Nuclear Structure Studies of Neutron Deficient Nuclei

   In nuclear structure physics we constantly want to probe the structure of nuclei further and further from stability. These nuclei are much more difficult to populate than nuclei nearer to stability. The study of very neutron deficient nuclei requires advanced detector systems that can consist of:

• A large high efficiency g-ray detection device.
• A charged particle detector.
• A neutron detection system.

   The GAMMASPHERE detector array (shown below) is a detector system based in the United States of America. It is a device which consists of approximately 100 high purity germanium (HPGe) detectors. The detectors are mounted in a spherical (4) configuration completely surrounding a target. The primary function of the array is the detection of -rays that are emitted following a nuclear reaction.

   A large contribution to the Doppler broadening of the -rays comes from the size of the detector opening angle , with the effect maximised in detectors at 90 degrees ( = 90) to the beam direction.

   The Doppler Broadening equation has important consequences for segmented germanium detectors:

E = E ( 0 ) (v/c) sin ( )

   The GAMMASPHERE array contains electrically segmented HPGe detectors at angles around 90 degrees to the beam direction. The segmentation effectively splits the detector into two halves, reducing the opening angle of the detector. The Doppler broadening of the detector is then reduced.

   The angle can be better defined reducing by using a segmented detector.

   The ratio R of the side channel energy to the high resolution energy gives:

• R > 90% - Right hit
• R<10% - Left hit
• 10% < R < 90 % - Centre hit

    Linear Polarisations can be deduced.

Charged-particle detection

   The MICROBALL detector system is an ancillary detector designed for use with the GAMMASPHERE array. It consists of 95 CsI scintillator detector units completely surrounding the target in a 4 configuration. It is mounted within the HPGe detectors and so much closer to the target which is mounted within the CsI detectors. The MICROBALL is used to detect and discriminate between protons and alpha particles. Particle discrimination is achieved by using Pulse Shape Discrimination (PSD) and Zero Crossing Time (ZCT).

• Pulse shape discrimination: The light output from the scintillator has two components, one has a mean decay time t = 0.4-1.0µsec, the amplitude and fall time of which depend on the particle detected. The second component has a decay time of 7µsec and is particle independent.

• Zero Crossing Time: This is obtained from the differentiated fast signal of the constant fraction discriminator. The reference time is taken as the RF time of the cyclotron.

   The MICROBALL also allows the Doppler broadening of -rays to be reduced. This is afforded by kinematically reconstructing each event. The direction of the recoiling compound nucleus can be reconstructed because the MICROBALL measures the energy and momentum of the protons and alpha particles that are emitted immediately following a nuclear reaction. The reduction in the peak Full Width Half Maximum (FWHM) is significant.

Neutron detection

Neutron detectors can be used as an important reaction channel selection device.

   The neutron detector system that was used to augment the GAMMASPHERE array consisted of 15 NE213 liquid scintillator neutron detectors; these were chosen because NE213 provides good n- discrimination. The neutron detectors were mounted at forward angles capturing the neutrons produced in the compound nucleus reaction. Neutron- discrimination was achieved by using Time of Flight (TOF) and Pulse Shape Discrimination (PSD).

• Pulse Shape Discrimination: In the same way as the MICROBALL the contribution to the total light output of the scintillator is dependent on what interacted in the scintillator (n or ). For neutrons the contribution to the total light signal is greater than for s. An example of the Pulse Shape Discrimination method is shown.
  
  

• Time of Flight: For low energy neutrons the TOF method provides better n- discrimination than PSD. Neutrons go slower than s so they arrive at the neutron detectors after the s. For low energy neutrons TOF discrimination is preferable.

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Last modified: 4 March 1998