Interpretation of the 21-ns Isomer inHg190as(ν

Abstract

A previous assignment of the 21-ns isomer 190 Hg as (ith n / 2 )io+~2 is investigated by means of a ^-factor measurement. The experimental^ factor, -0.21±0.02, determines the configuration of the isomer as (^ 13/2^2 anc^ suggests a renewed study of the level scheme of 190 Hg.The i 13/2 neutron orbital plays an important role in the description of high-spin states in the mass region A = 150-208. For example, the first backbend found in many deformed nuclei is explained as an alignment of a pair of t 13/2 neutrons (see, e.g., Stephens 1 ). Also, in light lead isotopes the level obtained by coupling two neutron holes to the maximum allowable angular momentum of 12 units occurs as a low-lying isomer. 2 * 3 In contrast, published level schemes of 190 " 196 Hg and 194 Pt exhibit a low-lying isomeric 10 + level while the 12 + level is shown at somewhat higher excitation. 4 " 7 Such a level sequence does not arise naturally in the (W 13 / 2 ) 2 configuration, and the first 10 + level in 190_196 Hg and in 194 Pt has therefore been explained as a {irh n / 2 ) 10 + " 2 state. 4 * 6,8 Some additional information on the level structure in odd gold and mercury isotopes has been invoked in support of these ideas. 9 A measurement of the nuclear g factor should easily distinguish between these possibilities in a model-independent way. This is true because the g factor for a 7r/2 ll/2 excitation is close to unity, whereas the g factor of an t 13/2 neutron is about -0.2. Since the half-lives of the isomers in i9o,i92 H g are re ported 5 as 24 ±3 and 16 ±3 ns, respectively, a measurement using the time-differential spin-rotation method should be feasible. However, no convincing measurement of the g factor of the "10 + level" in any of these nuclides has so far been published.A major difficulty here is that many convenient target materials form hexagonal lattices. In such an environment the spin alignment produced in the reaction can be destroyed rapidly. However, 181 Ta has a body-centered-cubic lattice and small paramagnetism suggesting that spin relaxation should be slow. Consequently, the chances of preserving the spin alignment during the isomeric decay of 190 Hg formed in a ( 14 N, 5n) reaction on 181 Ta should be good.In the present experiment 94-MeV 14 N ions from the Oak Ridge isochronous cyclotron were used to produce 190 Hg nuclei in a 0.114-mm-thick foil of 181 Ta. The target was annealed for 30 min at 1100 °C prior to bombardment to relieve internal stresses caused by the rolling procedure. It was then mounted in the target chamber between the pole tips of a magnet capable of producing a 2.0-T magnetic field. Following the reaction, the y rays were detected in three coaxial Ge(Li) detectors located at 0° and at ±135° relative to the beam. Pulse heights from the three detectors and the times relative to the cyclotron beam bursts were stored in list mode on magnetic tape. The time of arrival of a beam burst was obtained from a device 10 containing a thin plastic scintillator inserted in the path of the beam. In this way ...

Gyromagnetic ratios of the 4+ and 6+ rotational states of184W

“…In the quasi-rotational bands of the oblate even mercury nuclei, however, a strong backbending reduces the spacing of the states with spin 8 +, 10 + and 12 + so much that the low transition energies lead to lifetimes of the t0 + and 12 + states in the nanosecond region. Small negative gfactors have been observed [14,15] for these states in agreement with the expectation for an aligned i~3/2 neutron pair.…”

“…By use of these values we derive from the measured spin rotation frequency in the 2 + state and the measured integral rotational angles in the 4 + and 6 + states the ratios of the gR-factors: gR(2+): gR(4+): gR(6+)= 1:0.99(4): 0.98 (15).…”

Nuclear g factors and structure of high-spin isomers in 190,192,194Pt and 196,198Hg

“…The g factors of most of these isomers have been measured in the mercury nuclei [1][2][3][4]. They have helped to establish the structure of the isomeric levels even when the experimental precision is poor.…”

Coupling of one and two quasiparticles to a Bohr-Mottelson core in195,196,197,198Hg

“…states are (h~i/2) 2 protons and " 2 (t~3/2) neutrons. The recent measurements of the gyromagnetic factors [3,4] for 10~ indicate that the two neutron q.p. state is energetically preferred.…”