Transmission Efficiency of the Recoil Mass Spectrometer EMMA at TRIUMF

TL;DR

EMMA at TRIUMF was used to measure the mean transmission efficiency as a function of kinetic energy/charge and scattering angles. Measurements with a monoenergetic $^{148}$Gd alpha source were compared to ion-optical (GIOS) and GEANT4 simulations, revealing noticeable discrepancies that motivated an empirical description. The authors develop a piecewise two-dimensional Gaussian fit for $\epsilon(\delta T,\theta,\phi)$ across nine $\delta T$ settings, enabling interpolation for arbitrary kinematics and quantified uncertainties. This empirical approach, integrated with GEANT4 or LISE++ recoil simulations, allows accurate mean transmission estimates for absolute cross-section measurements in EMMA experiments.

Abstract

The mean transmission efficiency of the EMMA recoil mass spectrometer at TRIUMF has been measured with 6 different angular apertures at 17 kinetic energy/charge deviations with respect to the central, reference trajectory. Measurements performed using a 148Gd alpha source installed at the target position of the spectrometer are compared to ion-optical calculations and Monte Carlo simulations. The transmission efficiency as a function of angle and kinetic energy/charge is described empirically using piecewise Gaussian functions whose parameters are fit to the data.

Figures (17)

• Figure 1: Plan view of the Electromagnetic Mass Analyzer EMMA, showing the target chamber, 4 quadrupole magnets (Q1-4), 2 electrostatic deflectors (ED1-2), dipole magnet (D), and the focal plane detector enclosure that contains the parallel grid avalanche counter (PGAC). The arrows indicate the trajectory of an ion moving along the optic axis.
• Figure 2: Schematic rendering of the angular apertures used to measure the transmission efficiency of EMMA, indicating their orientations and relative sizes. The 4 off-axis apertures are shown on the left, while the 2 on-axis apertures appear on the right.
• Figure 3: Measured mean transmission efficiency as a function of $\delta T$ with statistical errors. The solid curve represents the predicted transmission efficiency calculated with GIOS. The dotted curve shows the transmission efficiency calculated with a GEANT4 simulation.
• Figure 4: Measured mean transmission efficiency as a function of $\delta T$ with statistical errors. The solid curve represents the predicted transmission efficiency calculated with GIOS. The dotted curve shows the transmission efficiency calculated with a GEANT4 simulation.
• Figure 5: Measured mean transmission efficiency as a function of $\delta T$ with statistical errors. The solid curve represents the predicted transmission efficiency calculated with GIOS. The dotted curve shows the transmission efficiency calculated with a GEANT4 simulation.