Isomeric 26 Al beam production with CRIB

We performed an experiment to measure proton resonant elastic scattering of a mixed 26m,gAl beam with a thick target in inverse kinematics by using CNS RI beam separator, located at RIKEN Nishina Center. It aimed to search for strong proton resonances and determine level properties of low spin-parity states in 27Si. Diagnosis of the 26mAl purity of the beam by annihilation radiation are discussed.


Introduction
Galactic abundance of the 26 Al radionuclide provides a unique window to the ongoing nucleosynthesis in the Milky Way. 26 Al is known as the first detected specific radioactivity that decays along with its characteristic β-delayed γ-ray and it has been directly observed by astronomical telescopes [1].Despite a lot of effort over the past three decades, particular sites of galactic 26 Al are still poorly understood, and there is a discrepancy between observations and theories on estimations of its abundance.
However, this problem is complicated by its isomer, 26m Al, which has a low spin J π = 0 + and a short lifetime T 1/2 = 6.35 s compared with T 1/2 = 0.72 Myr and J π = 5 + of ground state, and thus it directly decays to a stable state without emitting γ-ray.It is proposed that the ground state of 26 Al can communicate with the isomeric state through thermal excitations under high temperature environment such as core-collapse supernovae [2], therefore the reactions of the isomer might play an important role in the production of 26 Al in our Galaxy.Though the isomeric state of 26 Al might be a key to solve the discrepancy, only a little experimental information on the isomer has been published and theoretical calculations based on Hauser-Feshbach theory are used in the reaction networks so far.For this reason, the production of 26m Al beam is a useful experimental approach for measuring nuclear properties and cross-sections as inputs to calculations of stellar reaction rates.

Production of 26m Al
We performed an experiment with the Center for Nuclear Study low-energy RI Beam separator, CRIB, which is located at RIBF of the RIKEN Nishina center.

RI beam production with CRIB
A primary beam of 26 Mg 8+ was supplied by an azimuthally varying field (AVF) cyclotron with radio frequency at 65 MHz and the kinetic energy of 172.9 MeV (6.65 MeV/u) and typical primary intensity of 20 − 50 pnA.A cryogenic hydrogen gas target system was used as a proton target to induce the 26 Mg(p, n) 26 Al reaction in inverse kinematics.The secondary beam was bent by two dipole magnets, and by selecting the beams with a slit which is placed at the momentum-dispersive focal plane between two dipole magnets, the beam was purified according to the magnetic rigidity Bρ and the slit width.The beam spot as well as the number of particles was measured by parallel plate avalanche counters at dispersive-focal plane after that.The isotopes in the beam were identified by energy versus time-  1.Through the Wien-Filter, 26 Al 13+ was purified to more than 90% at the experimental target position, and typical intensity was around 1.5 × 10 5 pps, and typical energy was 114-120 MeV.However, because the ground state and the isomeric state of 26 Al have only a small mass difference (228 keV), they could not be distinguished event-by-event by the CRIB separator.By varying the secondary beam production target, H 2 gas pressure, several conditions of 26m Al beam were produced during the experiment.Since CRIB uses a in-flight method and has a typical fight time of 26 Al on the order of 500 ns, the second excited state of 26 Al (417 keV) decays enroute to the traget, contributing to the yield of 26g Al.

Determing purity of the isomers
To obtain the absolute purity of the isomers, the beam was in a pulsing mode with a cycle of 24 s for implantation, where the beam was on for 12 s for implantation and off for 12 s independent from scattering measurement.The RI beam was implanted into several targets such as CH 2 film and thick blocks, and when the beam was off, the radiation was measured by 10 NaI scintillators placed above the center of the target, in order to measure annihilation radiation from 26m Al.The half life of the radiation at 511 ± 60 keV, where 60 is about 3σ of the energy resolution of the detectors, was measured to be 6.359 ± 0.150 s (cf.known T 1/2 of 26m Al is 6.35 s).Therefore it is confirmed that almost all of the decay events originated from 26m Al.
As 26 Al decays, positrons emitted from the inside target isotropically can escape to all the surrounding material and made a precise counting annihilation radiation difficult.To estimate such complicated situation, we defined the intrinsic efficiency η of the detectors as η = (511 keV counts)/( 26m Al decays) which depends on solid angle of detectors, quantum efficiency and geometry of the experimental chamber, and the Monte-Carlo simulation framework, Geant4, was used to evaluate it.By comparing simulation results and experimental data, the validity of the simulation was confirmed within an uncertainty of 5%.

Result of the decay measurement
Using the value of η from the simulation, the isomeric purities determined are tabulated in Tab. 1 under several conditions.These differences will be used to evaluate the contribution of 26g Al to the cross section of the resonant scattering.

Conclusion
The isomeric RI beam 26m Al was produced by CRIB with the purity of at maximum 56.4% and at minimum 43.0% according to the evaluation with the Monte-Carlo simulation.The energies of RI beams can cover the region of interest of high temperature environments such as core-collapse supernovae.With these isomeric 26 Al beams, proton elastic scattering was measured with a thick target in inverse kinematics method, in order to encompass the uncertainty of recent 26 Al problem connected with the observation of the γ-ray and the reaction networks of nuclear astrophysics.

Figure 1 .
Figure 1.RI species identified at focal plane after the dipole magnets