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Isomeric level in Y90
Nuclear Physics 27 (1961) 344--347 ; @ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or raicrof1m without written permission from the publisher
ISO
E
C LEVEL
9°
J. M. FERGUSON U.S . Naval Radiological Defense Laboratory, San Francisco 24, California Received 4 April 1961 Abstract : A 3.5 h activity was produced by fast neutron bombardment of yttrium and zirconium. The evidence indicates this activity is Y9om . Two gamma rays in cascade were found with energies of 200 keV and 493 keV . It is felt that the isomer consists of a 7-{- level at 693 keV decaying to the 3 -- level in Y9 e at 200 keV.
1. Introduction The jj-coupling shell model predicts 1) an isomeric level in Y9°. If we regard as a closedshell nucleus, then 3sY'* has one proton and one neutron S outside closed shells . The two lowest states available for the 39th proton are the pi and gi states . The 51st neutron is expected to be in a dl. state. rfrhus there should. be the following two sets, of levels in Y9 ° : the two levels of the pi, di configuration and the six levels of the gI, di. configuration. Since the energy difference between the + and -levels in a9Y~ is 915 keV, the "centre of gravity" of the gi, di levels in Y9° should be about 915 keV above that of the pi, levels . Also, the 2+ and 7 + states should be the lowest of the g#, dl. configuration 1) . G. A. Bartholomew et al. have studied the Y9o level structure using the Y99 (n, y) reaction 2). They found the two lowest levels of Y9° to have characters of 2- and 3- ; these are presumably the states of the pi, dl. configuration . Also they found a level at 776.7 keV which probably has a 2-}-- character. This would be a level of the E;I, di configuration. The other states of this configuration would not be excited in the Y89 (n, y) reaction using thermal neutrons . If the 7-$- level hes below the other levels of the g, d.i configuration, as predicted, its decay would be strongly hindered by its high angular momentum . In this work we have obtained evidence for such an isomeric level.
2
2
Experimental
Results
The Zr" (n, p) and Y89 (n, y) reactions were used to search for the isomer. To form a 7-{- level with these reactions it is necessary to supply at least 3 or 4 units of angular momentum to the nucleus. Thus there is a rather large centri fugal barrier for the formation of Y9°m . At high energies, however, the penetra344
ISOMERIC LEVEL IN
Y90
345
tion of high angular momentum neutrons into the nucleus should be comparable to that of low angular momentum neutrons 3) . Hence we used the highest energy neutrons available. In the original runs samples of Zr (OH) 4 and Y2 03 were irradiated with 14.5 MeV neutrons. The gamma-ray spectra from these samples were studied with 10 cm high, 10 cm diam NaI (Tl) crystals and a 100-channel pulse-height analyzer . The samples were placed directly on top of the crystal housing to obtain maximum counting rates. The pulse-height spectrum from a yttrium sample is shown in fig. 1 . The background and the contribution from long-lived Y88 has been 2000 1800
200 k0V
1600 1400 1200 N F O Q
1000
"
490 keV
800 600
1
400 200
0
LA " 20
I "
«De
.
40 60 CNANNFI NUMBER
"" ~e"
1
80
100
Fig. 1 . Gamma-ray pulse-height spectrum from a yttrium sample bombarded with 14 .5 MeV neutrons . The sample was placed directly on top of the crystal housing for counting . The pulse height spectrum due to long-lived Y88 and the background has been subtracted . The peak at channel 85 is a sum peak due to simultaneous detection of both gamma rays .
subtracted from this spectrum . The two larger peaks correspond to gamma energies of 200±5 keV and 493±5 keV. The peak at channel 85 corresponds to 693 MeV and is a sum peak (see below), indicating the two gammas are in cascade. The 200 keV and 493 keV peaks correspond to gammas of about equal intensity, after correcting for the crystal efficiency. The half period of the activity is 3.5±0 .5 h. The following seven neutron reactions are energetically allowed : Y89 (n, y)Y9°, Y89 (n, 2n) Y88, Y89 (n, p) Sr89, Y89 (n, n') Y89r:, v89 (n, a) Rb86, Y89 (n, d)Sr88 and . Y89 (rl, t) Sr" In a chemical analysis the activity stayed with the yttrium, hence it i:, either Y883n, Y89m or Y9om. The same activity was produced by 14.5-MeV neutron bombardment of
346
J.
