Lab Report: NE-2204
Department of Nuclear Engineering
Lab Number:
LAB TITLE: Demonstration and explanation of different gamma ray peaks for Co-60 sample.
Prepared by:
Student Name: Saad Islam
Roll: 106
Registration No. : 2013-312-224
Department of Nuclear Engineering
University of Dhaka, Bangladesh
DU
Objective: To demonstration and explain different gamma ray peaks for Co-60 sample.
Equipment:


NaI(Tl) detector (SPA38) with Multi Channel Analyzer (MCA UCS30-1K)
Radioactive sources: 60Co (orange)
Introduction: Gamma rays, photons of a very high-frequency electromagnetic wave, are emitted upon
the transition from an excited energy state of a nucleus to a lower energy state. For the sources that will
be observed, the gammas are emitted following the decay of a radioactive nucleus by processes such as
alpha decay (α), beta (β−) decay, positron (β+) decay, or electron capture (EC). A schematic graph of
some of these nuclear transitions for 60Co is shown in the below figure. Note that the vertical axis is a
mass-energy axis in units of keV (=1.602 x 10-16 Joules), and the horizontal axis is the atomic number, Z,
the number of protons in the nucleus. Gammas are emitted on transitions between energy states,
where the transition is represented by a vertical arrow. Diagonal arrows represent α, β−, β+, or electron
capture transitions. The objective is to collect the gamma spectra from 60Co radioactive sources and
measure the gamma energies and the characteristics of the peaks. Study the nonlinear response of the
NaI(Tl) detector.
Figure 1 Energy level diagram for 60Co nuclei. The vertical axis is the mass energy while the horizontal axis is
the atomic number.
Procedure:
1. Start the MCA, the calibration is done by selecting Auto Calibrate in the UCX x64 java program
provided by SpecTech. The auto calibration is done with a 137Cs source. The observed photopeak
should be centered near 661.6 keV. If it is not within 10 keV of the accepted value, the system
should be recalibrated. Use 2 point (or 3 point for quadratic channel to energy relationship )
calibration:
Channel
keV
695
1173.21
789
1332.47
then by going to Settings>Amp/HV/ADC, set the high voltage to 550 V (click “on”), the coarse
amplifier gain to 4, the fine amplifier gain to about 1.5, the conversion gain to 1024, and the LLD
to 0. Look at Mode Menu to be sure Pulse Height is checked. Also check the Display Menu to be
sure Calibration and ROIs are both unchecked. In the Settings menu, Clear all ROIs.
2. Get the 60Co source from the box and place it directly under the crystal end (2nd shelf) of the
detector. Take data by pressing the Go button.
3. Acquire a gamma spectrum for 60Co radioactive source. Set several region of
interest (ROI) by going to Settings>ROI>Set ROI. You can then use the cursor to highlight
ROI. Place a ROI around each peak, Record a data of the spectrum. Also record the centroid
energy, channel number, and FWHM for each photopeak.
Observations:
Figure 2: The raw data obtained from the module is analyzed in CAS
Cobalt-60 is a man-made isotope with a half-life of 5.27 years. It decays emitting an electron with a
maximum energy of 318 keV (β decay) into an excited state of the stable nickel-60. From this state a
transition into another excited state takes place with emission of a 1173.21 keV γ quantum, then the
ground state is reached whereby a γ quantum of 1332.47 keV is emitted.
The jacket of the preparation used in the experiments absorbs the β particles. Therefore only γ quanta
can be observed.
When doing γ spectroscopy with a scintillation counter, the Compton edge of the line with the higher
energy 1333 keV is at 1119 keV, i.e. it is located in the low-energy edge of the second line at 1173 keV
and distorts its shape.
A Compton collision outside the detector leads to an energy loss of the γ quantum before it reaches the
detector. A continuum of scattered photons arises with energies from Eγ (661.6 keV) down to the
energy after 180° backscattering (94.18 keV in the spectrum). Because of the angular dependence of the
scattering coefficient (Klein-Nishina formula), the probability of 180° backscattering is enhanced. This
leads to the backscatter peak at channel 58 the spectrum.
In the detector the γ quantum with the energy Eγ can be completely absorbed (photopeak), but a
Compton effect can occur as well so that the γ quantum escapes from the detector and only the energy
of the electron is detected. The energy of this electron lies between zero and the maximum value, which
corresponds to 180° backscattering, leading to a continuum from zero up to the Compton edge.
Result: The spectrum and analyzed and the analysis is coherent as expected.
Precautions:
1. The electric power must be provided only after making all appropriate connections.
2. The applied HV across PMT, ADC’s coarse and fine gain and quadratic scaling are twitched to get
a neat spectrum.
3. Never to leave the live module unattended.