(updated: october 2012)
experiment: E622s at GANIL / SPIRAL (LIRAT/IBE)
Date: march-april 2012
Purpose of the experiment
The prupose of this experiment is to measure the branching ratio of the super-allowed Fermi transition in the decay of 18Ne.
During the experiment, the half-life of 18Ne was also addressed, and the half-life of 19Ne as well to check the procedures. The separator was also tuned to measure 17F and 14O half-lifes.
The 18Ne radioactive beam was produced by fragmentation of a 95 MeV/A stable beam of 20Ne on a beryllium target.
The ions were separated with SPIRAL1 and sent to the IBE room, where the collection and detection setup was installed. The radioactive isotopes were collected on the mylar tape of a transport system (fig.1). After collection, the tape drive was moving the activity to a decay measurement station. Once the decay measurment was over, the tape was moved again to evacuate the activity before starting a new cycle (fig.2).
A plastic scintillator (from LPC Caen) is used at the decay station to measure the beta particles. For the branching ratio measurement, the germanium detector from CENBG (Gebdx) is used, and an additional detector is installed for gamma-gamma coincidences (Gedown). The collection point is equiped with 1 germanium detector (Geup) to monitor the activity during the collection.
Figure 1: Setup of the experiment: the collection point, with Geup detector (Camberra), is located at the upper station (low energy radioactive beam is coming from behing on the picture), and the decay station is the bottom one, with the precision detector (Gebdx)) in the front and the other germanium detector in the back. The plastic scintillator to measure beta particles from the decay is located in front of the tape, in the chamber, at the decay station.
Two VME based data acquisition systems were used in parallel (a GANIL VXI/VME DAQ and a mobile GANIL VME from CENBG). The dead-time for event processing is fixed to 200 us (this is a minimum, since data transfer and other processor tasks may make it longer in few occasions). In addition, the multiscaler DAQ from CENBG was also used to build time decay spectra, for checking purposes mainly, with fixed dead-times of 2 and 8 us.
The data acquisitions were triggered according to the phase of the cycle. During collection (COL) and tape move (MOV), the trigger is Geup (singles), that is divided to reduce the counting rate. During the initial background (BG) phase and the decay (DEC) phase, the trigger is the beta-gamma (plastic-Gebdx) coincidence.
Figure 2: Typical cycle from the experiment : the red curve shows the beta-gamma coincidences (BG and COL), and the blue one show the singles from Geup (COL and MOV).
The first online analysis of 18Ne were resulting in a half-life shorter than what was previously known (figure 3). The reason is that the Neon evaporates from the tape, and part of the activity is lost during the cycle. We measured also 19Ne for which the effect is the same (same chemical element), but with a much longer half-life: this allows to estimate the evaporation rate, and try to correct for it.
Figure 3: Beta*Gamma triggers in the decay of 18Ne. The lower plot shows a single cycle (black) with dead-time correction (red), and the upper plot shows the summed cycles over the run.
This problem also lead us to try to measure the decay of other isotopes, in the same context of weak interaction studies via beta decay: 14O for super-allowed Fermi transitions and 17F in the context of mirror transitions.
Figure 4: Fit of decay 14O for a single cycle: uncorrected time spectrum fit with a function taking into account a fix dead-time (red), and dead-time corrected spectrum fit with a standard function (blue).
Branching ratio estimate
The 0+ —> 0+ transition in the decay of 18Ne populates an excited state at E = 1042 keV . The branching ratio (BR) of this transition is determined from the number of single beta counts in the decay phase, compared to the number of gamma counts (from beta-gamma coincidence triggers) in the 1042 keV peak (see figure 5).
The BR estimate requires a precise correction of the gamma efficiency, which requires a very well calibrated gamma detector for absolute efficiency, and an acquisition dead-time correction.
Figure 5: Exemple of fit (one run file) of the 1042 keV peak in the decay of 18Ne: the black histogram is the full gamma spectrum, the red one contains only events in the analysed decay window, and the blue curve is the peak fit.
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