Program - Brookhaven National Laboratory

good timing characteristics.Examples of recent precision measurements using a combined LaBr3-HpGe
array based at the tandem van de Graaff accelerator, Bucharest, Romania will be presented [1,2]. The
presentation will also discuss the development of a multidetector array based on LaBr3 detectors for future
studies of very exotic nuclei produced at the upcoming Facility for Anti-Proton and Ion Research (FAIR)
[3]. ∗ This work is supported by grants from the Engineering and Physical Sciences Research Council
(EPSRC-UK) and the Science and Technology Facilities Council (STFC-UK).
[1] P.J.Mason et al., Phys. Rev. C 85, 064303 (2012) [2]T.Alharbi et al., App. Rad. Isotopes 70, 1337
(2012) [3] P.H. Regan App. Rad. Isotopes 70, 1125 (2012)
HD 4 4:40 PM
Further Test of Internal-Conversion Theory with a Measurement in 119 Sn m
N. Nica, J. C. Hardy, V. E. Iacob
Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA
Internal conversion is a general property of the atomic nucleus and its electronic shells, which has become of
increased interest, particularly in the last decade. In order to balance decay schemes, one needs to know the
internal conversion contribution to each transition as expressed by its internal conversion coefficients (ICC).
For some transitions used in critical applications or basic nuclear science, very precise and accurate values
of ICC’s are crucial. However, ICC’s are only rarely measured and are instead taken from calculations,
so for decades people have relied on tabulated ICC’s. Unfortunately the tabulated values can differ quite
significantly from calculation to calculation so precision is hard to guarantee. Thus one must seek guidance
from very precisely measured ICC’s, which can distinguish among the various calculations. But in 2002,
when a survey of measured ICC’s was made [1], only about 20 out of hundreds of published measured values
were found with a reported precision of 2% or better, so an extended set of 100 ICC values of precision not
worse than 6% was finally adopted for comparison with theories. Of particular importance for calculations
is the treatment of the electronic vacancy, which is known to survive long enough after the electronic
conversion takes place that it should really be taken into account in determining the potential seen by
the outgoing conversion electron. However the 2002 survey showed better agreement with calculations
that ignored the electronic vacancy and, as a result, the most recent published tables [2] were deliberately
calculated without the inclusion of the vacancy. This apparent contradiction prompted us 2003 to begin a
series of ICC measurements aimed at high precision ( 1%) that focuses on transitions that are particularly
sensitive to the theoretic treatment of the atomic vacancy. Our method consists of measuring αK ICC’s
based on the ratio of the intensity of the K x rays produced as a result of internal conversion, and
the corresponding gamma-ray intensity. This is possible using an HPGe detector whose high-precision
calibration of the detection efficiency has been reported in [3]. The method is suitable for radioactive
sources with a single transition that converts in the K shell. We use neutron activated sources, which
introduces some impurities that need to be dealt with. For energies below 50 keV there are also important
contributions from the scattered radiation that contribute to the K x-rays peaks, which we characterize
specifically from case to case. We use redundancy checks and simulation calculations to address these
issues. With this method we have measured [4] αK’s in 134 Cs, 137 Ba, 193 Ir, and 197 Pt. Here we report on a
new ICC measurement, the 65.7-keV, M4 transition in 119 Sn for which we find αK = 1604(30). This is in
agreement with the calculated value that includes the vacancy (by the so-called ”frozen-orbital” method),
1618, and differs from the calculated value that ignores the vacancy, 1544. The same conclusion resulted
from all the cases we have measured previously, thus demonstrating that the calculations including the
atomic vacancy should be used for tabulated values.
[1] S. Raman et al., Phys. Rev. C bf 66 (2002) 044312. [2] I.M. Band et al., At. Data Nucl. Data Tables,
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