,-- 3 5 --,,, cm cm
127 ¢rn
,,.
Fag 1 Beta-spectrometer wlth (1) pneumatic tube, (2) propomonal counter, (3) plastic scmtdlator, (4) photomultlpher 58 AVP and (5) rabbit
The fl-? coincidences were measured between a 12.7 x 12.7 cm e plastic scintillator and a 7.6 x 7 6 cm z NaI(T1) detector To determine the fl-ray absorption (both for fl-singles and fl-7 coincidence measurements), fl-radlators of different thicknesses. equivalent to the thickness of the Zn sample, wlth well-known end-point energies were compared with the fl-end-polnts of 70aCu, 70bCu and 67N1. The samples used for the cahbratmg procedure were
168
W L. REITER et al
(a) 6ZCu (2.923_+0007 MeV fl-end-pomt energy4)), produced by the reaction 63Cu(n, 2n)62Cu, (b) ~SRb (with fl-end-polnts at 5 17___0.12 MeV, 3 33_+0.12 MeV and 2.44_+0 12 MeV, ref. 5)), after Irradiating Sr(NO3)2 wa the reaction 88Sr(n, p) 88Rb, (c) 16N(with fl-end-points at 10.44 MeV and 4.27 MeV, ref. 6)), after irradmtmg (C5H802) n, via the reaction 160(n, p)16N. Thus the fl-spectrometer could be cahbrated in a wide energy range, from 2 5 MeV up to 10.4 MeV for thick samples The fl-end-point energies were determined from F e r m l - K u n e plots. A Fortran leastsquare fit program was used for the analysis of fl-spectra*. The region for the best fit to the data of each fl-component was determined by a g2-mmimum procedure. The evaluation of fl-spectra with Fermi-Kurle plots was tested by fl-spectra of the calibration sources (see above) The data given in literature could be reproduced
3. Results 3.1. GAMMA-RAY SPECTRA To keep activation of long-living nuchdes low, the 7°Zn sample was irradiated for 10 s. The counting periods were 15 s and 100 s in succession. To Improve statistics, 120 irradiahon cycles were recorded for one spectrum. Excellent agreement with the results of Taft et al 1) was achieved. Thus our results are not reproduced here and furthermore we will refer to the 7-data obtained bk Taft et aL because of their better statistics. The 7-ray at an energy of 884 8 keV was used for the determination of half-lives; two components with 4_+ 1 s and 52 + 5 s half-lives could be determined 3 2 SINGLES fl-RAY SPECTRA After an irradiation time of 10 s, the fl-actlVltles were stored successively in four sections of a multlchannel analyzer ( N D 2200) for 15 s, 45 s, 100 s and 100 s to separate the different half-life components. The procedure was repeated 143 times to give suitable statistical accuracy. In the first two counting periods fl-end-points of 6.09_+0.16 MeV and 3 83_+0.09 MeV were found In the last two sections fl-endpoints with 4 5 5 _ 0.12 MeV and 2.57_+ 0.11 MeV were determined. Besides the two above-mentioned half-byes, multlscahng of the fl-achvltles showed a third half-life component with 16_+4 s. The correction for background fl-achvitles due to oxygen contamination in the polyethylene rabbit was determined experimentally. The spectra are shown in fig 2 3 3 BETA-GAMMA COINCIDENCE MEASUREMENT The spectra were observed in coincidence with a 440 keV broad region around the ),-peak formed by the two hnes at 884.8 keV and 901 7 keV The measurement was t The calculatJons were done at Interfakultares Rechenzentrum d Umv Wlen
7°Zn
o
x
x
.-1
169
n..
13.
