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A new holmium activity, 159Ho
J. Inorg. Nucl. Chem., 1958, Vol. 7, pp. 1 to 4, Pergamon Press Ltd., London
A NEW HOLMIUM ACTIVITY, 159Ho* KENNETH S. TOTH Radiation Laboratory and Department of Chemistry, University of California, Berkeley, California (Received 21 November 1957)
Abstract--A new activity, 15aHo, has been produced by an (~,4n) reaction on stable terbium. The nuclide decays primarily by electron capture with a half-life of 33 min. Evidence has been found to support the proposal by Mn-mLtCHand co-workers that the ground state of le°Ho is short-lived compared to its metastable state whose half-life is 5 hr. The half-life of the ground state seems to be about 28 min. BOMBARDMENTby 0~-particles of terbium in the Berkeley 60-in. cyclotron has resulted in the discovery and mass-assignment of a new isotope, 159Ho. The new activity was seen in a full energy (48 MeV) helium-ion bombardment, but was absent in an experiment that was carried out at 37 MeV. The latter energy is below the (0c,4n) threshold. This information leads one to believe that the new activity must have been made by such a reaction on terbium. Because the element consists of a single stable isotope, lSgTb, the nuclide observed must be 159Ho. EXPERIMENTAL The terbium was irradiated in the form of Tb20 a. The powder was placed in a platinum " h a t " which fitted into a standard target assembly. A platinum foil was placed over the "hat", and the thickness of the foil was varied to allow bombardment by impinging ~-particles of different energies. The rare earths were separated from one another by an ion-exchange method described elsewhere31) After elution, the holmium fraction was studied for total radioactivity in a Geiger counter and for X-ray and 7-radiation in a 100-channel NaI (T1) scintillation spectrometer. RESULTS The resolution of the Geiger counter decay curves yielded only two activities in each of the two bombardments (48 MeV and 37 MeV 0~-particles on 159Tb), one short-lived and the other having a 5 hr half-life. The short-lived activity was resolved to have a 33 4- 3 min half-life in the full-energy bombardment and a 28 -+- 3 min half-life in the second case. When the two shorter activities were compared in abundance relative to the amount of the 5 hr nuclide seen in each case, it was found that the 28 min one was present in a much smaller quantity. The factor between them was at least 30. The 7-spectra obtained at the two different bombarding energies are shown in Figs. 1 and 2. It is readily noticed that there are four photon peaks present in the full-energy bombardment that are absent in the second experiment. The energies of * This work was done under the auspices of the U.S. AtomicEnergy Commission. ~1~ S. G. THOMPSON,B. G. HARVEY, G. R. CHOPPINand G. T. SEABORG, Y. Amer. chem. Soc. 76, 6229 (1954).
2
KENNETH S. TOTH
the four transitions are: 125, 180, 250, and 305 keV. The rest of the spectrum is identical to the one obtained in the 37 MeV bombardment, except that in Fig. 2 there is a 200 keV transition that is missing in Fig. 1. This particular y-ray does appear in the full-energybombardment after the 180 keV photon decays. The energies of the transitions shown in Fig. 2 are 90, 200, 650, 730, 890, and 970 keV. The 7-transitions have been seen by others~,3, 4) and assigned to 16°Ho. I00 000
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FIG. 1.--Gamma spectrum obtained at 48 MoV.
