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Category: Chemical Engineering Seminar Topics

Gamma Ray Spectroscopy


What a gamma-ray will interact with its medium is considered one of three exclusive approaches: photoelectric absorption, Compton scattering, and pair production. These more than a few interactions trade their probability of occurring depending on the power of the gamma-ray and the tiny quantity of the fabric.

Photoelectric Absorption

In the photoelectric absorption, the incident photon disappears, and a photoelectron will be provided from one of the electron shells of the absorber. The kinetic energy that this electron carries off is Ee− = h − Eb, where Eb is called the binding response of the liberated electron in its original shell. Electron rearrangement will immediately fill this empty spot in the electron shell. This process causes the binding energy, Eb, to be released as well. This power shared in the form of a single X-ray or an Auger electron.

The photoelectric absorption interaction is the ideal interaction for gamma-ray spectroscopy. The photoelectron moves on most of the gamma-ray energy, and then an X-ray or Auger electron runs on the remaining kinetic energy. Assuming an ideal detector, the sum of these powers will equal to the strength of the gamma-ray.

Here is desired for gamma-ray spectroscopy because we are interested in knowing the energies of the various gamma-rays that are transmitted by a source. We see what the ideal photopeak created by mono-energetic gamma-rays of single power looks similar.

Compton Scattering

The Compton scattering communication is the scattering of a gamma-ray off of a free or unbound electron, thus generating a scattered gamma-ray photon and a recoil electron. The energy of the incoming photon is distributed between the scattered photon and the recoil centre by a connection that is dependent on the scattering angle. There are two advanced positions recorded by this equalisation: When = 0, the scattered photon holds all of its energy and the recoil electron accumulations no power. When =, the incident gamma-ray is backscattered, and the recoil electron moves along the direction of the number. In this case with the maximum energy transfer between the incoming gamma-ray and the electron.

In the detector, all scattering angles from 0 to will occur. Because of this, a continuum of energies can convey to the electron. This power has a range from 0 all the way to the maximum.

Pair Production

Pair result is a gamma-ray that turns into an electron-positron pair. That occurs when the gamma-ray is in the intense electric field near the nuclei of the engaging material. There is a minimum amount of gamma-ray force that is required for this method to take place.

Scintillation Detector

A block diagram for a typical scintillation detection system. The scintillation detector is illustrated. Our sensor has a 44 inches round NaI scintillation crystal which started with about 1 part in 103 thallium impurities. Through various processes, a gamma ray passing into the glass may interact with it creating many visible and ultraviolet photons (scintillations). Oscilloscope Linear PM Base Amplifier NaI (Tl) Detector Source HV Power Supply Preamp Multi-Channel Analyzer Figure 1: Block diagram for a scintillation detector system.

To determine the scintillation photons, the crystal is set near to a photomultiplier tube (PMT), and the scintillator/PMT (detector) is inserted in a reflective, light-tight housing. For breaking the consequences of history gamma radiation, the sensor is surrounded by a thick lead shielding tube with the desired gamma rays entering at the scintillator end of the tunnel. The PMT consists of a photocathode followed by a series of dynodes (6-10 is typical) followed by and ending with a collection anode. Scintillation photons are striking the photocathode eject electrons via the photoelectric effect. A high voltage (HV) power supply and a resistor chain (not shown) bias the cathode, dynodes, and anode to stimulate electrons from the cathode into the first dynode, from one dynode to the following, and from the final dynode to the anode collector. Each incident electron strikes a dynode with sufficient energy to eject around 5-10 which is called secondary electrons from that dynode. For each primary photoelectron, by the end of the chain, there are on the order of 106 particles reaching the anode.

The anode consolidated to a charge-sensitive preamplifier which saves the collected charge to a proportional voltage pulse. The preamp beat is when shaped and amplified by a linear amplifier before processing proceeds. Because the amount of light produced in the scintillation crystal is directly proportional to the amount of gamma-ray energy initially absorbed in the glass, so also are the number of photoelectrons from the cathode, the final anode charge, and the amplitude of the preamp and amplifier voltage pulses. The overall effect is that the last pulse height is proportional to the gamma ray energy absorbed in the crystal.


Analyzer Pulse Height

As per name is shows that a pulse height analyzer (PHA) estimates the height of each input pulse. Special circuitry, including a sample and grip amplifier and an analogue to digital converter, determines the maximum positive height of the pulse—a peak voltage as might be read off an oscilloscope trace. From the pulse height, a corresponding channel number is determined. For example, for a PHA having 1000 channel capability and a pulse height measurement range from 0 to 10 V, a pulse of height 1.00 V would correspond to channel 100, one of 2.00 V would communicate to channel 200, one of 8.34 V would correspond to direct 834, etc. After the correct channel for a given input pulse has been prepared, the PHA then increments the count in that circuit.

Our PHA can separate pulse heights in the variety 0-10 V and shall be mounted to style them into 1024 channels. After many pulses of different sizes have made, a plot of the numbers in each channel versus the channel number may also be exhibited to exhibit the distribution of pulse heights. With some caveats to be described quickly, the heartbeat height distribution from a scintillation indicator may represent as a plot of the number of gammas versus the energy of the gammas from the source, i.E., a gamma-ray spectrum of the (radioactive) supply. Tiers of pure isotopes may also be discovered in references and compared with a source frequencies to assess the nuclear composition of the amount.

Gamma Interactions

For knowing about the pulse height distribution connected with the gamma rays from a radioactive source, it is essential to realise that only a fraction of the gamma rays interacts with the scintillator; several do not interact at all and merely pass right through. Furthermore, when a gamma does communicate, the size of the pulse from the detector depends on whether all or only part of the gamma-ray energy stored in the scintillator. For a given amount of energy stored in the scintillator, the output pulse height will be well-defined, but every pulse will not be the equal size. Because of statistical variations in light production, photon collection, photoelectron production, and electron multiplication, the pulse heights will show a distribution of values with some pulse heights more significant and some smaller than the average. Typical variations with our detector are in the range of 5-10 percent.

Gamma Ray Spectroscopy

The pulse height distribution for a source emitting only single energy gamma rays typically appears as in Fig. 3. The considerable height at the far right is called the photopeak and arises when all the gamma-ray energy is stored in the scintillator. Note the 5-10% width of this height due to statistical fluctuations. The most likely interaction to collect 100% of the gamma-ray energy is the photoelectric effect. The incident gamma essentially gives up all its power to eject a bound inner shell electron from one of the crystal atoms. The emitted electron then has the significant kinetic energy (the gamma-ray energy less the small binding energy of the atomic particle, on the order of 10 keV) and loses this power by exciting and ionising more crystal atoms.



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