Measurement of radiation

Question 1

what is intrinsic conduction?

Answer 1

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

Question 2

where does intrinsic conduction take place

Answer 2

semi conductors

Question 3

what is the depletion layer

Answer 3

a region with no free electrons

Question 4

what are p-type conductors

Answer 4

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

Question 5

in n type what is the current flow due to?

Answer 5

electrons

Question 6

what is extrinsic conduction

Answer 6

adding impurities with 5 valence electrons

Question 7

how can the number of conduction electrons be altered?

Answer 7

adding atoms from other material

Question 8

what do n type contain

Answer 8

free electrons and are electrically neutral (electrons = neutrons)

Question 9

what happens with the n and p type conductors join?

Answer 9

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

Question 10

how is a continuous flow achieved

Answer 10

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

Question 11

describe the forward bias

Answer 11

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

Question 12

describe the reverse bias

Answer 12

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

Question 13

what happens at RT

Answer 13

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

Question 14

what are LEDs

Answer 14

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.

Question 15

what are photodiodes?

Answer 15

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.

Question 16

when are X-ray tubes most efficient

Answer 16

when the PD is unidirectional

Question 17

describe the conduction within an x-ray tube

Answer 17

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

Question 18

how does n-type differ

Answer 18

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

Question 19

how does p-type differ

Answer 19

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

Question 20

what is scintillation

Answer 20

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.

Question 21

what is found in a photo-multiplier tube

Answer 21

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

Question 22

what is relative dosimetry

Answer 22

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

Question 23

what is absolute dosimetry

Answer 23

measurements made at a specific depth for reference machine parameters

Question 24

what does a calibration chain look at

Answer 24

the accuracy and consistency

Question 25

describe what happens to the light photon

Answer 25

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.

Question 26

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

Answer 26

- 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

Question 27

exposure....

Answer 27

- 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

Question 28

how can biological measurements of radiation be assessed?

Answer 28

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

Question 29

how is radioactivity measured

Answer 29

from the radionuclide within blood and faeces

Question 30

what is KERMA

Answer 30

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)

Question 31

what is absorbed dose

Answer 31

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

Question 32

describe the role of electrons within the band theory of conduction

Answer 32

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

Question 33

what is the forbidden gap

Answer 33

where no free electrons exist, there is NO band

Question 34

what is the valence band

Answer 34

the highest band level

Question 35

what is the conduction band

Answer 35

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

Question 36

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

Answer 36

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

Question 37

describe the charged particle equilibrium

Answer 37

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

Question 38

what is extrinsic conduction

Answer 38

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

Question 39

what is intrinsic conduction

Answer 39

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

Question 40

what is Thermo Luminescent Dosimetry

Answer 40

- 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