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Physics > Measurement of radiation > Flashcards

Measurement of radiation Flashcards

1
Q

what is intrinsic conduction?

A

electrons move towards a + potential and the holes towards the -

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2
Q

where does intrinsic conduction take place

A

semi conductors

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3
Q

what is the depletion layer

A

a region with no free electrons

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4
Q

what are p-type conductors

A

conductors which carry a current due to the holes. Adding impurities increases the number of holes, adding atoms with 3 valence electrons

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5
Q

in n type what is the current flow due to?

A

electrons

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6
Q

what is extrinsic conduction

A

adding impurities with 5 valence electrons

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7
Q

how can the number of conduction electrons be altered?

A

adding atoms from other material

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8
Q

what do n type contain

A

free electrons and are electrically neutral (electrons = neutrons)

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9
Q

what happens with the n and p type conductors join?

A

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

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10
Q

how is a continuous flow achieved

A

via an external battery
n type is connected to the negative terminal
p type is connected to the positive terminal

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11
Q

describe the forward bias

A

when the p type is positive and the n type is connected to the negative
SEMI CONDUCTOR CONDUCTS

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12
Q

describe the reverse bias

A

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

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13
Q

what happens at RT

A

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

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14
Q

what are LEDs

A

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.

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15
Q

what are photodiodes?

A

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.

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16
Q

when are X-ray tubes most efficient

A

when the PD is unidirectional

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17
Q

describe the conduction within an x-ray tube

A

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

18
Q

how does n-type differ

A

it has one extra electron in its outer orbital (5 electrons)

19
Q

how does p-type differ

A

it has one less electron in its outer orbital (3 electrons)

20
Q

what is scintillation

A

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.

21
Q

what is found in a photo-multiplier tube

A
  • a photo cathode: which releases electrons when illuminated
  • evacuated tube
  • dynodes (+ potential)
  • multiplication effect
22
Q

what is relative dosimetry

A

dose measured under specific conditions which are compared to the dose given at specific reference depth for reference depth

23
Q

what is absolute dosimetry

A

measurements made at a specific depth for reference machine parameters

24
Q

what does a calibration chain look at

A

the accuracy and consistency

25
Q

describe what happens to the light photon

A

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.

26
Q

describe the use of the Geiger counter in the gas multiplication effect

A
  • 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

27
Q

exposure….

A
  • 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
28
Q

how can biological measurements of radiation be assessed?

A
  • through blood
  • gastrointestinal/ cerebrovascular and cutaneous systems
  • radiation biomarkers: chromosome aberrations
  • AD in enamel, bone, nails, hair
29
Q

how is radioactivity measured

A

from the radionuclide within blood and faeces

30
Q

what is KERMA

A

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)

31
Q

what is absorbed dose

A

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

32
Q

describe the role of electrons within the band theory of conduction

A

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

33
Q

what is the forbidden gap

A

where no free electrons exist, there is NO band

34
Q

what is the valence band

A

the highest band level

35
Q

what is the conduction band

A

it is a energy level above the valence band, with the free lecterns not being connected.

36
Q

describe the band width in insulators, semi-conductors and conductors

A

insulators = large gap between bands
semi conductors = small gap between bands
conductors = no gap between bands

37
Q

describe the charged particle equilibrium

A

it is when each charged particle is replaced with an identical particle so there is the same amount of energy entering

38
Q

what is extrinsic conduction

A

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

39
Q

what is intrinsic conduction

A

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

40
Q

what is Thermo Luminescent Dosimetry

A
  • 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

Physics (25 decks)

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