Imaging Nuclear Reactions Zhon Butcher 2006 REU Program Cyclotron Institute Mentor: Dr. Robert Tribble

Applications of Nuclear Imaging  Space Telescopes – Cosmic radiation identification and direction of origin.  Imaging reactions in the nuclear physics laboratory.

How Imaging Works in the Lab  Several detectors are placed around the reaction site covering a given solid angle.  Detectors determine particle identity and position.  The resulting image gives a picture of the reactions that took place in the chamber.

Particle Identification  Telescopes: Front detector and rear detector. Front detector picks up energy loss as the particle passes through. Rear detector picks up residual energy.  Particle identification determined by:

Methods for Position Determination  Many small detectors coupled with a large amount of electronics (clustering).  Resistive strip detectors.  Double sided strip detectors.  Resistive sheets.

1-D Position Sensitive Detector Q1Q1 Q2Q2 Q tot

Resistive Strip Detectors  Consist of many resistive strips placed alongside one another.  Good resolution in the X direction, poor resolution in the Y direction (or vice versa depending on orientation).

PSSDs

Double Sided Strip Detectors  Two sheets of strips placed one in front of the other so the strips form a grid. Results in better position resolution  Washington University team had detectors with 32 strips in each direction. 64 strips per detector x 4 detectors = 256 channels for position reconstruction

Double sided PSSDs

Resistive Sheets  A single resistive sheet spans the entire active area of the detector.  Advantages Fewer signals to process. Less electronic equipment.  Detector Types: Duo-lateral: Generates two signals from each face of the detector, two from the front and two from the back. Tetra-lateral: Generates five signals, one from each corner of the resistive side, and one signal from the back.

Tetra Lateral Detectors 10 k  Bias 10 k  Schematic diagram of the detector 1 M  Particle impinging position calculated by:

Signal Processing Detector Preamplifier Spectroscopy Amplifier ADC Discriminator Gate Generator Computer Preamplifier Spectroscopy Amplifier Spectroscopy Amplifier Spectroscopy Amplifier Timing Amplifier Rear signal

How Silicon Detectors Work

Current Through Semiconductor

Doped Semiconductor What is doping?  Doping is the integration of impurities into the lattice structure of the semiconductor. This allows extra electron and hole energy levels which will increase the conductivity of the semiconductor.

Experiment  To characterize the Micron Semiconductors tetra-lateral detectors in terms of energy and position resolution as well as non-linearity in position reconstruction.  Three tetra-lateral type PSDs were investigated. One 200 m and one 400 m thick detectors with a resistive strip around the active area, and one 200 m without a resistive strip.  Optimal strip resistance is approx. 1/10 th the resistance of the detector active area.

Setup  The detectors were placed in a vacuum chamber with a radioactive source. ( 241 Am and 228 Th were used)  The distance between the source and the detector was approx 25cm for 241 Am and 10cm for 228 Th

Calibration Masks  Two masks were used to cover the detectors.

Position Reconstruction 200  m Position reconstruction of impinging alpha particles for the 200 m thick detector with and without a resistive strip. Without resistive strip: With resistive strip:

Position Reconstruction 400  m Position reconstruction of impinging alpha particles with and without a mask for the 400 m thick detector with a resistive strip. Without mask: Slit mask:Holes mask:

Energy Resolution  Energy Spectrum of alpha decay from 228 Th with 400m detector: Energy Resolution: Approx 10%

Results  The position resolution was determined to be around 3-4 mm and energy resolution of 8% for both the 400 m and 200 m thick detectors with the resistive strip.  The resistive strip has a major contribution in reducing the position reconstruction distortion. * *For more information see T.Doke et.al. NIM A261 (1987) 605

Conclusion  The position resolution for the tetra-lateral PSDs strongly depends on the resistivity of the resistive sheet, electrode termination resistors, the filter components of the preamplifiers, and the shaping times of the amplifiers.  The measurements done were employing the use of Indiana University preamplifiers and CAEN amplifiers (3 s shaping time). Further investigation of these dependencies is ongoing.

Acknowledgements Special thanks to:  Dr. Robert Tribble  Dr. Livius Trache  Dr. Adriana Banu  Matthew McCleskey