Speaker
Description
When studying the decay properties of heavy elements isolated using recoil separators the nuclei of interest are typically implanted into a highly ''pixelated'' Double-sided Silicon Strip Detector (DSSD). Upstream of the implantation DSSD, a tunnel-like arrangement of silicon detectors is used to measure internal conversion electrons (ICE) emitted from the decay of excited nuclear states. They form a crucial element in determining the electromagnetic character of the observed decay, hence the quantum nature of the states. Surrounding the silicon detectors are high-purity germanium detectors that measure X-rays and $\gamma$-ray quanta. An example of such a system is GABRIELA [1].
Alpha particles emitted at backward angles (''escape alphas'') that are observed in the tunnel share their total energy between the implantation and tunnel detectors. This phenomenon can mask real $\alpha$-ICE (implantation-tunnel) coincidences. One such example are the conversion electrons emitted from the decay of excited states in $^{251}$Fm, populated via the $\alpha$-decay of $^{255}$No. These can be contaminated by a low energy escape alpha background from the more intensely populated $^{254}$No [2] (when using the $^{48}$Ca + $^{208}$Pb reaction channels).
Reverse injected Silicon detectors have been used to distinguish protons from deuterons [3] and different incident heavy ions [4] using analogue electronics. An analysis of the pulse shapes of the digitised output from a current sensitive preamplifier was performed for the first time for atomic clusters in reference [5]. Two methods were developed: the simplest plotted the maximum current as a function of the charge (integral of the current). The second, more robust, method used the averaged current signals of each cluster type as a basis function with which to classify individual pulses based on a $\chi^2$ analysis. The simplified method has also been applied to distiguish Z$\leq$ 2 ions down to $\sim$3 MeV [6] using a 500 $\mu$m NTD-Si (crystal orientation $<$1 1 1$>$) and a dual output (I and Q) preamplifier [7].
Pulse shape analysis has been performed to compare conversion electrons from $^{133}$Ba and $^{207}$Bi sources with alpha particles from a mixed $^{239}$Pu, $^{241}$Am and $^{244}$Cm source after passage through a degrader foil to obtain alpha particles in the 0 - 1 MeV energy range. The $\chi^2$ and single parametric methods of reference [5] have been applied with success and the results will be briefly presented since they will feature in a publication already. While the $\chi^2$ template matching method is an extremely powerful off-line analysis method it is not efficiently implemented in an online DAQ context. In this talk I will focus on results from our effort to perform a multi-parametric discrimination of ICE from escape alphas using Neural Networks. These networks are small enough to be implementable online providing a non-destructive metric of "electroness" to flag a large percentage of the background.
\vspace{0.5cm}
\textbf{References}\newline [1] R. Chakma, \textit{et al.}, European Physics Journal A \textbf{56} (2020) 245.
\newline [2] K. Rezynkina, Ph.D Thesis, Université Paris Saclay, 2016:\ ~~~https://theses.hal.science/tel-01394373
\newline [3] C.A.J. Ammerlaan, \textit{et al.}, Nucl. Instr. and Meth. \textbf{22} (1963) 189.
\newline [4] M. Mutterer, \textit{et al.}, IEEE Trans. Nucl. Sci. \textbf{47} (2000) 756.
\newline [5] M. Chabot, \textit{et al.}, Nucl. Instr. and Meth. B \textbf{197} (2002) 155.
\newline [6] J.A. Due\~nas, \textit{et al.}, Nucl. Instr. and Meth. A \textbf{676} (2012) 70.
\newline [7] H. Hamrita, \textit{et al.}, Nucl. Instr. and Meth. A \textbf{531} (2004) 607.