From the evidence presented, one can draw the following conclusion: an activity that had at least four characteristic y-rays was seen in the holmium fraction at full bombarding energy. Each of the photopeaks decayed with a half-life of about 33 min. The four y-transitions were missing in the holmium fraction when the experiment was carried out below the (~,4n) threshold. The new activity must therefore be 159Ho. The half-lives associated with all the photopeaks will now be discussed because some interesting conclusions can be drawn about 16°Ho and 16°mHo. The y-rays belonging to 159Ho, as mentioned above, decayed with a 33 min half-life. The remainder of the y-rays shown in Fig. 1 decayed at first with a somewhat similar half-life. A few hours after bombardment time, however, a 5 hr component appeared in the decay of these y-rays. The explanation was first proposed that all the y-transitions seen immediately after chemical separation (Fig. 1) belonged to 159Ho. Then a few hours later, when this short-lived isotope had decayed, 16°Ho (produced in sufficient amount by an (,,3n) reaction on lSgTb) was seen, as evidenced by the 5 hr component and the isotope's characteristic y-rays. That a number of y-rays presumably belonging to ~SgHo were similar in energy to transitions assigned to Xr°Ho by other workers was thought to be a coincidence. The explanation was found to be incorrect, because in the second bombardment only the four transitions (125, 180, 250, and 305 keV) were absent. One other f2) W. E. NERVIK and G. T. SEABORG, Phys. Rev. 97, 1092 (1955). Cs~ T. H. HANDLEY,Phys. Rev. 94, 945 (1954). (4~ j. W. MIHELICH, B. HARMATZ and T. I-I. HANDLEY, Nuclear Spectroscopy of Neutron-Deficient Rare Earths (Th through Hf) ORNL-63M57, (1957); Phys. Rev. 108, 989 (1957).
A new holmium activity, ~SgHo
difference was the appearance of a 200 keV photopeak in place of the 180 keV one. All of the y-rays shown in Fig. 2 decayed with the same short half-life and again had a 5 hr component. Because the experiment was carried out below the (e,4n) threshold, l~gHo could not have been made, and so these y-rays could not belong to a59Ho, Rather they had to be assigned to ~6°Ho in spite of their initial short half-lives. Also the energies of the photopeaks did not change as the 5 hr component appeared.
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FIG. 2.--Gamma spectrum obtained at 37 MeV.
Therefore, it seemed reasonable to assume that the short half-life (28 min) and the 5 hr one belonged to one and the same isotope. MII-IELICIa et al. in a recent paper ~4) found that 16°Ho had a metastable state whose half-life was 5 hr and which decayed to the ground state by an E3 transition of 60.1 keV. Their conversion-electron spectra showed no evidence of growth in the intensities of transitions converting in dysprosium. The decay rate of these lines seemed to be identical to that for the lines of the 5 hr metastable state. They proposed two alternatives: (a) either there was no electron capture from the isomeric state and the ground State had a short half-life, or (b) the isomeric state underwent electron capture to levels in 16°Dy, in which case the half-life of the ground state was either short or very long compared to 5 hr, or it was 5 hr. They preferred the first alternative. They also suggested that the half-life of the ground state was 22 min as reported by HANDLEy.ta)
Our data certainly support the first alternative. We believe that the short half-life seen in the decay of the transitions of 90, 200, 650, 730, 890, and 970 keV is that of the ground state of 16°Ho. In the bombardments, both of the isomers were produced but predominantly the short-lived ground state. The 60.1 keV transition in holmium would be obscured by the K X - r a y peak. Then as most of the ground state decayed and as the two activities came into equilibrium, the 5 hr isomer would become noticeable because it would now be controlling the half-lives of the y-transitions. It should be indicated that our results show the ground state to have a half-life of about 28 ~ 3 min rather than 22 rain as reported by HANDLEY. One other piece of information should be added. The resolution of the K X-ray
4
Kr~,mTri S. TOTH
peak decay curve in the second experiment yielded a 2-5 hr component. This activity is 161Ho. It was not seen in the full-energy experiment. Also a 20 keV y-ray was seen in the lower-energy bombardment and not at full energy. It is not shown in Fig. 2 but was seen in another energy setting of the y-analyser. Its half life was 2.5 hr, so it presumably belongs to 161Ho as has been previously suggested by MIHELICH et aL ~4~ The cross-section for the (0~,2n) product, 16trio, at 48 MeV must be rather low because the isotope was not observed at that particular energy. Analogously then, one would not expect from cross-section systematics to observe the (~,n) product, ~62Ho, at either 48 or 37 MeV. The statement is certainly borne out experimentally by the fact that the 67 min half-life of ~62Ho was not present in any of the decay curves at either of the two bombarding energies. Acknowledgement--I should like to express my appreciation to Professor J. O. RASMUSSENfor the constant interest and guidance shown during these and other studies.
