Nuclear Physics - Chap 7 Radiation Detectors

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Chap 7 - Radiation Detectors

When a beta particle interacts with the gas of a gas-filled detector, the energy required to produce one ion pair is approx. 30 eV. If the particle's energy is 1 MeV, find the number of the electron pairs, the total charge produced.

Total charge produced (1 MeV) divided by 30 eV / ion pair is approx. 33,333.33 ion pairs.

Explain the role of quenching gas in an ionization detector. (p 96)

Solves the problem of a single ionizing radiation event in a GM counter going into a pulsating series of discharges by (1) giving up electrons easily so they neutralize the positive ion cloud created by the primary gas; (2) when the quenching gas molecules are neutralized by electrons entering higher energy orbits, they de-energize themselves by dissociating into molecular fragments rather than emmitting UV photons; and (3) they are strong absorbers of UV radiation, so the few UV photons that are released during neutralization of the positive ion cloud are quickly absorbed before they can set off another avalanche.

Name the types of gas-filled detectors (p. 89)

Gas-filled detectors include (1) ionization chambers, (2) proportional counters, and (3) Geiger-Muller (GM) counters.

Describe the principle of operation of an ionization chamber (Draw a diagram - p. 90)

Electrical charge liberated by ionizing radiation is collected by the positive anode and negative cathode electrodes. The applied voltage exceeds saturation voltage to ensure complete collection of liberated charge. Device is a chamber with cathode and anode electrodes connected to a voltage source and a current measuring device.

Describe the use of ionization chambers in nuclear medicine (p. 91)

(1) Survey meters - used to monitor radiation levels for radiation protection purposes; and (2) dose calibrators - used to assay activity levels.

Describe how a pocket dosimeter works (p. 92)

Device records total charge collected over time. The ionization chamber electrodes are a central charging electorde and the outside case. They and insulated electrically fron one another and form an electrical capacitor. The capacitor is first charged with a reference voltage connecting the charging rod to a separate charging unit. When the capacitor is exposed to radiation, electrical charge is collected by the electrodes, discharging the capacitor. Change in voltage across capacitor is measured and related to the amount of electrical charge collected by the ionization chamber electrodes.

How do the proportional counters operate? (p. 94)

A proportional counter is a special constructed ion chamber designed to optimize the gas amplification effect, both in terms of the amount of amplification and the uniformity of this amplification within the chamber. They are filled with gases that allow easy migration of free electrons because this is critical for the amplification effect. The size of the electrical signal produced by an individual ionizing radiation event is much larger those produced by ionization chambers. .

How are proportional counters used?

They are useful for detecting and counting individual radiation events, and they can be used for energy-sensitive counting because the size of the pulse is proportional to the amount energy deposited by the radiation event.

What is "Townsend avalanche/avalanche ionization? (p. 94-5)

Electrons liberated by radiation gain such high velocities and energies when accelerated toward the positive electrode that they cause additional ionization in collisions with other atoms in the gas, initiating a cascade process.

Draw a diagram and explain how a Geiger-Muller counter works. (p. 96-7)

The center wire (anode) is maintained at a high positie voltage relative to the outside cylinder (cathode). The tube, with a thin window at one end, is sealed and filled with a special gas mixture, typically argon and a quenching gas. When ionization occurs, electrons are accelerated toward the center wire. Gas amplification occurs. In addition to ionizing gas molecules, the accelerating electrons cause excitation of gas molecules through collisions. The excited molecules quickly return to ground state emitting UV radiation. UV photons interacts via photoelectric absorption releasing another electron, triggering an avalanche. This is process is terminated by the positive ions that reduce the effective electric field around the anode. A constant electric charge is created that is large and easily detected with electronic circuits.

In a given GM counter, what does the threshold voltage depend on? (Fig 7-10)

GM counter threshold voltage depends on the conditions defined for the circuit, including the type of gas mixture in the tube.

