An Introduction to Gamma Ray Spectrum AnalysisAn experiment to determine the Gamma Detector Resolution

Tenicka T. TurnquestAdvanced LaboratoryDepartment of Physics and EngineeringMorehouse CollegeAtlanta, Georgia

Abstract

This article gives a brief look into the history of gamma -rays and their applications. This includes a brief description of the

gamma ray spectrum, gamma-ray production and resolution. More precisely, the wavelength and energy of gamma rays is

depicted as a part of the electromagnetic spectrum. The resolution of gamma-rays is analogous in size to the ratio of the distance

between points on the total absorption peak and the pulse height at the center of the total absorption peak. Within this experiment

the resolution of the NaI (TI) detector for gamma rays is analyzed.

IntroductionThe electromagnetic spectrum is the collection of all electromagnetic radiation. It can be viewed

as a collection of photons all moving at the speed of light with different energy and wavelengths.

The wavelength, energy and frequency of each type of radiation differ and have specific values

over a known range of values. The frequency of a photon is the quotient of the speed of light and

the wavelength of photon (c/). The period of a photon is the inverse of its frequency. The

energy of a photon in the electromagnetic spectrum is given by the quotient of the product of

Planck’s constant and the speed of light and wavelength of the photon (hc/).

Gamma-raysGamma-rays exhibit wavelengths between 10 and 100 femto-meters with energies from 124 keV

to 12.4 MeV.(Moore, p33)

Ex. Photon in (Red light) Visible light portion of the Electromagnetic spectrum

Gamma radiation is

The energy of a photon is inversely proportional its wavelength, it follows naturally that the

energy of a photon with a relatively small wavelength (gamma-rays (0.001-100 fm)) would have

abundantly more energy than a photon with relatively large wavelengths (visible (400-900nm)).

Gamma Ray DetectionRadioactive materials decay into alpha, beta and gamma particles. In order to detect gamma rays,

there has to be a mechanism which absorbs the alpha and beta particles, but which allow the

gamma particles to pass through. In general to detect gamma-rays, electrons are captured during

a decay process by a radioactive sample like sodium iodide (NaI (Tl)) or Germanium Ge (Li).

Sodium Iodide is the more prevalently used material for scintillation detectors because

Germanium has to be housed in a container using liquid nitrogen to keep the density profile of

the lithium drifting within this radioactive sample. There are two important quantities which are

determined when detecting gamma rays: the energy from the gamma source and the number of

gamma rays which are liberated from the source.

High Voltage Power Supply

Linear Amplifier

Multi-channel

Analyzer

Printer

Plotter

Teletype

PM Tube

Figure 1. Components of a Gamma Spectrometry System.

Compton EffectMost solid substances have crystalline structures. A crystal is a solid made up of a collection of lattice points with a known motif and pattern. More precisely, a crystal structure is a space lattice (3-dimensional organization of molecules in space) made up a periodically repetitive unit cells. Where a unit cell is defined by the points in space about which particles are free to vibrate in a crystal. There are exactly fourteen (14) distinct ways to construct a pattern of points without altering the basic symmetry elements of respective cells. The 14 possible ways of constructing a crystal structure with allowable symmetry are called Bravais lattices. A Bravais lattice is a space lattice with a distinct rotation axis, center of symmetry and rotary inversion axis.

Results

Cesium 137 Photopeak

0

10000

20000

30000

40000

50000

60000

40 45 50 55 60 65 70 75 80 85

Channel

Cen

troi

d C

hann

el

Counts

y = -136.9x2 + 17052x - 494948R2 = 0.7671

Discussion

Bibliography

G.F. Knoll, Radiation Detection and Measurement .Wiley, NY.1979

Moore, Thomas A., Six Ideas That Shaped Physics: Unit E.2 nd ed McGraw Hill, NY.2003

Quantum 8 Preliminary Operation Manual The Nucleus, Oak Ridge, TN. 1982.