Orateur
Description
Transfer reactions are powerful probes to provide single-particle and collective properties of nuclear states, such as excitation energies, spin-parity assignments, and spectroscopic factors, extending even to unbound states. The extensive use of CH$_2$/CD$_2$ plastic and H$_2$/D$_2$ cryogenic targets has boosted the study of the neutron wave functions and thus research fields such as neutron shell evolution, neutron capture rates for nuclear astrophysics, and neutron-neutron pairing. Conversely, the experimental challenges posed by their counterparts for proton transfer have proven to be a significant hindrance to the study of the proton shell evolution, the spectroscopy of unbound proton-rich nuclei, the study of neutron-proton pairing in self-conjugate ($N=Z$) nuclei, and proton-capture surrogate reactions for nuclear astrophysics.
Indeed, the unavailability of sufficiently thick $^3$He targets (about $10^{20}$at/cm$^2$, see ref.[1] and references therein) represents a significant obstacle to the study of the one-proton stripping ($^3He,d$) and deuteron-stripping ($^3He,p$) reactions. Few have been developed for reaction measurements, with a focus on high energy beams (particularly for RIKEN) [2]. The only compact $^3$He target specifically designed and employed for transfer reactions experiments with low-energy (around 10 MeV/u) and low-intensity (10$^4$-10$^6$ pps) beams is the HeCTOr target [1]. It was used during the MUGAST-AGATA VAMOS campaign at GANIL [3], to study the proton structure at $N=28$ via the reaction $^{46}$Ar($^3He,d$)$^{47}$K.
The ATRACT project aims to produce, in addition to an active $^3$He target, a $^3$He cryogenic target that overcomes the shortcomings of the HeCTOr target, such as ice growing on the windows at a rate of 11 $\mu$m/window/day (for a vacuum of 10$^{-6}$ mbar), non-negligible background reactions, large energy losses and additional straggling due to the gas cell size and window material (3.8$\mu$m thick Havar foils). These limitations also include a significant absorption of the low energy and long half-life $\gamma$ rays, as well as an important LHe consumption to cool down the $^3$He cell.
The new strategy incorporates the use of a cryogen-free technique (pulse-tube type cryocooler), combined with innovative solutions for window materials and de-icing protocols. This will be achieved through several phases of testing samples and the prototype target, at room and cryogenic temperatures. This procedure allows us to study the properties of the windows: resistance to pressure, surface deformation, He leak tightness, but also the time constant and efficiency of different de-icing techniques.
To guide the technical decisions that are made, mechanical and thermal calculations, as well as simulations of the relevant reactions (using the NPTool framework[4]), are performed. The latter are crucial to determine the energy thresholds, the energy straggling into the target and its windows, the excitation energy resolution for the particle measurements and the absorption of $\gamma$-rays for each physics case. The final cryogenic $^3$He target is planned for use during the future GRIT-AGATA-VAMOS at GANIL(2029-2030).
In this presentation, I will show the results of tests conducted on porosity to helium, resistance to pressure, and the measurement of the deformation of the selected windows, in addition to the current status of the target development. Detailed simulations of typical physics cases will also be shown.
$[1]$ F. Galtarossa et al., NIM A 1018, 165830 (2022).
$[2]$ H. Ryuto et al., Nucl. Instr. and Meth. in Phys. Res. A 555, 1 (2005).
$[3]$ D. Brugnara et al., submitted to PRL, https://arxiv.org/pdf/2506.23228 .
$[4]$ A. Matta et al., J. Phys. G: Nucl. Part. Phys. 43, 045113 (2016).