Week 5: Radiation detection (part 2) Flashcards
How are electrons in a semiconductor crystal arranged?
Thy are arranged in energy bands. The valence band is full, and above that is a conduction band which is empty.
This makes the semiconductor and insulator.
The energy gap is usually ~1 eV, which is small compared to a typical insulating crystal.
Draw the band structure in a semiconductor detector.
(29)
Explain how semiconductor detectors work.
A semiconductor detector is like a solid ionisation chamber.
Radiation creates many electron-hole pairs which are collected by the contacts.
The charge collected is closely proportional to the energy deposited by the radiation.
This primarily occurs between the valence and conduction bands.
Why is the energy resolution in a semiconductor detector very good?
The low amount of energy needed per electron-hole pair means that each particle of radiation produces many pairs.
The high number of charge carriers means that errors are small and the width of the peak and energy resolution are very good.
Eunc/E ~ sqrt(N)/N = 1/sqrt(N)
What does it mean if the detector is an intrinsic semiconductor?
The material is perfectly pure, such that the only way electrons could be in the conduction band is from excitation and that all conduction band electrons have a matching valence band hole.
How is a semiconductor detector made?
By combining two types of semiconductor: n-types and p-types.
What is an n-type material?
Where the charge carriers are negatively charged. i.e. electrons are excited into the conduction band where they are mobile.
What is a p-type material?
Where the charge carriers are effectively positive. i.e. Electrons are easily excited from the valence band which are mobile. The hole left is effectively a positive charge carrier.
What is the depletion region?
When a junction is made between the two types of semiconductor, electrons from the n-type migrate across the boundary and combine with holes in the p-type.
This creates a region consisting of compensated material devoid of either electrons and hole and will behave like an intrinsic semiconductor.
This zone is the depletion region.
Why does a contact potential form at an n-p junction?
The movement of negative charge leaves a layer of positive charge behind, with the n-type material being at a higher potential than the p-type.
Why does the migration of charge come to a stop at an n-p junction?
The electric field from the contact potential stop the migration of charge.
How does a reverse bias affect a p-n junction?
The application of a reverse bias increases the potential difference, creating a larger depletion region and hence a bigger detector.
How do semiconductors detect radiation?
If radiation deposits energy in the depletion region, it creates free charges which are immediately swept to the contact, resulting in a charge pulse that is proportional to the gamma ray energy.
How can a semiconductor be made to behave like it is intrinsic?
By forcing lithium atoms into the impure material, in a process known as drifting, the impurities are neutralised, thus making the semiconductor behave intrinsically.
List the feature of silicon semiconductor detectors.
Good for charged particles and X-rays.
Eunc ~ 10keV for 5 MeV alpha.
Finite life due to heavy ions damaging the crystal structure.
Cooling not needed.
Lithium drifting.
List the feature of HPGe semiconductor detectors.
Used for gamma rays.
Eunc = 2 keV at 1.33 MeV.
Liquid nitrogen cooling required.
High purity, large volume crystals available.
Expensive.
Not very portable.
List the feature of CZT semiconductor detectors.
Used for gamma rays and X-rays.
Good at room temperature, no cooling required.
Easily portable.
Worse energy resolution than Ge due to poor crystal quality.
Only small crystal volumes.
What is the photopeak?
The full energy peak produced when all of a photons energy is registered in the detector.
What is the Compton continuum?
The peak produced from the detected recoil electron from a Compton scattering interaction.
What is the Compton edge?
The sharp drop-off after the Compton continuum.
It corresponds to the photon being scattered back the way it came.
i.e. transferring the maximum amount of energy to the electron.
What is the origin of the single escape peak?
If pair production occurs, then the annihilation of the positron will produce two 511 keV photons, leading to a peak at this energy.
If one of these photons escapes the detector, but all of the remaining energy is deposited, the single escape peak will appear 511 keV below the photopeak.
What is the origin of the double escape peak?
If pair production occurs, then the annihilation of the positron will produce two 511 keV photons, leading to a peak at this energy.
If both annihilation photons escape the detector, the energy deposited is seen in the double escape peak.
What is the origin of the backscatter peak?
Where photons are scattered back into the detector.
This occurs when interactions that change the direction of travel by ~180 degrees occur.
Why do X-ray peaks occur?
Photoelectric absorption in material around the detector will result in the emission of low energy X-rays that may be detected.
Photoelectric interactions inside the detector but near the surface can allow the characteristic X-ray to escape, also giving a peak a little below the full-energy peak.