M . FERGUSON
zirconium. The cross section for. the production of the activity was found to be larger by a factor of 4 for zirconium than for yttrium. An elimination process makes Y9om appear to be the most likely assignment . If the activity were Y88m it would be produced by the Y (n, 2n) (11 .8 MeV threshold) and Zr(n, t) (13 MeV threshold) reactions. The Zr(n, t) cross section would be expected to be much smaller than the Y (n, 2n) cross section. If the activity were Y89 m it would be produced by the Y (n, n') reaction (0.5 MeV threshold) and the Zr (n, d) reaction (7 MeV threshold) . Again, the Zr (n, d) reaction 105 ~-
I m O
Y, 1 CM ABOVE CRYSTAL Y, 4 .5 CM ABOVE CRYSTAL
m 0 u
10
3
1oz
0
I
I
20
I
I
I
I
40 60 CHANNEL NUMBER
I
I
80
I
I
100
Fig. 2. Gamma-ray pulse height spectrum from a yttrium sample bombarded by fast neutrons . The peaks at channels 17, 46 and 67 are from the same activity as shown in fig. 1. The peak at channel 35 is from Y87m and the peak at channel 88 is due to Y88. In the upper curve the sample was i cm from the crystal face and in the lower curve the sample was 4 .3 cm from the crystal face . The large reduction of the 693 keV peak indicates it is a sum peak .
would be expected to be smaller than the (n, n') reaction . For Y9°m the reactions are Y89 (n, y) (exoergic) and Zr(n, p) (2 MeV threshold) . While it is harder to compare these two reactions, since one is radiative decay of the compound nucleus ~.,nd the other is particle emission, it is not unreasonable to have the (n, p) cross section larger than the (n, y) cross section at 14 MeV. The 200±5 keV energy of the lower gamma ray also indicates that the activity is Y"m. Bartholomew 2) found the first level of Y90 to be at 202.4±0.3 keV.
ISOMERIC LEVEL IN Y90
34 7
Intense sources containing the 3 h activity were produced by bombarding yttrium with cyclotron produced neutrons. Because of the higher energy of the neutrons additional contaminating activities were produced, notably Y87 and Ys7am from the (n, 3n) reaction. However, the more intense source was useful for verifying that the 200 keV and 493 keV gamma rays are in cascade. Fig. 2 shows the pulse-height spectrum from the sample . The upper spectrum. was taken with the samples placed I cm above the crystal face, while the lower curve was taken with the sample 4 .3 cm above the crystal face. In the upper curve, the peak at 693 keV could be the sum peak of the 200 keV and 493 keV peaks, or a cross-over gamma transition. It is seen that the 693 keV peak disappears in the lower curve. This verifies that it is a sum peak and not another gamma ray, for if it were another gamma ray it would have about the same height relative to the other peaks in both runs . Because of the differences in the efficiencies of the two positions the sum peak should be reduced by a factor of 8 in the second run, and that is what is indica±ed by the data . To summarize, the data indicate an isomeric level in Y91,m at 693 keV which decays through a 493 kel' or 200 keV level. It is very likely that the intermediate level is the 200 keV level found by Bartholomew. 3. Discussion The reduced lifetime 4) of the 493 keV transition indicates it is an M4 transition . Since Bartholomew found that the 200 keV level has a character of 3-, the 693 keV level would be 7-E- . The 693 keV level is presumably the 7-}- level of the gf, d configuration . According to the shell model the ratio of lifetimes of this transition to the lifetime of the 9 -f- level in Y89m should be just proportional to the 9th power of the energy ratios. On this basis the predicted lifetime; of the Y90m level should be 1 .2 h. The discrepancy of a factor of 3 between this predicted value and the observed 3.5 h half period is not unreasonable in the comparison of gamma lifetimes. The author would like to express his gratitude to Dr. A. de-Shalit for pointing out the possibility of a Y90 isomer and for some very enlightening discussions. Also, the author wishes to thank Dr . D. L. Love for making the chemical separations, efei-ences 1) A. de-Shalit, private communication 2) G. A. Bartholomew, P. J. Campion, J. W. Knowles and G. Manning, Nuclear Physics 10 (1959) 590 3) H. Feshbach, Nuclear spectroscopy (Academic Press, Inc., New York 1960) part B, capt . V.A . 4) M. Goldhaber and A. W. Sunyar, Beta and gamma-ray spectroscopy (Interscience Publishers, Inc., New York (1955) p. 453
ISO
E
C LEVEL
9°
J. M. FERGUSON U.S . Naval Radiological Defense Laboratory, San Francisco 24, California Received 4 April 1961 Abstract : A 3.5 h activity was produced by fast neutron bombardment of yttrium and zirconium. The evidence indicates this activity is Y9om . Two gamma rays in cascade were found with energies of 200 keV and 493 keV . It is felt that the isomer consists of a 7-{- level at 693 keV decaying to the 3 -- level in Y9 e at 200 keV.
