gas detectors (IN PROGRESS)

most detection instruments use electrons from ionization events and electronics to

convert energy into readable information

all detectors can be affected by

source geometry, so detector and source will change amount of interaction; typically simplified into 4pi and 2pi geometries

three types of gas detectors

ionization chamber

proportional counter

geiger-mueller counter

gas detectors are called such because

signal is produced when charged particles ionize the gas in the detection chamber

if gas is neon…

positive member of each ion pair is a neon ion while negative member is a freed electron

if detector gas is air…

positive is a positively charged nitrogen molecule and negative member of the ion pair is a freed electron

in the absence of an electric field, ion pairs created in gas of the detector…

recombine

if ionization occurs bw 2 oppositely charged electrodes and the electric field is sufficiently strong…

positive and negative members of the ion pairs will separate and be collected at the electrodes

anode

positively charged electrode

cathode

negatively charged electrode

parallel plate electrodes

only used for ion chambers

coaxial electrodes

more common due to geometry achieving much higher electric fields for the same applied voltage

average energy lost by a charged particle per ion pair

about 34 eV

a typical electron/beta particle produces ___ ion pairs per cm of travel

100 ion pairs

bc of gas detector size, beta particles transfer…

small portion of energy to detector gas

a typical alpha particle produces ___ ion pairs per cm

40000 ion pairs

bc of gas detector size, it’s possible for some alpha particles to

expend all energy in detector gas

photons might interact directly w detector gas but are more likely to

interact w detector wall (cathode)

when photon interacts w cathode, an electron is…

knocked off of detector wall; this traverses the gas inside the chamber and might have sufficient energy to cause further ionization

magnitude of detector signal depends on

number of ion pairs produced in gas by incident charged particle

largest to smallest signal produced (generally) is

alpha (largest)

beta

photons

modes of operation detector gas

current, pulse mode

ion chambers usually operate in which mode

current; bc pulses are too small to be easily distinguished from electronic noise

proportional counters and geiger meuller detectors usually operate…

in pulse mode

current mode of operation

system measures current from chamber rather than counting pulses; calibration relates the current to a quantity such as exposure rate

current (I) is measured in units of..

amperes (A)

I = Q/t

collected charge (Q) is measured in

coulombs (C)

Q = n e

the current from a radiation detector is very small, often

on the order of picoamperes (pA)

pulse mode of operation characteristics

voltage (V)

as long as detector gas is not ionized, potential difference bw anode and cathode is…

constant

during movement of electrons to positive ions to electrode, the potential difference..

decreases

after some time, voltage stops decreasing and…

returns to baseline voltage

decrease and recovery = pulse

maximum change in voltage is

pulse height (amplitude)

when a detector operates in pulse mode, we might…

count pulses and/or measure amplitude

for pulse height to carry useful info abt energy deposited in detector gas…

must be directly related to collected charge

a proper choice of the time constant is

necessary for pulse heigh to be related to collected charge

pulse width indicates

the charge collection time

the potential difference (voltage) reflects the

instantaneous charge in the circuit rather than the total collected charge

τ« charge collection time: pulse height does NOT reflect

energy deposited in detector gas; in other words it carries no useful information

large resistance to current and large capacitance of electronics means…

current is “held up”, causes potential difference (volts) to build up to potential max voltage

(τ) » charge collection time: max pulse height reflects

energy deposited in detector gas; output pulse from detector is negative (voltage decreases)

six region curve

plot pulse height as a function of high voltage applied to a gas detector

gas amplification

number of ion pairs created by voltage from each primary ion pair created

all gas filled detectors operate on same principle of

ionized e- creating an electric current, but voltage applied across gas chamber changes how they react

six sections of six region curve

recombination

ionization

proportional

limited proportionality

geiger-mueller

continuous discharge

recombination region

at lowest voltages, strength of electric field is sufficiently low that many primary ion pairs created simply recombine

is no electrons reach anode

there is no pulse

ionization region

electric field just strong enough to prevent recombination of ion pairs produced in gas

ionization region: resulting current is referred to as

saturation current

why are alpha pulses correspondingly larger

alpha produces considerably more ion pairs than a beta

ion chambers used to measure

relatively high exposure rates, like over 1 mR/hr

ion chambers read accurately no matter

energy which they are calibrated at, bc of flat energy response

ion chambers measure dose accurately when it has

chamber material w tissue equivalent to stimulate rad interaction w tissue

dose calibrators

ion chambers that convert charge collected to activity for a selected isotope

amount of charge collected is different for each isotope per amount of activity, so…

ion chambers have multiplication factors for each isotope they use