Excitation
energies in 33Cl using 32S(p,γ)
Introduction
Experiment
Conclusions
One
of
the many
interesting scenarios to investigate experimentally, is to look for
deviations from the known V-A structure of the weak interaction. Such
tests provide sensitive probes for physics beyond the Standard
Model. Nuclear beta decay of atomic nuclei offers excellent
opportunities for such tests. In particular, 0+→ 0+
(superallowed-Fermi
type) beta-transitions
are sensitive to scalar interactions that may be mediated via the weak
interaction.
The
Standard
Model, as well as its left-right
symmetric generalizations predicts that for a superallowed Fermi decay,
the leptons are emitted with opposite helicities, such that, they will not be emitted in opposite
directions (so that conservation of angular momentum is respected).
However, in the
event that a rare process occurs, such as the exchange of a scalar
boson instead of the usual W± vector boson exchange,
the leptons can be emitted with the same helicities and emitted in
opposite directions. The angular distribution is given by,
Precise measurements
of the β-ν
correlation coeffcient sets bounds on scalar contributions to the weak
interaction.
Detailed analysis of the shape of β-delayed proton
groups from the decay of 32Ar provides information about
the recoil of the daughter nucleus and hence the β-ν
angular
correlation. The figure on the left shows a Monte Carlo simulation of
the shape of a proton spectrum (assuming vanishing Fierz
interference). The light (shaded) curve is assuming purely scalar
interactions and the broad (unshaded) curve represents the shape of the
spectrum assuming a purely vector interaction.
β-delayed protons from 33Ar
are used for energy calibration purposes. This energy calibration is
based on excitation energies in 33Cl from well known 32S
+ p resonances.
An improved measurement of excitation energies and widths of the
relevant states in 33Cl may play an
important role in the β-ν
angular
correlation determination.
Experiment
The
32S(p,
γ)
experiment
was done using the Van de Graaf accelerator at CENPA. Stable proton
beams at various energies (with an energy spread of ≈
1.8 keV) were impinged on various 32S targets of varying
thicknesses to populate the states of interest in 33Cl. The
corresponding γ transitions were
registered by a 50% HPGe detector at 0°
to the beam. The Doppler shifts on the γ energies were
determined by careful Monte Carlo simulations. Excitation
function data to determine level widths were determined using thin
Sm2S3 targets evaporated onto thin (≈ 300 μg/cm2)
gold foils and varying the proton energy in steps of ≈ 0.5 keV.
Results & Conclusions
As an example, the
figure on the left shows the γ rays
corresponding to transitions from the lowest T = 3/2 state in 33Cl.
Our results for the excitation energies are in agreement with previous
measurements, but with much improved precision.
Previously, there existed a
discrepancy for the width of the J3π
= 3/2+ state in 33Cl (at Ex ≈ 3971 keV)
obtained from a previous 32S(p,γ) measurement
and the delayed proton spectrum from 33Ar. The 32S(p,γ) measurement
reported the width as Γ = 5
± 3
keV. The proton spectrum
however, indicates a width of Γ ≤ 0.2 keV. Our
excitation function data resolves this discrepancy. As shown in the
figure below(left), the width of the resonance is comparable to another
known narrow resonance, indicating an upper limit of 0.3 keV for the
widths of both the states.
As byproducts of this work, we also determine the relative gamma
branches from the Ex = 3971 keV state. Our results differ
significantly
from the previously determined values.
