Coulex-multipolarimetry with relativistic heavy-ion beams

Benchmarking the PreSPEC@GSI experiment for Coulex-multipolarimetry on the $$\pi (p_{3/2})\rightarrow \pi (p_{1/2})$$ spin-flip transition in $$^{85}\hbox {Br}$$

“…Since the ratio of Coulomb excitation cross sections between M 1 and E2 excitations scales with the incident beam velocity with β 2 [14], the E2/M 1 multipole-mixing ratio δ is accessible via measurement and comparison of Coulomb-excitation γ-ray yields for two different well chosen incident beam energies. For a sensitive range of multipole mixing ratios, here 0.01 δ 0.1, it is possible to access δ experimentally via γ-ray yield measurements [8]. To minimize the necessary beam time, the use of two consecutive targets with one incident beam energy instead of a single one with two beam energies was proposed.…”

“…To increase the selectivity of the used Mars Gamma-ray Tracking MGT [33][34][35], the χ 2 limit for accepted photons, referred to as χ 2 Max , has to be chosen such that the best ratio ξ [see Eq. (8)] can be achieved. The acceptance limit χ 2 Max is used to compare measured energies E dep,i at the respective interaction points x with their expected deposited energies stemming from geometrical angles between consecutive interaction points x i calculated via the Compton-scattering formula.…”

“…γ-decay intensity due to isotropically emitted γ rays. Hence, a novel experimental technique, called Coulex-multipolarimetry [8] was proposed. It was designed such that a direct measurement of the E2/M 1 multipole mixing ratio δ and the lifetime could be accessed via comparison of different experimental γ-ray yields in Coulombexcitation reactions.…”

“…Due to uncertainties arising during the Doppler-correction process, the peak width of the peak at 1191 keV is expected to be significantly broader than for γ rays emitted from nuclei at rest. In addition, the expected peak shape of the Doppler-corrected 1191 keV γ rays is complex, differing from a Gaussian distribution [8]. Assuming a Gaussian shape with a FWHM ≈ 20 keV, calculated via Geant4 simulations, as an approximation, the minimum number of counts A P necessary to identify a peak that is above the background level is approximately given by [29] A P,Ti = 4.65 ∆A B,Ti = 4.65 A B,Ti .…”