Z
O
.5 "
U
1
(d)
5
.~
I0
20
30
0
\,
10
20
30 40 50 CHANNEl_ NUMBERx 6
7b
s 60 CHANNEL NUMBERx
Fig 2 Smg|es E-ray spectra after irradmtlon o f ~°Zn, irradmtlon time I0 s: (a) first section, accumu|atlon tlme IO s, (b) fourth section, accumulation time 100 s, (c) second sectmn, accumu]atmn tlme 45 s, (d) third sechon, accumu|atlon time lO0 s
carried out as a two-parameter experiment The ]?-spectra were digitized into 128 channels, the y-spectra into 8 channels, four of these groups were counted m succession to get information of the time dependence of the coincidence events. To give sufficient statistics, 120 repetitions were accumulated cychcly. The Irradlatmn time was 10 s, the achvlties were counted for 10 s, 20 s, 40 s and 300 s. Beta endpoint energies with 5.42___0.25 MeV for the short-lived achvlty and 4 62_+0.22 MeV, 3.37_+0.17 MeV and 2 . 3 3 + 0 15 MeV for the long-lived activity were found
4. Discussion
4 1 THE DECAY OF 7°=Cu AND 7°bCu The assignment of the 4 s and the 52 s activity to two isomers of 7°Cu is supported by the present results. Using the measured ]?-slngle~ and ]?-? coincidence end-point energies in addition to the associated y-rays, best Q-values and best values for the energy differ-
170
W L REITER et al
TABLE 1 Experimental fl-end-pomt energtes and best Q-values, end-point energies and energy d~fference 70bCu.70aCu
fl-transmon 7OCu(1+) -:~ 7OZn(O+) 7OCu(1+) --~ 7OZn(2+) 7OCu(5-) -~ 7OZn(4+) 7°Cu(5-) ~ 7°Zn((4, 5, 6)-) 7OCu(5-) --~ 7OZn(5-)
Measured end-points (MeV) singles comc
Best values (MeV)
6 0 9 ± 0 16 4 55±0.12 2 574-0 11
5.42±0.25 4.62±0.22 3.374-0.17 2 3 3 ± 0 15
5 29:]:0 11 4 53±0.11 3.27±0.11 2 5 2 ± 0 11
Q-value 7°~Cu(l +) ~ 7°Zrl(O+) 7ObCu(5- ) -+ 70Zn(O+ ) 7°bCu'7°aCu
6.17~0.11 6.31 ±0.11 0 24±0.16
0 1 0 ± 0 09
0 1 4 ± 0 08
ence of both isomers have been determined. Since the asstgnment of the lowest energetic fl-btanch is somewhat tentative, it has not been used. Here, a mean value of 2.52+0.11 MeV for singles and coincidence end-points was found. All other best value fl-end-point energies have been determined flom the Q-values using the 7-energies. A compilation of the measured fl-smgles and coincidence end-point energies and their best values, the Q-values and the energy difference for the 7°Cu isomers is given m table 1. The errors gwen are standard deviations including systematic errors. The Q-values for 7°aCu and 7°bCu are in fair agreement with the calculated decay energy of 6.54 MeV for 7°Cu from the mass table of Garvey et al. 7). The 6.09+0.16 MeV//-branch (measured m the first time section of the single fl-measurement) with a branching ratio of (46+ I 0 ) ~ is assigned to the ground state transition of 7°~Cu. The transition seems to be allowed (log J? = 5.2), hke the branch to the first excited 2 + state (log f t = 4.5) with 5.29+0.11 MeV end-point energy and a transition rate of (54+ 10) ~ . Thus from the possible spin-panty values, (1 +) has been assigned to the 7°Cu state with a 4 s half-life. The 4.53+0.11 MeV branch (measured in the last t~me section of the fl-smgles and fl-~ coincidence measurement) from 70bCu,with a branching ratio of (30_+ 3) %, is very likely to feed the level at 1786.5 keV. The comodence spectrum showed fl-end-pomts at 3 37+0.17 MeV and 2.33-+0 15 MeV for the 52 s 7°bCu asomer. The branching ratios of these two fl-rays are (60-+ 5) }o and (10-+ 5) %, the logft values 4.9 and 4 3, respectively. With regard to the energy balance, the 3.37 fl-transltlon feeds the level at 3038 2 keV. [According to Taft et al ~) one can assume that the levels i n 7 ° Z n up to an excitation energy of 3.5 MeV should be populated by fl-branches.] Cooper et al 8) found a level at 3048 keV m their (e, e') scattering work (with no spin-panty assignment), which can be Jdentxfied with the level at 3038 2 keV given by ref. a), but no level of this energy has
7°Zn
171
been found by Alpert et al. 9) m their s-particle scattering work and with the (p, p') reaction studied by Calderbank et al. lO). A comparison of the transition rate of the 2.33___0.15 MeV fl-branch (measured m the coincidence experiment) to that of the 7-transitmn of 1108.7 keV (9_+2) ~o given mref. ~) suggests a 7°Zn level at 4146.9 keV, which decays to the level at 3038 2 keV. An in&cation of a level m 7°Zn around 4200 keV is gwen by the (~, ~') scattering experiment of Alpert et aL 9) with spin-parity 5-. This weak transltmn has not been analyzed m the fl-V coincidence measurement of Taft et al. 1). (5") (1 +)