1. Introduction The jj-coupling shell model predicts 1) an isomeric level in Y9°. If we regard as a closedshell nucleus, then 3sY'* has one proton and one neutron S outside closed shells . The two lowest states available for the 39th proton are the pi and gi states . The 51st neutron is expected to be in a dl. state. rfrhus there should. be the following two sets, of levels in Y9 ° : the two levels of the pi, di configuration and the six levels of the gI, di. configuration. Since the energy difference between the + and -levels in a9Y~ is 915 keV, the "centre of gravity" of the gi, di levels in Y9° should be about 915 keV above that of the pi, levels . Also, the 2+ and 7 + states should be the lowest of the g#, dl. configuration 1) . G. A. Bartholomew et al. have studied the Y9o level structure using the Y99 (n, y) reaction 2). They found the two lowest levels of Y9° to have characters of 2- and 3- ; these are presumably the states of the pi, dl. configuration . Also they found a level at 776.7 keV which probably has a 2-}-- character. This would be a level of the E;I, di configuration. The other states of this configuration would not be excited in the Y89 (n, y) reaction using thermal neutrons . If the 7-$- level hes below the other levels of the g, d.i configuration, as predicted, its decay would be strongly hindered by its high angular momentum . In this work we have obtained evidence for such an isomeric level.
2
2
Experimental
Results
The Zr" (n, p) and Y89 (n, y) reactions were used to search for the isomer. To form a 7-{- level with these reactions it is necessary to supply at least 3 or 4 units of angular momentum to the nucleus. Thus there is a rather large centri fugal barrier for the formation of Y9°m . At high energies, however, the penetra344
ISOMERIC LEVEL IN
Y90
345
tion of high angular momentum neutrons into the nucleus should be comparable to that of low angular momentum neutrons 3) . Hence we used the highest energy neutrons available. In the original runs samples of Zr (OH) 4 and Y2 03 were irradiated with 14.5 MeV neutrons. The gamma-ray spectra from these samples were studied with 10 cm high, 10 cm diam NaI (Tl) crystals and a 100-channel pulse-height analyzer . The samples were placed directly on top of the crystal housing to obtain maximum counting rates. The pulse-height spectrum from a yttrium sample is shown in fig. 1 . The background and the contribution from long-lived Y88 has been 2000 1800
200 k0V
1600 1400 1200 N F O Q
1000
"
490 keV
800 600
1
400 200
0
LA " 20
I "
«De
.
40 60 CNANNFI NUMBER
"" ~e"
1
80
100
Fig. 1 . Gamma-ray pulse-height spectrum from a yttrium sample bombarded with 14 .5 MeV neutrons . The sample was placed directly on top of the crystal housing for counting . The pulse height spectrum due to long-lived Y88 and the background has been subtracted . The peak at channel 85 is a sum peak due to simultaneous detection of both gamma rays .
subtracted from this spectrum . The two larger peaks correspond to gamma energies of 200±5 keV and 493±5 keV. The peak at channel 85 corresponds to 693 MeV and is a sum peak (see below), indicating the two gammas are in cascade. The 200 keV and 493 keV peaks correspond to gammas of about equal intensity, after correcting for the crystal efficiency. The half period of the activity is 3.5±0 .5 h. The following seven neutron reactions are energetically allowed : Y89 (n, y)Y9°, Y89 (n, 2n) Y88, Y89 (n, p) Sr89, Y89 (n, n') Y89r:, v89 (n, a) Rb86, Y89 (n, d)Sr88 and . Y89 (rl, t) Sr" In a chemical analysis the activity stayed with the yttrium, hence it i:, either Y883n, Y89m or Y9om. The same activity was produced by 14.5-MeV neutron bombardment of
346
J.
