# Gamma Ray Spectroscopy

## Instructions

## Outline

### Introduction
* What is gamma ray?
* Why is the nucleus emitting it?

Gamma-ray emission is not, strictly speaking a decay process; it is a de-excitation of the nucleus resulting in the emission of high energy photons. After alpha or beta decay, generally the nuclei do not transition from one ground state to another, but rather end up in excited levels (as we've seen in the Shell model). The de-excitation from these levels emits high energy photons, aka gamma rays.

**NOTE:** Some nuclei are, however, *pure* beta emitters which transition directly between nuclear ground states, thus producing no gamma rays. As a result, these kind of nuclei cannot be detected by gamma spectroscopy.

* Is the spectra actually unique/why for each element? (basis of this 'sight')

Each gamma ray photon has a discreet energy, and this energy is characteristic of the source isotope. This forms the basis of gamma ray spectrometry – by measuring the energies of gamma ray photons, **we can determine the source of the radiation.** (so what we are SEEING, is the constituents of a sample of radionuclides by the indirect observation of its gamma emission spectrum, which is akin to a unique fingerprint to all nuclei of that type.)

### How is it being seen?

#### Interaction
* The mechanisms of interaction of gamma radiation with matter leads directly to an interpretation of the features within a gamma spectrum due to interactions within the detector itself and within the detector surroundings.

* The instrumental detection of any particle or radiation depends upon the production of charged secondary particles which can be collected together to produce an electrical signal.

* Gamma photons are uncharged and consequently cannot
do this. Gamma-ray detection depends upon other types of interaction which transfer the gamma-ray energy to electrons within the detector material. These excited electrons have charge and lose their energy by ionization and excitation of the atoms of the detector medium, giving rise to many electron–hole pairs. The absorption coefficient for gamma radiation in gases is low and all practical gamma-ray detectors depend upon interaction with a solid. As we shall see, the charged pairs produced by the primary electron are electron–hole pairs. The number produced is proportional to the energy of the electrons produced by the primary interaction. The detector must be constructed of suitable material and in such a way that the electron-hole pairs can be collected and presented as an electrical signal.

* OFCOURSE, to fuck shit up: As we shall see, not all interactions will effect a complete absorption of the gamma-ray. The result of this is that absorption curves lie somewhat below attenuation curves in the mid-energy range.

* Photoelectric interactions are dominant at low energy and pair production at high energy, with Compton scattering being most important in the mid-energy range.

#### The three main modes of energy loss

* **Photo-electric absorption:** arises by interaction of the gamma-ray photon with one of the bound electrons in an atom. The electron is ejected from its shell with a kinetic energy, Ee, given by:
$$E_e = E_{\gamma} - E_b$$
Eb is the energy of the bound electron.

The atom is left in an excited state with an excess energy of Eb and recovers its equilibrium in one of two ways. The atom may de-excite by redistribution of the excitation energy between the remaining electrons in the atom. This can result in the release of further electrons from the atom (an Auger cascade) which transfers a further fraction of the total gamma-ray energy to the detector.

Alternatively, the vacancy left by the ejection of the photoelectron may be filled by a higher-energy electron falling into it with the emission of a characteristic X-ray which is called X-ray fluorescence (see Figure 2.4(b)). This X-ray may then in turn undergo photoelectric absorption, perhaps emitting further X-rays which are absorbed, in turn, until ultimately all of the energy of the gamma-ray is absorbed.

It is normally assumed that photoelectric absorption results in the complete absorption of the gamma-ray. However, for those events near to the surface of the detector there is a reasonable probability that some fluorescent X-rays, most likely the K X-rays, might escape from the detector. **Because why the fuck not**.

* **Compton Scattering:** It is a direct interaction of the gamma-ray with an electron, transferring part of the gamma-ray energy. The energy imparted to the recoil electron is given by the following equation:

$$E_e = E_{\gamma} \left(1 - \frac{1}{1 + E_{\gamma}(1-\cos\theta)/m_0c^2} \right)$$

The inescapable conclusion here is that, at all scattering angles, less than 100 % of the gamma-ray energy is absorbed within the detector. May the lord save our souls.

* **Pair Production:** Unlike photoelectric absorption and Compton scattering, pair production results from the interaction of the gamma-ray with the atom as a whole. The process takes place within the Coulomb field of the nucleus, resulting in the
conversion of a gamma-ray into an electron–positron pair.

In a puff of quantum mechanical smoke, the gamma-ray disappears and an electron–positron pair appears. For this miracle to take place at all, the gamma-ray must carry an energy at least equivalent to the combined rest mass of the two particles – 511 keV each, making 1022 keV in all. In practice, evidence of pair production is only seen within a gamma-ray spectrum when the energy is rather more than 1022 keV.

