Literature Review on Fine Structure in the α Decay of 218At
📚 Main Source
Title: Fine Structure in the α Decay of 218At
Publication Year: 2019
Link: Physical Review C
🌐 Other Sources
- Stockholm α-Decay Conference (PDF)
- Fine Structure of α-Rays by G. Gamow
📝 Introduction
Many of the ongoing latest research in nuclear physics requires high energy to analyse the properties and behaviour of an atom. CERN-ISOLDE facility is in the frontier of the nuclear research. The Isotope mass Separator On-Line facility (ISOLDE) is a unique source of low-energy beams of radioactive nuclides, those with too many or too few neutrons to be stable. The facility permits the study of the vast territory of atomic nuclei, including the most exotic species.
Half-life is the time required for a quantity to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay.
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and decays into a different atomic nucleus.
📖 Description
The following study of alpha decay was conducted on 218At (Astatine) at the CERN ISOLDE facility. The 218At nuclei was produced by the spallation method — a process in which fragments of material are ejected from a body due to impact — involving a 1.4 GeV proton beam on a uranium carbide target.
Then the 218At atoms were selectively ionised by a three-step resonance laser ionisation scheme, and the ions were extracted by an applied electrostatic potential of 30kV. The mass-separated ion beams were measured for decay using silicon detectors. Additional gamma and X-ray data were collected using germanium detectors.
Older studies of alpha decay in 218At come from indirect study of the alpha decay chain of long-lived 226Ra isotope. There, they used magnetic spectrometers to measure the energy of alpha particles and identify decay lines. Upon calculation, the half-life of 218At comes to be 1.3 seconds at ground state. Similar studies had happened in the past, but this one is the first to be conducted by direct methods (not via decay chain of another element).
The alpha decay spectra shown in Fig. 1 was collected by silicon detectors. It clearly shows the spectra of 218At between 6600 < Eα < 6800 keV, and also the decay of 218Fr at Eα > 7000 keV, which was used for energy calibration of the detectors as the decay of 218Fr is already well studied and understood.
The collected data from detectors was analysed by fitting with the Ball function, which is a probability density function that consists of a Gaussian core and a power-law tail. All data analysis was done on a computer running the MINUIT package in Fortran, developed at CERN for particle physics studies.
The alpha decay energies were identified at:
- Eα = 6655 keV
- Eα = 6694 keV
- Eα = 6741 keV
Figure 2 shows the decay scheme deduced from the work done in this paper.
Nuclear states have an intrinsic spin and a well-defined parity, η = ±1, defined by the behaviour of the wavefunction for all the nucleons under reversal of their coordinates with the centre of the nucleus at the origin.
The spin and parity of the nuclear ground state for 218At was determined as:
J^π = (3−)
✅ Conclusion
Ground-state spin and parity assignment of J^π = (3−) is proposed for 218At. The half-life of 218At was determined from the decay data analysis. The decay curve was fitted with an exponential function and constant background, resulting in a high-precision result:
T₁/₂ = 1.27(6) sec