M . FERGUSON
zirconium. The cross section for. the production of the activity was found to be larger by a factor of 4 for zirconium than for yttrium. An elimination process makes Y9om appear to be the most likely assignment . If the activity were Y88m it would be produced by the Y (n, 2n) (11 .8 MeV threshold) and Zr(n, t) (13 MeV threshold) reactions. The Zr(n, t) cross section would be expected to be much smaller than the Y (n, 2n) cross section. If the activity were Y89 m it would be produced by the Y (n, n') reaction (0.5 MeV threshold) and the Zr (n, d) reaction (7 MeV threshold) . Again, the Zr (n, d) reaction 105 ~-
I m O
Y, 1 CM ABOVE CRYSTAL Y, 4 .5 CM ABOVE CRYSTAL
m 0 u
10
3
1oz
0
I
I
20
I
I
I
I
40 60 CHANNEL NUMBER
I
I
80
I
I
100
Fig. 2. Gamma-ray pulse height spectrum from a yttrium sample bombarded by fast neutrons . The peaks at channels 17, 46 and 67 are from the same activity as shown in fig. 1. The peak at channel 35 is from Y87m and the peak at channel 88 is due to Y88. In the upper curve the sample was i cm from the crystal face and in the lower curve the sample was 4 .3 cm from the crystal face . The large reduction of the 693 keV peak indicates it is a sum peak .
would be expected to be smaller than the (n, n') reaction . For Y9°m the reactions are Y89 (n, y) (exoergic) and Zr(n, p) (2 MeV threshold) . While it is harder to compare these two reactions, since one is radiative decay of the compound nucleus ~.,nd the other is particle emission, it is not unreasonable to have the (n, p) cross section larger than the (n, y) cross section at 14 MeV. The 200±5 keV energy of the lower gamma ray also indicates that the activity is Y"m. Bartholomew 2) found the first level of Y90 to be at 202.4±0.3 keV.
ISOMERIC LEVEL IN Y90
34 7
Intense sources containing the 3 h activity were produced by bombarding yttrium with cyclotron produced neutrons. Because of the higher energy of the neutrons additional contaminating activities were produced, notably Y87 and Ys7am from the (n, 3n) reaction. However, the more intense source was useful for verifying that the 200 keV and 493 keV gamma rays are in cascade. Fig. 2 shows the pulse-height spectrum from the sample . The upper spectrum. was taken with the samples placed I cm above the crystal face, while the lower curve was taken with the sample 4 .3 cm above the crystal face. In the upper curve, the peak at 693 keV could be the sum peak of the 200 keV and 493 keV peaks, or a cross-over gamma transition. It is seen that the 693 keV peak disappears in the lower curve. This verifies that it is a sum peak and not another gamma ray, for if it were another gamma ray it would have about the same height relative to the other peaks in both runs . Because of the differences in the efficiencies of the two positions the sum peak should be reduced by a factor of 8 in the second run, and that is what is indica±ed by the data . To summarize, the data indicate an isomeric level in Y91,m at 693 keV which decays through a 493 kel' or 200 keV level. It is very likely that the intermediate level is the 200 keV level found by Bartholomew. 3. Discussion The reduced lifetime 4) of the 493 keV transition indicates it is an M4 transition . Since Bartholomew found that the 200 keV level has a character of 3-, the 693 keV level would be 7-E- . The 693 keV level is presumably the 7-}- level of the gf, d configuration . According to the shell model the ratio of lifetimes of this transition to the lifetime of the 9 -f- level in Y89m should be just proportional to the 9th power of the energy ratios. On this basis the predicted lifetime; of the Y90m level should be 1 .2 h. The discrepancy of a factor of 3 between this predicted value and the observed 3.5 h half period is not unreasonable in the comparison of gamma lifetimes. The author would like to express his gratitude to Dr. A. de-Shalit for pointing out the possibility of a Y90 isomer and for some very enlightening discussions. Also, the author wishes to thank Dr . D. L. Love for making the chemical separations, efei-ences 1) A. de-Shalit, private communication 2) G. A. Bartholomew, P. J. Campion, J. W. Knowles and G. Manning, Nuclear Physics 10 (1959) 590 3) H. Feshbach, Nuclear spectroscopy (Academic Press, Inc., New York 1960) part B, capt . V.A . 4) M. Goldhaber and A. W. Sunyar, Beta and gamma-ray spectroscopy (Interscience Publishers, Inc., New York (1955) p. 453