52.+5s
Eo
7°bc u
10 9 f t
52-+011
10±5
43
27-+011 53+-0.11
60+-5 30+-3
4.9 5 5
2 9 -+ 011
64+-10
4 5
17+-0.11
46-_10
5.2
5") --
617-+011
~'o
-r- - - 4146.9
338.2
631-'011
4+)
8848
L=---L--__
0
7OZn F]g. 3 Decay scheme of 7°Cu isomers, thin work. For 7'-ray energies see ref 1)
The Q-values for 7°aCu and 7°bCu as mean values of all fl-transltlons plus v-ray energies (weighted with the reclprocal square of the statistlcal errors) are 6.17+0 11 MeV and 6.31 _0.11 MeV, respectwely. In the same way, one gets 0 14___0.08 MeV for the difference between the decay energies of the two isomers Thus the longhvlng isomer has been assigned to be the energetically higher one. This is supported by the spin-panty assignment of (5-) for 7°bCu given by Taft et al. 1), because it is unhkely that the 70Cu ground state is not (1 +) (strong Nordhelm rule) Furthermore, in accordance with the (5-) assignment of the 7°bCu and (4 +) for the level at 1786 5 keV m 7°Zn [for the &scusslon of the (4 +) assignment of this level see Taft et al 1)] the log f t value of the 4.53+0.11 MeV fl-branch has been determined to be 5.5, suggesting a first-forbidden transition. The level at 3038.2 keV may be ass]gned neg-
172
w . L . REITER et al
atlve p a n t y with spin 4, 5 o r 6, since 70bCu decays to this level with a n allowed log f t value o f 4.9. O u r p a r t m l decay scheme for the 7°Cu isomers IS shown in fig. 3. 42 THE DECAY OF 67N1 T h e mass &fference for 67N1 a n d 67Cu, given by G a r v e y et al. 7) is 4 17 M e V T a f t et al 1) c o n s i d e r e d their 3.7 _ 0.4 M e V fl-branch to be the g r o u n d state transition. This w o r k gives a value o f 3 . 8 3 + 0 . 0 9 M e V ( l o g f t = 4.5). W a h one n e u t r o n in the p~ subshell one c a n expect ( ½ - ) f o r the g r o u n d state o f 67N1, in a n a l o g y to the spins o f 6aCu a n d 65Cu [ref. 11)], (½)- IS a p o s s i b l e a s s i g n m e n t for 67Cu. This w o u l d be in a c c o r d a n c e with a n a l l o w e d t r a n s i t i o n to the g r o u n d state. T h e first excited state r e p o r t e d f r o m the (~, p ) r e a c t i o n 12) is 0.71 M e V with spin ½. T h e difference between the fl-energy observed in this w o r k a n d the Q-values r e p o r t e d by G a r v e y et al. 7) is 430 keV, which p r o v i d e s n o s t r o n g a r g u m e n t a g a i n s t a fl-branch to the first excited level. T a f t et al. l ) find a t r a n s i t i o n with a n energy o f 708.5 keV. T h e d e c a y scheme o f 67Ni p r o p o s e d b y M e a s o n a n d K u r o d a 2) c o u l d n o t be affirmed. O n the o t h e r h a n d , their o b s e r v a t i o n o f the long-lived 67Cu activity (T~r = 62 h ) is a p r o o f t h a t 67N! is f o r m e d d u r i n g fast n e u t r o n i r r a d i a t i o n o f 7°Zn. T h e a u t h o r s w a n t to t h a n k Prof. B. K a r h k for h e r k i n d interest a n d s u p p o r t o f this w o r k W e w a n t to t h a n k Dipl. Ing. G. Stengl for help d u r i n g the experiments. W e t h a n k P r o f H V o n a c h for r e a d i n g the m a n u s c r i p t a n d f o r useful discussions.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
L M Taft, B K. S Koene and J. van Khnken, Nucl Phys. A164 (1971) 565 J L. Meason and P K Kuroda, Phys. Rev 138 (1965) B1390 W. L Relter, Thesis, Umv. Wlen, 1974 H Verheul, Nucl. Data Sheets for A = 62, Nucl Data B2-3 (1967) 11 C D Goodman et al, Nucl. Data Tables A8 (1970) 353 C. M. Lederer, J M. Hollander artd I. Perlman, Table of isotopes (Wdey, New York, 1968) G T Garvey, W J Gerace, R. L. Jaffe, I Talml and I. Kelson, Rev. Mod Phys. 41 (1969) SI L R. Cooper, H T Easterday and C D. Zafiratos, Nuovo Clm 69A (1970) 545 N. M. Alpert, E J Martens and W. Pickles, Bull. Am. Phys. Soc. 15 (1970) 62 M. Calderbank, E J Burge, V E. Lewis, D A Smith and N. K. Ganguly, Nucl Phys A105 (1967) 601 11) Nucl Data B2-6 (1968) 75 12) S. C Panchoh and W B. Ewbank, Nucl Data Sheets for A = 67, Nucl Data B2-6 (1968) 71