Imaging Nuclear Reactions

Zhon Butcher2006 REU Program Cyclotron InstituteMentor: 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:totE

mz

dx

dE 2

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

Q1Q2

Qtot

LQ

Qx

tot*

1

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

10 k

10 k10 k1

0 k

10

k

10

kSchematic diagram of the

detector

1 M

2*

)(

)()( L

DCBA

DABCX

2*

)(

)()( L

DCBA

BADCY

Particle impinging position calculated by:

Signal Processing

Detector

PreamplifierSpectroscopy

Amplifier

ADC

DiscriminatorGate

Generator

Computer

Preamplifier

Preamplifier

Preamplifier

Preamplifier

SpectroscopyAmplifier

SpectroscopyAmplifier

SpectroscopyAmplifier

TimingAmplifier

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. 1/10th the resistance of the detector active area.

Setup

The detectors were placed in a vacuum chamber with a radioactive source. (241Am and 228Th were used)

The distance between the source and the detector was approx 25cm for 241Am and 10cm for 228Th

Calibration Masks

Two masks were used to cover the detectors.

Position Reconstruction 200m

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 228Th with 400m 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