Gama Ray Spectrometry

8/12/2019 Gama Ray Spectrometry

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-KAVERI

-PRIYANKA

GAMMA RAY SPECTROMETRY



Introduction

• The particular energy levels of nuclei are characteristic ofeach species.

• The photon energies of the gamma rays emitted, which

correspond to the energy differences of the nuclei, can be

used to identify particular elements and isotopes.

• Distinguishing between gamma-rays of slightly different

energy is an important consideration in the analysis of

complex spectra : spectral resolution

• Semi-conductor detectors, based on cooled germanium orsilicon detecting elements are extensively used.

http://en.wikipedia.org/wiki/Spectral_resolution

http://en.wikipedia.org/wiki/Germanium

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Germanium

http://en.wikipedia.org/wiki/Spectral_resolution



Sources of gamma radiation

Potassium (40K)

Uranium (238U and 235U and their daughters)

Thorium (232Th and its daughters)

Artificial radio-isotope: 137Cs with photopeak at 0.662 MeV and has a

half-life of about 30 years



All radioactivity counting problems involve:

1. Source of ionizing radiation

2. A detector sensitive to the radiation

The radiation is emitted in all directions and israndom.

Decay law:

dN/dt=- N λ where N=number of radioactive isotopes present

λ=decay constant



The detector response

• Thallium-doped sodium-iodide scintillation

crystals NaI(TI).

• These detectors modify the spectrum considerably.

•

The main aspects of the detector response are :1. Detector efficiency

2. Energy resolution

3. Dead time



• Detector efficiency relates to how well the detectorabsorbs gamma rays.

• The detector energy resolution is a measure of a

detector’s ability to distinguish between twogamma rays of only slightly differing energy.

• Dead time refers to the finite time required for the

spectrometer to process individual photons.

• Spectrum photopeaks have Gaussian shapes.

• This is mainly due to the limited energy resolution

of NaI detectors.



Counting efficiency

The ratio the counting rate to the number ofdisintegrations per unit time

E= cps/dps

Factors that affect overall efficiency of countingsystem

1. Geometry

2. Absorption

3. Scatter

4. Intrinsic detector efficiency





Geometry Location, size and shape of the sample in relation

to the sensitive volume of the detector

Inverse square law

As the detector moves away from a point source,

the count rate decreases by 1/(R*R) and vice versa. If R is reduced to 0, the counting rate does not

become infinite.

A closer approximation to the true geometric

efficiency:

½(1-cosA)

where a=one-half the angle subtended by the

crystal



Even greater detector efficiency is achieved by placing the source within the source :

geometry of a well counter

100% geometric efficiency can be obtained bysuspending the source completely within the

detector :

liquid scintillation counting





Absorption

Not all of the radiations emitted from theradioactive source and subtended by the detector

reach the detector.

Some are absorbed by means of interaction withmaterial of the source(self-absorption), in the

medium between the source and the detector and in

the window of the detector itself.

Absorption outside the detector always decreasescounting rate.



Scatter

Scattered photons in the source may either increaseor decrease the detector efficiency.

Compton scattered photons

Scattering reduces photon energy, but may increasethe total counts observed.





Intrinsic efficiency

Ratio of the number of radiations that interact inthe detector to the total number of incident

radiations.

Nearly 100% for alpha and beta particles due toshort path length.

For gamma rays, it is dependent upon the photon

energy and the size, shape and composition of the

sensitive detector volume. Detector efficiency decreases logarithmically with

increasing photon energy.





Instrument dead time

The period following the occurrence of a pulse during

which the instrument is insensitive to incoming

radiation

At high counting rate some pulses are lost Some detectors have built in dead time correction

circuitry

Usual solution: avoid dead time errors by limiting thesource activity

True counting rate: Nt=n0/(1-n0t)



COUNTING BETA PARTICLES -vely charged and are emitted in a continuous energy

distribution ranging from zero to maximum energy

characterstic of the particular nuclide.

Readily interact with matter,easily absorbed and

scattered.

Most commonly used beta counters are liquid

scintillation counters.

Advantage of liquid scintillation counting system over

others is the radioactive sample intimately mixes with

the liquid fluors resulting in high geometric efficiency

and overall efficiency for low energy beta.

Beta can be counted with geiger-muller tubes fitted

with thin mica windows.

Gas flow proportional counters may be used for low

energy beta.



GAMMA RAY COUNTING

Gamma ray counting are preferred because: -Easier to detect and quantitate.

-Improves radiation protection.

-Administered dose is easily calculated.

Attenuation of a beam of No photons is: N=Noe^(-ux)

Where N=final no. of photons

u=linear attenuation coefficient

Photon attenuation or absorption within the

scintillation crystal occurs by the threeprocesses of photon interaction:

-Photoelectric effect

-Compton scattering

-Pair production



Gamma ray spectrometry



Measurement Of Radiation



Variable portion of absorbed energy converted toheat rather than light photons.

Variable self-absorption of light within the crystal.

Variable photon absorption within the

photocathode,which may be irregular over its

surface.

Imperfect dynode amplification.

Fluctuating background or noise throughout theelectronics.

Variable amplification gains.



MAXIMUM ENERGY IS IMPARTED TO ACompton electron when the electron is

scattered at 180 degrees to the incident

photon & is given by relation:

Compton edge(KeV)=E^2/(E+256)

Backscatter peak(KeV)=256E/(E+256)



Types of scattering which degrade

the spectrum

Iodine K

escape





Scattering within the source is most commonorigin of low energy pulses.(photon g,h)

Collimator Scatter is a source of 2 different kinds

of scatter.(Photon e,f)





Application

Astronomical Spectrometers.

Planetary Gamma Ray Specrometers.

Radioelement Mapping.

Water Detection.

W d i



Water detection

By measuring neutrons, it is possible to

calculate the abundance of hydrogen, thus

inferring the presence of water.

The neutron detectors are sensitive to

concentrations of hydrogen in the upper

meter of the surface.

When cosmic rays hit the surface of Mars,neutrons and gamma-rays come out of the

soil.

The GRS measured their energies.

Certain energies are produced by

hydrogen. Since hydrogen is most likely present in

the form of water ice, the spectrometer will

be able to measure directly the amount of

permanent ground ice and how it changes

with the seasons.



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Gama Ray Spectrometry

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