Measurement of radiation Flashcards
what is intrinsic conduction?
electrons move towards a + potential and the holes towards the -
where does intrinsic conduction take place
semi conductors
what is the depletion layer
a region with no free electrons
what are p-type conductors
conductors which carry a current due to the holes. Adding impurities increases the number of holes, adding atoms with 3 valence electrons
in n type what is the current flow due to?
electrons
what is extrinsic conduction
adding impurities with 5 valence electrons
how can the number of conduction electrons be altered?
adding atoms from other material
what do n type contain
free electrons and are electrically neutral (electrons = neutrons)
what happens with the n and p type conductors join?
some of the free electrons in the n type, move to the p type. Some of the holes move from the p type to the n type. The electrons which move and combine with the holes form negative ions. The holes which move and combine with the free electrons form positive ions. This continues until the charge builds up on either side of the junction repelling further holes or electrons
how is a continuous flow achieved
via an external battery
n type is connected to the negative terminal
p type is connected to the positive terminal
describe the forward bias
when the p type is positive and the n type is connected to the negative
SEMI CONDUCTOR CONDUCTS
describe the reverse bias
when the p type is connected to the negative and the n type is connected to the positive
SEMI CONDUCTOR DOESNT CONDUCT
depletion zone gets wider preventing electron flow
what happens at RT
some of the covalent bonds when supplied to the covalent bonds in semi conductor, the electron is able to acquire enough energy to transition from the conduction band leaving a hole behind. This hole is filled by an electron from the valence band in a neighbouring atom which leaves a positive charge
what are LEDs
light emitting diodes are used by x-ray devices which emit light when forward biased, the combo of holes and electrons results in light emission.
what are photodiodes?
this is held in the reversed bias state in which the light electrons have gathered enough energy to jump the forbidden gap into the conduction band, changing the diode into a conducting state.
when are X-ray tubes most efficient
when the PD is unidirectional
describe the conduction within an x-ray tube
it has a modified form of vacuum diode valve, if the alternating current output of a high tension transformer is connected directly across a X-ray tube it will conduct a current, during the half cycles when the anode is + =. During inverse half cycles the anode is - so the flow is blocked - SELFRECTIFICATION
how does n-type differ
it has one extra electron in its outer orbital (5 electrons)
how does p-type differ
it has one less electron in its outer orbital (3 electrons)
what is scintillation
it is the release of visible light photons from high energy ionising photons, which occurs in a scintillation crystal. The quantity of IP can be measured by the number of flashes. A photo-multiplier tube is paired with the crystal. The counter measures the radioactivity.
what is found in a photo-multiplier tube
- a photo cathode: which releases electrons when illuminated
- evacuated tube
- dynodes (+ potential)
- multiplication effect
what is relative dosimetry
dose measured under specific conditions which are compared to the dose given at specific reference depth for reference depth
what is absolute dosimetry
measurements made at a specific depth for reference machine parameters
what does a calibration chain look at
the accuracy and consistency
describe what happens to the light photon
it travels directly through the tube, hitting the photon cathode, releasing electrons when it hits light. A potential difference is found across the tube. A series of mini anodes and dynodes increases the positive pd. Once it hits the first dynode it causes a cascade effect, as each electron gains energy with each dynode releasing even more electrons than previously increasing the positivity. The number of electrons increases so that the electrical current can be measured.
describe the use of the Geiger counter in the gas multiplication effect
- IC detects high energy emissions
- it becomes gas filled at low pressures
- an ionising photon, ionises a gas molecules
- ion pair accelerates and interacts with other gas molecules which ionises them.
- creating a cascade effect (gas multiplication effect)
- a current is formed and detected
- each ionising event causes a click
after the detection of an emission the gas ions are quenches. Recombination allows for a fraction of a sec. No further ionisations occur in the shirt time
exposure….
- exposure rate measures beam intensity
- At a particular point Q/M
- only defined for air and only for gamma/ x-rays
- formation of electrical charges in the air by ionisation
how can biological measurements of radiation be assessed?
- through blood
- gastrointestinal/ cerebrovascular and cutaneous systems
- radiation biomarkers: chromosome aberrations
- AD in enamel, bone, nails, hair
how is radioactivity measured
from the radionuclide within blood and faeces
what is KERMA
Kinetic Energy Released per unit MAss
which is the energy imparted to a medium which is then deposited at a specific amount in tissue (increasing absorbed dose)
what is absorbed dose
measurement of the quantity of electrical charge produced in air by ionisation. It is the difference between the KERMA and energy carried away by the secondary particles (as long as equilibrium exists). It refers to the energy of the absorbing tissue
describe the role of electrons within the band theory of conduction
electrons are found at specified energy states, when these are close enough they influence each others electrical field. These separate levels form a band when there is a large number of atoms in close proximity. Within the band there are different energy states
what is the forbidden gap
where no free electrons exist, there is NO band
what is the valence band
the highest band level
what is the conduction band
it is a energy level above the valence band, with the free lecterns not being connected.
describe the band width in insulators, semi-conductors and conductors
insulators = large gap between bands
semi conductors = small gap between bands
conductors = no gap between bands
describe the charged particle equilibrium
it is when each charged particle is replaced with an identical particle so there is the same amount of energy entering
what is extrinsic conduction
it is when a doping agent/impurity is deliberately added to the semi-conductor, which adds another energy level in the forbidden gap.
n type = electrons step up to CB
p type = allows holes to up into the CB
what is intrinsic conduction
at low temperatures, there is not enough energy to cross the gap. At room temp some electrons gain enough energy to jump across. As the temp increases the number of electrons crossing increases (increasing conduction). Electrons are then able to move from the VB –> CB creating a hole which acts as a carrying charge agent
what is Thermo Luminescent Dosimetry
- crystalline semi conductors (phosphors)
- band theory
- a doping agent introduces a luminescent centre and an electron trap
- stimulating the phosphor causes electrons to jump to CB falling into the trap
- they have to jump to the CB in order to return to the VB,
- electron traps have a long distance from CB making it difficult to jump
- electrons must gain enough energy to jump from CB to get to VB
