Department of Physics

In-beam conversion electron spectroscopy

Home page of the JYFL electron spectroscopy group

In Collaboration with the University of Liverpool Nuclear Structure Group

 

Personnel

Peter Butler, Liverpool
Paul Greenlees, JYFL
Rodi Herzberg, Liverpool
Graham Jones, Liverpool
Peter Jones, JYFL
Rauno Julin, JYFL
Panu Rahkila, JYFL
 

News:
First in-beam tests carried out at RITU
First production runs completed successfully
Evidence for high-K bands in 254No

Introduction

In heavy nuclei, the process of internal conversion competes strongly with gamma-ray emission at low transition energies. When internal conversion becomes the dominant decay mode, measurement of internal conversion electrons is preferable to gamma-ray spectroscopy in order to probe the structure of the nucleus. To perform such measurements, the "SACRED" (Silicon Array for ConveRsion Electron Detection) device was installed at the target position of RITU in August 2000. The SACRED array allows the detection of cascades of conversion electrons, such that electron-electron coincidence techniques may be employed. The SACRED device was brought to Jyväskylä from the U.K. in the spring of 1994, and has been used as a stand-alone device with a square 25 element Si detector (refer to NIM A381 433 (1996) for a detailed description). The magnetic field to transport electrons from the target to detector was provided by a large super-conducting solenoid. The sensitivity of the device is further improved by the use of recoil-gating or recoil-decay tagging (RDT) to select a subset of events of interest.

Description of SACRED at RITU

The SACRED device consists of a 25 element annular Si detector, supplied by Hamamatsu Photonics, and a system of 4 normal-conducting coils to provide a solenoidal magnetic field. The prompt internal conversion electrons produced at the target are transported to the Si detector upstream of the target by the solenoidal magnetic field. The device is operated in (close to) collinear geometry, such that the solenoid field axis is at an angle of approximately 2.5 degrees to the beam axis, the beam passing on one side of the detector array to the target.
One experimental difficulty is the reduction of the background flux of low-energy delta electrons produced with large cross sections by the interaction of the beam with the target. This flux is suppressed by use of an electrostatic barrier, operated at a potential of around -35 kV. The barrier must operate in a high vacuum (around 10-6 mbar), and a system of 2 carbon foils is used to separate the high vacuum region from the He gas used to fill RITU. The efficiency of the device is estimated to be approximately 10% for the detection of single electrons at typical conversion electron energies of 50 to 350 keV. Further details can be found in our recent article: NIM A534, 503 (2004).

Schematics and Photographs:


Rendered Drawing of Gas Containment Window System:

Photo of Hamamatsu Annular 25-Element Detector Mounted in Chamber


Photos of SACRED Installed at RITU