Gamma Ray Spectroscopy-Physics-Lab Report, Exercises of Physics

Download Gamma Ray Spectroscopy-Physics-Lab Report and more Exercises Physics in PDF only on Docsity!

Gamma Ray spectroscopy using NaI(Tl) detector

Submitted to:

Dr. Asloob Ahmed Mudassar

Submitted by:

Yasir Ali

M.Phil. Physics DPAM

PIEAS

Introduction: -. Gamma-ray spectroscopy is the quantitative study of the energy spectra of

gamma-ray sources, both nuclear laboratories and also in other part of science. Gamma rays are the highest-energy form of electromagnetic radiation; they are exactly like all other forms of electromagnetic radiations (e.g., X rays, visible light, infrared, radio) the only difference is their higher photon energy and frequency, and shorter wavelength. More over they are produced as a result of nuclear transitions. Because of their high energy, gamma-ray photons are generally counted individually. To measure gamma ray energy, a number of methods are used. A Geiger counter or Gamma Probe are used which determine only the count rate (i.e. the number of gamma rays interacting in the detector in one second). Another method is gamma ray spectrometer. A gamma-ray spectrometer also determines the energies of the gamma-rays photons emitted by the source.

Gamma ray spectrum:-. Spectrum is energy distribution. In spectrum energy plot versus

intensity is drawn. It shows how much photons are emitted for certain energy. Each element has its own characteristic spectra. Gamma rays emitted from nucleus of different elements can be shown in the form of spectrum.

Many radioactive isotopes decay by β-particle emission. However, the decay is often not to the ground state of the daughter isotope, but to excited states. These will then decay to lower energy states by emitting gamma rays until the ground state is reached. These rays can be detected, and their energies measured, using several types of detectors. In our experiment we used scintillation detector.

Scintillation detector:-. A scintillator is a special material, which exhibits scintillation—the

property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light. Light emitted in this process has information of the ionizing particle struck the detector. This should be converted into current and amplified so that electronic equipments can handle it.

A scintillation detector is coupled to an electronic light sensor such as a photomultiplier tube (PMT) or a photodiode. PMTs absorb the light emitted by the scintillator and reemit it in the form of electrons via the photoelectric effect. The subsequent multiplication of those electrons (sometimes called photo-electrons) results in an electrical pulse which can then be analyzed and yield meaningful information about the particle that originally struck the scintillator.

NaI(Tl) detector:-. In 1948, it was discovered that there was a large scintillation

phenomenon caused by adding a trace quantity of Thallium (Tl) into a crystal of Sodium Iodide (NaI). Development of the NaI (Tl) crystal started shortly after NaI (Tl) was discovered. In the

1. Place gamma ray source near the scintillation detector and measure count rate.
2. Counts per some duration of time are measured from electronic part attached with detector. Set time duration of about one minute.
3. Starting from some small voltage measure count rate. Increase the voltage in small steps and measure count rates for different voltages.
4. In beginning there will be an abrupt change in count rate for small voltages. For some voltage count rate will show small change when voltage is changed. Operating voltage lie in this region.

Using data (shown below) observed in above procedure is used to plot a graph as shown.

Counts taken per 10 seconds for calculating optimized operating voltage Voltage (Volts)

Counts 1

Counts 2

Counts 3

Average Counts/10 sec

Voltage (Volts)

Counts 1

Counts 2

Counts 3

Average Counts/10 sec

From this graph we can find best operating where counter has less noise and signals are detected efficiently. Operating voltage can be approximated as 850 volts as average of plateau region.

Using single channel analyzer:-..

Introduction and Theory:

Single-channel analyzer (SCA) is one of the conventional methods used for the determination or mapping of the gamma energy spectrum of a radioactive (gamma active) material. It uses a

scintillation detector with an inbuilt photo-multiplier tube (PMT). The gamma particles (photons having energy of the order of a few MeVs) emitted by the radioactive materials (like Cs -137, Na

• 22, etc) are converted to the visible photons (or scintillations) by the scintillator and these visible photons are then used to knock out the electrons from the atoms, which are then multiplied using a photo-multiplier tube to give a detectable pulse/signal. Each such pulse or signal is counted as one gamma.