The electron and positron created share the excess gamma-ray energy (i.e. the energy in excess of the combined electron-positron rest mass) equally, losing it to the detector medium as they are slowed down.

#### Detection
Types of detectors used:
* * **Scintillation detector:** The sensitive volume of a scintillation detector is a luminescent material (a solid, liquid, or gas) that is viewed by a device that detects the gamma-ray-induced light
emissions [usually a photomultiplier tube (PMT)].

When gamma rays interact in scintillator material, ionized (excited) atoms in the scintillator material “relax” to a lower-energy state and emit photons of light. The scintillation light is emitted isotropically; so the scintillator is typically surrounded with reflective material (such as MgO) to minimize the loss of light and then is optically coupled to the photocathode of a PMT. (See Figure 3.2.) Scintillation photons incident on the photocathode liberate electrons through the photoelectric effect, and these photoelectrons are then accelerated by a strong electric field in the PMT. As these photoelectrons are accelerated, they collide with electrodes in the tube (known as dynodes) releasing additional electrons. This increased electron flux is then further accelerated to collide with succeeding electrodes, causing a large multiplication of the electron flux from its initial value. The magnitude of this charge surge, is proportional to the initial amount of charge liberated at the photocathode of the PMT. Furthermore, by virtue of the physics of the photoelectric effect, the initial number of photoelectrons liberated at the photocathode, is proportional to the amount of light incident on the phototube, which, in turn, is proportional to the amount of energy deposited in the scintillator by the gamma ray.

As discussed above, however, the spectrum of deposited energies is quite varied, because of the occurrence of the photoelectric effect, Compton effect, and various scattering phenomena in the scintillation medium and statistical fluctuations associated with all of these processes.

* * **Solid-state detectors:** In solid-state detectors, the charge produced by the photon interactions is collected directly. The gamma-ray energy resolution of these detectors is dramatically better than that of scintillation detectors.

The sensitive volume is an electronically conditioned region (known as the
depleted region) in a semiconductor material in which liberated electrons and holes move freely. Germanium possesses the most ideal electronic characteristics in this regard and is the most widely used semiconductor material in solid-state detectors. As Figure 3.3 suggests, the detector functions as a solid-state proportional counter, with the ionization charge swept directly to the electrodes by the high electric field in the semi-conductor, produced by the bias voltage. The collected charge is converted to a voltage pulse by a preamplifier.

#### Interactions within the detector
Gamma-ray interactions with the detector surroundings produce features which can be assigned as follows:
* Fluorescent X-rays (usually lead), photoelectric absorption and emission of fluorescent radiation.
* Backscatter peak – Compton scattering through a large angle, giving rise to a broad distribution at about
200 keV.
* Annihilation peak (511 keV) – pair production within the detector surroundings, followed by escape of one of the annihilation gamma-rays in the direction of the detector.

Be aware that many neutron-deficient nuclides may emit positrons, the annihilation of which will also give rise to counts in the annihilation peak. The larger the detector, the greater the probability of complete absorption of the gamma-ray and hence a larger full energy peak and lower Compton continuum (i.e. higher peak-to-Compton ratio). Sources emitting high-energy beta-particles are likely to give rise to a bremsstrahlung continuum at low energy.

#### How do we see?
Info on the actual gamma emission spectrum observed, and how it is identified to the radionuclide that produced it.

#### And last but not the least, is it possible to become Hulk?!

#### Schematic

Gamma ray spectrometers use the direct proportionality between the energy of an incoming gamma ray and the pulse amplitude at the output of the detector.

The gamma ray method is unusual in that it requires the consideration of many factors. The source intensity and the source-detector geometry affect observed gamma ray fluence rates.

Thallium-doped sodium-iodide scintillation crystals are the most common detectors used in natural radioelement mapping. These detectors modify the spectrum considerably. The main aspects of the detector response are detector efficiency, directional sensitivity, energy
resolution and dead time.

#### Misc. points
* The majority of ambiguities that arise in allotting nuclides to energies would probably be overcome if the energy resolution of detector systems were improved. (In passing, it must be said that a significant improvement in the resolution of germanium detectors is unlikely.)

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## General Sources
* [Intro to gamma spec (video)](https://www.youtube.com/watch?v=NoumT_8YWQw&t=2714s)
* [Guidelines for radioelement mapping using gamma-ray spectrometry data ](https://www-pub.iaea.org/MTCD/Publications/PDF/te_1363_web/PDF/Contents.pdf)
* [Gamma-Ray Detectors](https://sgp.fas.org/othergov/doe/lanl/docs1/00326398.pdf)
* [A report on gamma spec](http://www-personal.umich.edu/~ianrit/gammaspec.pdf)
* [A comprehensive book](http://library.lol/main/70B2194B370226B85AEB4727DC715846)