What are the disadvantages of the semiconductor detectors that limit their use in nuclear medicine? (p. 99)

(1) Both Si ad Ge conduct a significant amount of thermally induced electrical current at room temperature that creates a background "noise current" that interferes with detection of radiation-induced currents; and (2) Impurities enter into and disturb the regular arrangement of silicon and germanium atoms in the crystal matrix. These disturbances create "electron traps" and capture electrons released in ionization events resulting in a substanital reduction in the amount of electrical signal available and limits the thickness of a practical detector to about 1 cm, thus they are only efficient for detection of gamma rays.

What is the effect of the "coldfinger" in a semiconductor detector? (p. 100 - Fig. 7-12)

Minimizes thermal conductivity to reduce cool the detector and some of the preamplifier electronic circuitry to reduce electronic noise levels.

Explain the mechanism by which a scintillator emits visible light (p. 100)

Radiation from radioactive materials interacts with matter causing ionization and/or excitation of atoms and molecules. When ionized or excited products undergo recombination or de-excitation energy is released. Most of the energy is dissipated as thermal energy, however, in some materials (called scintillators) a portion of the energy is released as visible light.

What is the role of photomultiplier tubes (PMT) in a scintillation detector (p. 101)

They produce a pulse of electrical current when stimulated by very weak light signals, such as the scintillation produced by gamma ray or beta particle in a scintillation detector. They overcome the limitations of counting speed and accuracy from trying to observe and count scintillations in a darkened room.

What is quantum efficiency in a scintillation detector? (p. 101, Fig 7-14)

The conversion efficiency for visible light to electrons is typically 1 to 3 photoelectrons per 10 visible light photons striking the photocathode.

NaI is an "impurity-activated" scintillator. Explain what this means. (p. 103)

The NaI crystals contain small amounts of "impurity" atoms of other elements. Impurity atoms cause disturbances in the crystal matrix' normal structure that are responsible for the scintillation effect.

NaI is the most commonly used scintillator for detectors in nuclear medicine. List the advantages of NaI as a scintillation material. (p. 105)

(1) It is dense and has an element with a high atomic number (Iodine), so it is a good absorber and very efficient detector of penetration radiations; (2) it is a relatively efficient scintillator; (3) it is transparent to its own scintillation emissions so there is little los of scintillation light caused by self-absorption; (4) it can be grown relatively inexpensively; and (5) the scintillation light is well-matched in wavelength to the peak response of the PM tube photocathode.

List the disadvantages of NaI as a scintillation material p. 105)

(1) it is quite fragile and easily fractured which will create opacifications within the crystal that reduce the amount of scintillation light reaching the photocathode; (2) exposure to moisture or humid atmosphere causes a yellowish surface discoloration that impairs light transmission to the PM tube, thus requiring hermetic sealing; (3) at highter gamma-ray energies (above 250 keV) the predominant mechanism of interaction is by Compton interaction, thus requiring larger volumes of NaI(Tl) for adequate detection efficiency.

What is the role of Tl in a Nal(Tl) scintillation? (p. 103-104)

Tl is the impurity added to create activator centers in the crystal, causing the NaI to become efficient scintillators at room temperature.

BGO is the scintillator of choice for detector of eneregies above 511 keV. Calculate the effective atomic number of BGO. (p. 106)

Math Problem, See Example 7-1. Use Bi (Z=83), Ge (Z=32) and, O (Z=16). BGO = Bi-4 Ge-3 O-12; Total of 524 g. Use wi = (miZi) / sum (i=1 to n) of miZi; Use Zeff = (wiZ1^(x) + w2Z2^(x) + ... wnZn^(x))^(1/x)

What is the basic difference in the mechanisms of inorganic and organic scintillators? (p. 107)

In contrast with inorganic scintillators, like NaI, the scintillation process in organic scintillators is an inherent molecular property. The scintillation mechanism in organic scintillators is one of molecular excitation by absorbing energy from a gamma-ray or beta particle, followed by a de-excitation process in which visible light is emitted.

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