Setup:-. The single channel analyzer consists of a Scintillation detector with an inbuilt

photomultiplier tube. The high voltage required for the acceleration of electrons is provided to the PMT. Whereas the output pulse/signal from the PMT is pre-amplified and passed through a discriminator and pulse shaper and then finally to an electronic counter. The electronic counter counts the incoming pulses and shows them on its display screen. As evident the signal from the PMT carries a lot of noise which is primarily removed by the discriminator circuit. The pulse shaper changes the incoming bell shaped pulse to a square pulse so that it can be counted properly by the electronic counter. Here pulse is changed into logic pulse.

Integral spectrum with upper level of 10 volts Lower level (V)

Counts rate

Lower level (V)

Counts rate

Lower level (V)

Counts rate

Lower level (V)

Counts rate

Lower level (V)

Counts rate 0.1 20090 0.9 14152 1.7 9274 2.5 3618 3.3 611 0.2 19050 1 13481 1.8 8792 2.6 3083 3.4 564 0.3 18224 1.1 13041 1.9 8028 2.7 2815 3.5 567 0.4 17467 1.2 12454 2 7351 2.8 1830 3.6 537 0.5 16635 1.3 11542 2.1 6794 2.9 897 3.7 462 0.6 15850 1.4 11169 2.2 6331 3 820 3.8 416 0.7 15224 1.5 10552 2.3 5852 3.1 708 3.9 367 0.8 14437 1.6 9997 2.4 5288 3.2 664 4 359

Differential spectrum of Co^60 using MCA:-. In this part we will use multi channel

analyzer (MCA) to find spectrum of gamma-ray of Co^60. MCA will be run as a pulse-height analyzer (PHA), where each channel (or windows as we chose in SCA) corresponds to a small range of energies. Each event in the detector produces a voltage proportional to the energy of the gamma-ray which caused the event. The event is then added to the channel corresponding to its voltage (energy) range, and the resulting spectrum is thus a plot of intensity (number of counts) versus voltage (energy).

Procedure:-

To measure photo peaks or peaks i.e. 1.17 MeV and 1.33 MeV photo peaks from Co^60 are used

to calibrate the NaI (Tl) detection system.

1. Set up the electronics as shown in Figure 1. There are two parameters that ultimately determine the overall gain of the system: the high voltage that is furnished to the phototube and the gain of the linear amplifier. Their gain of the photomultiplier tube is quite dependent on its high voltage. A general rule for most phototubes is that a 10% change of the high voltage will change the gain by a factor of 2. The high voltage value depends on the phototube being used.
2. Set the high voltage to 850 V.
3. Start the Maestro MCA program.
4. Place the Co^60 source in front of the NaI(Tl) crystal.

Now electronic setup does the entire job. Gamma rays from cobalt-60 strikes the NaI(Tl) detector where they produce some current. The current is proportional to energy of gamma rays photons. This current is initially very small, so a photomultiplier tubes are used to increase this current. This current then passes through preamplifier and amplifier for further amplification and reshaping. Then it passes through single channel analyzer and it is converted into logic pulses. These pulses have height corresponding to energy of photons. They are sent to multi channel

analyzer. Multi channel analyzer has 1024 channels or windows. Each window has certain energy range and accepts pulse of certain height. Pulses with higher or lower height are not accepted or counted by a window. It will count only those pulses which have height or energy in range of that window.

This process is continued for sufficient large time so that computer makes a beautiful graph of gamma ray spectrum of cobalt.

As viewed that Co^60 has two photo peaks in its gamma ray spectrum.

Spectrum of Cs^137 :-. To find spectrum of Cs^137 , similar procedure to cobalt is repeated. MCA

again will count photons of different energies in different channels whose energy correspond to their energies. Spectrum obtained is given as.