Transcript Physics 490 - San Francisco State University

Lab B4: The Creation and
Annihilation of Antimatter
SFSU Physics 490
Spring 2004
Professor Roger Bland
Lab Partners
 Yvette Martinez
 Michael Hoffman
 Elizabeth Manrao
Experiment Description
 In this experiment you can observe
evidence for production and absorption of
gamma and beta rays, and for the creation
and annihilation of antimatter! Processes
involving gamma rays will be interpreted
using Feynman diagrams.
Learning Goals
 Understand the sodium-iodide scintillation
detector.
 Learn how a pulse-height analyzer works.
 Learn the different ways in which gamma
rays interact with matter.
 Understand the energy-level and decay
schemes for cobalt-60.
3 Ways Gamma Rays Interact
 The Photo-Electric
Effect
 Compton Scattering
 Pair Production
Experimental Physics, Dunlap, Oxford 1988 pg. 282.
The Photo-Electric Effect
Definition: The emission of
an electron from a surface as
the surface absorbs a photon
of electromagnetic radiation.
Electrons so emitted are
termed photoelectrons.
Source: http://ctd.grc.nasa.gov/dictionary/p.html
Compton Scattering
Definition: The scattering
of photons from charged
particles is called
Compton scattering after
Arthur Compton who was
the first to measure
photon-electron
scattering in 1922.
Source: http://www2.slac.stanford.edu/vvc/glossary.html
Pair Production
Definition: An absorption
process for X-ray and gamma
ray radiation in which the
incident photon is annihilated
in the vicinity of the nucleus
of the absorbing atom, with
subsequent production of an
electron and positron pair.
Source:
http://roland.lerc.nasa.gov/~dglover/dictionary/p.html
Experimental Procedure
Equipment List
 Tracerlab Detector #45292
 Computer: Halley 2 (IP 130.212.16.76)
 Oscilloscope: SN# 23042030, dual trace
 Canberra 816 Amplifier & High Voltage Power
Supply #70501
 Radioactive sources:
Co-60, Cs-137, Na-22
Experimental Procedure
Diagram of Apparatus
NaI Scintillation Detector
 Scintillation detectors detect light emitted by electrons when they
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change energy levels.
We use ionizing radiation from radioactive sources to provide the
electrons sufficient energy to move into a higher energy shell.
The electrons do not remain in the higher energy for long. Right
away they fall back to their original level and, as they do so, they
emit photons of visible light.
The number of photons of light emitted, and the intensity of the light,
is proportional to the energy of the incoming radiation.
Scintillation detectors are used to detect radiation and to separate
out the energies.
Scintillation Detectors
 Photomultiplier tubes are necessary in scintillation
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circuits to convert photons of light from the scintillator
into electrical pulses.
They are also used to amplify the size of the original
signal.
The incident radiation interacts with the crystal to
produce a light photon. This light photon then hits a
photocathode.
The energy from this light photon is absorbed by an
electron in the light sensitive material and this electron
gains enough energy to leave the photocathode.
The ejected electron forms the basis of the electrical
signal and is amplified at approximately four electrons
for every electron.
The high voltage power supply provides the stability the
circuit needs in order to operate consistently.
NaI Scintillation Detector
Source: http://www.physics.isu.edu/radinf/naidetector.htm
Multi-Channel Analyzer
 The pulses leaving the scintillation detector have
amplitudes proportional to the energy which the
particles or photons deposit in the detectors.
 These pulses are sorted according to their
height. This is equivalent to sorting the particles
or photons according to their energy.
 Electronic systems which do this are called
pulse height analyzers (PHA). Single channel
PHA's only count pulses of a given amplitude.
Multichannel analyzers (MCA's) can scan a
whole energy range and record the number of
pulses they count in each of the channels.
A Look at Cobalt 60
 Cobalt (Co) is a metal that may be stable or
unstable (radioactive, man-made). The most
common radioactive isotope of cobalt is cobalt60.
 Cobalt-60 is used in many common industrial
applications, such as in leveling devices and
thickness gauges, and in radiotherapy in
hospitals. Large sources of cobalt-60 are
increasingly used for sterilization of spices and
certain foods. The powerful gamma rays kill
bacteria and other pathogens, without damaging
the product (cold pasteurization).
Cobalt 60 Isotope diagram
Spectrum of Cobalt 60
Summary & Conclusion
 Using the Isotope diagrams we expected
to find 1.17 MeV, 1.33 MeV and 2.50 MeV
peaks on the MCA. After calibrating the
MCA we indeed found evidence
supporting this.
 We also found evidence for Compton
scattering.