A NEW HOLMIUM ACTIVITY, 159Ho* KENNETH S. TOTH Radiation Laboratory and Department of Chemistry, University of California, Berkeley, California (Received 21 November 1957)
Abstract--A new activity, 15aHo, has been produced by an (~,4n) reaction on stable terbium. The nuclide decays primarily by electron capture with a half-life of 33 min. Evidence has been found to support the proposal by Mn-mLtCHand co-workers that the ground state of le°Ho is short-lived compared to its metastable state whose half-life is 5 hr. The half-life of the ground state seems to be about 28 min. BOMBARDMENTby 0~-particles of terbium in the Berkeley 60-in. cyclotron has resulted in the discovery and mass-assignment of a new isotope, 159Ho. The new activity was seen in a full energy (48 MeV) helium-ion bombardment, but was absent in an experiment that was carried out at 37 MeV. The latter energy is below the (0c,4n) threshold. This information leads one to believe that the new activity must have been made by such a reaction on terbium. Because the element consists of a single stable isotope, lSgTb, the nuclide observed must be 159Ho. EXPERIMENTAL The terbium was irradiated in the form of Tb20 a. The powder was placed in a platinum " h a t " which fitted into a standard target assembly. A platinum foil was placed over the "hat", and the thickness of the foil was varied to allow bombardment by impinging ~-particles of different energies. The rare earths were separated from one another by an ion-exchange method described elsewhere31) After elution, the holmium fraction was studied for total radioactivity in a Geiger counter and for X-ray and 7-radiation in a 100-channel NaI (T1) scintillation spectrometer. RESULTS The resolution of the Geiger counter decay curves yielded only two activities in each of the two bombardments (48 MeV and 37 MeV 0~-particles on 159Tb), one short-lived and the other having a 5 hr half-life. The short-lived activity was resolved to have a 33 4- 3 min half-life in the full-energy bombardment and a 28 -+- 3 min half-life in the second case. When the two shorter activities were compared in abundance relative to the amount of the 5 hr nuclide seen in each case, it was found that the 28 min one was present in a much smaller quantity. The factor between them was at least 30. The 7-spectra obtained at the two different bombarding energies are shown in Figs. 1 and 2. It is readily noticed that there are four photon peaks present in the full-energy bombardment that are absent in the second experiment. The energies of * This work was done under the auspices of the U.S. AtomicEnergy Commission. ~1~ S. G. THOMPSON,B. G. HARVEY, G. R. CHOPPINand G. T. SEABORG, Y. Amer. chem. Soc. 76, 6229 (1954).
2
KENNETH S. TOTH
the four transitions are: 125, 180, 250, and 305 keV. The rest of the spectrum is identical to the one obtained in the 37 MeV bombardment, except that in Fig. 2 there is a 200 keV transition that is missing in Fig. 1. This particular y-ray does appear in the full-energybombardment after the 180 keV photon decays. The energies of the transitions shown in Fig. 2 are 90, 200, 650, 730, 890, and 970 keV. The 7-transitions have been seen by others~,3, 4) and assigned to 16°Ho. I00 000
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I000
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,.D
aJ
m
o
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I
I00 20
40
CHANNEL
60
80
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FIG. 1.--Gamma spectrum obtained at 48 MoV.
