JYFL-ACCLAB-R61 Probing the N = 126 shell closure: alpha-decay spectroscopy of 220Pu

R61: Probing the N = 126 shell closure: -decay spectroscopy of 220Pu

Spokesperson: K. Auranen

Liaisons: K. Auranen

Collaboration: JYFL + Liverpool + UWS + Bratislava + GSI

Date: 20.1.2025-3.2.2025
Nucleus: 220Pu
Beam (charge state): 40Ca(8+), 40Ca(9+), 36Ar(8+)
Target (thickness): natW (~680 ug/cm²) on natC (~100 ug/cm²) backing (C downstream of W). For calibration/sanity check purposes 159Tb (~890 ug/cm²)
Degrader (thickness): C (~ 30 ug/cm²) (Only used with Tb and Lu)
Beam energy: 201, 205, 214 MeV, additionally 184 MeV (36Ar)
Beam intensity: Typically 100 pnA , occasionally 125+ pnA, for 40Ca. 100-200 pnA for 36Ar
Beam hours delivered roughly: 290 h
Instrumentation: RITU, DSSD (BB20 300-um thick), BOX, MWPC, focal plane Ge-array

Objectives(less than 250 words): Detect and identify new plutonium isotopes in the vicinity of N=126

Achievements (less than 250 words): In the beginning of the beamtime the ECR was rather unstable, but finally on morning of 24.1. the ECR people found stable operation and since then the beam was very stable at 100-125 pnA range. We started with approximate barrier energy of 206 MeV for 40Ca(8+) and learned quickly that there are contaminants in the beam, for example 30-Si. We moved to 40Ca(9+) that is cleaner. We run approximately 5 days at average 100 pnA at this energy, corresponding to 10-15 pb sensitivity limit. We identified at least 217Th, 216Ac, 215Ra, 214Fr, 216Th, 218Ac, 219Th, 220Pa, 217Ac, 218Pa, 218U alpha decays in the data. We then reduced the beam energy to sub-barrier 201 MeV, and saw very similar alpha activities. This suggested that the alpha decays we've seen might come from 40-Ar contaminant in the beam that is impossible to avoid. PACE4 simulation supported this.

We then decided to go above the barrier at 214 MeV to search the 5n evaporation channel. Beam was on target about 3 days at 100+ pnA, no sign of Pu decays. We then decided to spend the weekend on something else, i.e. try to search isomeric states in 207Ac and 207Ra with 36Ar+175Lu at beam energy of 184 MeV. Turned out that 36Ar(8+) beam had 27Al(6+) contaminant.