NUCP 2371 Radiation Measurements II Gamma Spectroscopy NUCP 2371 Radiation Measurements II

Spectrometer Since electromagnetic radiation does not cause any direct ionization Detector system needs to do several things Reasonable probability to convert gamma rays to electrons Must be able to detect these electrons Able to separate signal output from detector

EM radiation interactions Since EM does not have any charge it must interact directly with the electron to cause ionization The more electrons to interact with the higher the probability to create an ion pair Usually if have more electrons will have more Protons More protons usually (but not always) leads to a higher density Density is main characteristic of how EM radiation will interact with mater

EM Interactions Rayleigh scattering- scattering of light by very tiny particles not very important because it does not cause ionization Photoelectric effect- electron scattering Comptom scattering- electron scattering and an extra gamma ray Pair Production- creation of two charged particles

Photoelectric Interactions Low energy process Electron absorbs all energy of incoming photon Electron gets ejected with the energy of the photon minus the binding energy of the electron Binding energies are several to tens of eV Kinetic Energy of all electrons should be the same as the incoming photon If all energy is captured in detector will get a single peak corresponding to the photons energy

Compton Interactions Medium energy Electron absorbs some the incoming photons energy and creates a new lower energy photon Energy distribution is based on the scattered angle All scattered angles will be in detector so electron will have a series of energies

Compton (cont) Leads to the Compton continuum, the misc energies of the photon that gets scattered at different angles which lead to different energy deposition in the detector Compton edge distance from photopeak is dependant of energy of photon but is usually limited to 256 keV

Compton Scattering λ1- λ0= h/mec (1- cosθ) λ1= wave after scattering λ0 = initial wave of EM radiation h/mec= Compton wavelength of electron 2.43 X 10 -12 m h=planks constant m= mass of electron c= speed of light θ = angle of scatter Energy= h f E= hc/ λ because λf=c h= planks constant= 4.13 E-15 eV s

Energy change Determine λ of initial EM radiation Calculate change in λ Add change in λ to original λ Then calculate the new energy of the scatter EM radiation Start with 500 keV photon that is deflected 45degrees What next?

Pair Production High energy process High energy EM interacts with the strong electric field of the nucleus Produces an electron and positron Energy of the two particles is energy of the EM wave minus 1.022MeV (rest mass energy of a β- and β +) Responsible for single and double escape peaks if energy of these particles escapes the detector

Gamma Spectroscopy The size of the signals generated from scintillators and solid state detectors are proportional to the energy deposited in the crystal. Signals can be organized with respect to energy Each time a signal of a certain magnitude is counted it is added to that energy category So more of the same size signals that get produced from the detector will result in a larger peak at that corresponding energy Result will be a chart (spectrum) of the energies of the gamma rays and their intensities Signals can be sorted by output strength and counted, when arranged in increasing energy order we can create a energy spectrum of the gamma ray energies emitted byt a sample. Then we can compare this energies to nuclides that emit these energies ands [possible identify the unknown nuclide

Gamma Spectroscopy Can be used to identify unknown radionuclides Can be used determine quantity of material present when calibrated Can be quite complicated if have many radionuclides present There are week long classes on how to properly read and interpret gamma spectra.

NaI vs HPGE NaI Higher efficiency Lower cost Larger sizes HPGE Better resolution Better for detecting weak sources But if need resolution in complex spectra no better than HPGe

NaI and Ge detectors Notice the difference in the gamma spectra from the two different detectors

Uranium Spectrum

Spectrum Photo peak Xrays Compton edge Single escape peak Double escape peak Sum peak 511 peak Ge escape peak FWHM

Photo Peak Large peak in the spectrum that correlates to the full energy of the EM radiation Ideally would be straight line if all energy from EM would be collected in detector Is a wider peak due to some of the energy of the EM is lost from the detector

Photopeak

Xrays Low energy peaks that corresponds to either characteristic X-Rays from the sample or the shield material Some of the energy from EM radiation will excite the electron in the shield material, will have prominent peak at the Xray energies of Lead

Xrays

Single/Double Escape peak Energy lost from the photo peak which correspond to the energy equivalent of an electron Only in high energy EM radiation (>1.022MeV) will escape peaks be present If have a small peak 511 keV lower than your main photopeak this is the energy of an electron escaping from the detector

511 keV 511 keV is a special energy It is the energy that is emitted when a positron gets annilated by an electron and 2 511 keV gamma rays are produced. If you get this peak in your spectrum there is a good chance you have a positron emitter in your sample Easy to spot

Peaks

Sum Peak If you have sample with much activity May get a peak that is twice the energy of the photo peak This is from the detector seeing 2 independent EM radiations as one large one and will add the energies of them together and have one count but at twice the energy

Sum Peak

Ge Escape Peak Ge x-rays gets subtracted from the photopeak Only important in low energy photopeaks In the order of 10-14 keV

FWHM Full width half maximum Measure of the thickness of a peak Take the width in channels at the point where the counts in those channels are ½ those of the maximum counts The lower the number the more compact and peaky the photo peak will be Can be used to determine if the detector is working correctly

Detector Sizes Small detector- Large detector- size of detector is small compared to the path of secondary gamma rays Will result in significant compton continuum and possible escape peaks Large detector- Detector is large compared to path of secondary gamma rays Will result in a single photo peak and no Compton continuum Most detectors are in between

Signal processing Detector-detects radiation and produces an electronic signal Amplifier- amplifies the signal so it can be better seen ADC or Shaper- turns signal from Gaussian function into step function 2 types used in counting Scaler- separates signals according to potential Counter- accumulates counts and displays results

Calibration Energy-match energy of photo peak to the energy on the screen Efficiency- match area of peak to activity of source

Energy Calibration Purpose- match up the energy of the photopeaks with the energy label on the spectrum How is it done- have a know spectrum of photo peaks and their energies Correlate the channel number to the photopeak energy Need to have at least 3 peaks(5 better) Create an energy curve Save data and apply it to all spectra

Efficiency Calibration Purpose- to have the area of the photopeak match the activity of the source How is it done- have a standard for which you know the activity and the % error Assign that activity to the area of the photo peak Need to have at least 5 peaks from low to high energies Create a efficiency curve that will be applied to all spectra Efficiency will then be used to convert areas to activities for all energies

Efficiency Calibtration Low energies will have low efficiencies EM can not effectively get through the protective covering of the detector High energies will have low efficiencies Higher energy EM will have a lower probability of interacting with the detector

Betas High energy beta will create brehmstrahhlung This will appear on the gamma spectrum as a large broad low energy mound Can interfere with small low energy peaks

Neutrons Neutron- too many neutrons will damage the detector Avoid neutrons

QUESTIONS?