From the evidence presented, one can draw the following conclusion: an activity that had at least four characteristic y-rays was seen in the holmium fraction at full bombarding energy. Each of the photopeaks decayed with a half-life of about 33 min. The four y-transitions were missing in the holmium fraction when the experiment was carried out below the (~,4n) threshold. The new activity must therefore be 159Ho. The half-lives associated with all the photopeaks will now be discussed because some interesting conclusions can be drawn about 16°Ho and 16°mHo. The y-rays belonging to 159Ho, as mentioned above, decayed with a 33 min half-life. The remainder of the y-rays shown in Fig. 1 decayed at first with a somewhat similar half-life. A few hours after bombardment time, however, a 5 hr component appeared in the decay of these y-rays. The explanation was first proposed that all the y-transitions seen immediately after chemical separation (Fig. 1) belonged to 159Ho. Then a few hours later, when this short-lived isotope had decayed, 16°Ho (produced in sufficient amount by an (,,3n) reaction on lSgTb) was seen, as evidenced by the 5 hr component and the isotope's characteristic y-rays. That a number of y-rays presumably belonging to ~SgHo were similar in energy to transitions assigned to Xr°Ho by other workers was thought to be a coincidence. The explanation was found to be incorrect, because in the second bombardment only the four transitions (125, 180, 250, and 305 keV) were absent. One other f2) W. E. NERVIK and G. T. SEABORG, Phys. Rev. 97, 1092 (1955). Cs~ T. H. HANDLEY,Phys. Rev. 94, 945 (1954). (4~ j. W. MIHELICH, B. HARMATZ and T. I-I. HANDLEY, Nuclear Spectroscopy of Neutron-Deficient Rare Earths (Th through Hf) ORNL-63M57, (1957); Phys. Rev. 108, 989 (1957).
A new holmium activity, ~SgHo
difference was the appearance of a 200 keV photopeak in place of the 180 keV one. All of the y-rays shown in Fig. 2 decayed with the same short half-life and again had a 5 hr component. Because the experiment was carried out below the (e,4n) threshold, l~gHo could not have been made, and so these y-rays could not belong to a59Ho, Rather they had to be assigned to ~6°Ho in spite of their initial short half-lives. Also the energies of the photopeaks did not change as the 5 hr component appeared.
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FIG. 2.--Gamma spectrum obtained at 37 MeV.
Therefore, it seemed reasonable to assume that the short half-life (28 min) and the 5 hr one belonged to one and the same isotope. MII-IELICIa et al. in a recent paper ~4) found that 16°Ho had a metastable state whose half-life was 5 hr and which decayed to the ground state by an E3 transition of 60.1 keV. Their conversion-electron spectra showed no evidence of growth in the intensities of transitions converting in dysprosium. The decay rate of these lines seemed to be identical to that for the lines of the 5 hr metastable state. They proposed two alternatives: (a) either there was no electron capture from the isomeric state and the ground State had a short half-life, or (b) the isomeric state underwent electron capture to levels in 16°Dy, in which case the half-life of the ground state was either short or very long compared to 5 hr, or it was 5 hr. They preferred the first alternative. They also suggested that the half-life of the ground state was 22 min as reported by HANDLEy.ta)
Our data certainly support the first alternative. We believe that the short half-life seen in the decay of the transitions of 90, 200, 650, 730, 890, and 970 keV is that of the ground state of 16°Ho. In the bombardments, both of the isomers were produced but predominantly the short-lived ground state. The 60.1 keV transition in holmium would be obscured by the K X - r a y peak. Then as most of the ground state decayed and as the two activities came into equilibrium, the 5 hr isomer would become noticeable because it would now be controlling the half-lives of the y-transitions. It should be indicated that our results show the ground state to have a half-life of about 28 ~ 3 min rather than 22 rain as reported by HANDLEY. One other piece of information should be added. The resolution of the K X-ray
4
Kr~,mTri S. TOTH
peak decay curve in the second experiment yielded a 2-5 hr component. This activity is 161Ho. It was not seen in the full-energy experiment. Also a 20 keV y-ray was seen in the lower-energy bombardment and not at full energy. It is not shown in Fig. 2 but was seen in another energy setting of the y-analyser. Its half life was 2.5 hr, so it presumably belongs to 161Ho as has been previously suggested by MIHELICH et aL ~4~ The cross-section for the (0~,2n) product, 16trio, at 48 MeV must be rather low because the isotope was not observed at that particular energy. Analogously then, one would not expect from cross-section systematics to observe the (~,n) product, ~62Ho, at either 48 or 37 MeV. The statement is certainly borne out experimentally by the fact that the 67 min half-life of ~62Ho was not present in any of the decay curves at either of the two bombarding energies. Acknowledgement--I should like to express my appreciation to Professor J. O. RASMUSSENfor the constant interest and guidance shown during these and other studies.