Instrumentation Flashcards
____ is sensed by detecting the ionizations of gas molecules produced by deposition of energy during radiation’s passage. (Gas Filled Detectors)
Radiation
_____ radiation interacts with the e- structure of an atom
Ionization
______ the energy required to knock e- out of its orbit ( the energy of the radiation must be greater than the binding energy of the e-)
Ionization energy
____ positive charged gas molecule and an e- that are produced as a result of ionization.
Ion pair
A gas filled detector contains 2 electrodes
-Positive electrodes(anode)
-Negative (cathode)
A gas filled detector is filled with what gases?
He,Ne,Ar,H or air
Why are noble gases used instead of of air
They are more dense- it gives them a greater probability of interaction better efficiency
The atmosphere of the gas is usually increased to ______
5 to 12 times more gas within the detectors
Voltage source does what to the electrodes
Applies a voltage
Current measuring device:
Measures current coming from the electrodes
How does a gas detector work ? Steps
- Radiation enters the chamber and causes an ion pair
- Ion pair is produced from the interaction of radiation with an e- from the gas
- Due to the voltage placed on the electrodes an positive gas molecule is attracted to the cathode
- E- is attracted to the anode
- When the e- and gas molecule reach the perspective electrodes a charge is produced
- The charges produce a current( amps)
- The current is being measured
- The amount of current measured is proportional to the strength of the radiation (stronger radiation means higher current)
There are 3 types of gas filled detectors that we use:
- Ionization chambers
- Proportional chambers
- GM Detectors
The voltage applied to the gas filled doctor determines the one we use
What does the voltage curve shows:
As you increase voltage what happens to the number of ion pairs produced(more are produced)
Each type of radiation has an amount of ionizing force a higher amount of ionizing force produces more
Ion pairs
Alpha has a higher ionizing force than
Beta( alpha produced 100 time more ion pairs than beta)
Beta has a higher ionizing force than
Gamma( Beta produces 100 time more ion pairs than gamma)
The curve shows the difference in ion airs produced
For each type of radiation
Voltage curve (region I) Recombination region ranges from ?
0-100 volts
In the recombination region what happens?
Radiation enters the chamber an ion pair is formed but the migration of the ions to their perspective electrodes depends on the voltage if 0 volts are applied the gas ions recombine and because they recombined they are not detected.
Voltage curve ( Region II) Ionization region volts ?
100-400 volts
Region II the radiation
- Enters the chamber
- An ion pair is formed
- The increase in the voltage allows all ion pairs produced to be collected
- A plateau occurs in respect to voltage vs # of ion pairs( increasing the voltage doesn’t produce more ion pairs)
- The plateau is called the saturaton voltage
For region 2 if increasing the voltage doesn’t increase the number of ions collected what voltage should we operate at ?
150 volts over 150 volts only make ions migrate to the electrodes faster and will waste battery
Equipment used in the region is called ____?
Ionization chambers
Dose rate meters and dose meters
Are ionization chambers the current measured by an ionization chamber is direct measurement of the # of ions collected( good for measuring high radiation areas)
What measures intensity of radiation
Dose rate meters = radiation/unit time
mrem/hr is an example
Cutie pie and ionization dose rate meters
Dose rate measures only the amount of radiation present like
Ci or Bq with dose calibrators and pocket dosimeter
Dose rate measures only the amount of radiation present like
Ci or Bq with dose calibrators and pocket dosimeter
Voltage curve region III is called what region
Proportional region
How many volts are in the region III (proportional)
400-800 volts
What happens in the III region
- Radiation enters the chamber
- An ion pair is formed
- The increased voltage increases the speed of migration of the ion pairs
4.The increase voltage will increase e- acceleration and kinetic energy
5.Because the e- are going faster they can cause further ionizations this is called gas amplification which causes more ion pairs to -e produced and collected. - A primary ion starts the reaction and the secondary ions are produced
- If the secondary ions create more ions, an avalanche occurs the voltage allows all ions to be collected
- The current will represent,ore radiation than actually entered the chamber
The actual amount of radiation that entered the chamber is called?(region III)
A factor of the current detected
Ex. 500 volts could have a factor of 1000 for every ion pairs that is created, 1000 more will be produced from the original
The instrument used in the region(III)
Proportional counter which measure alpha and beta particles which is not used in nuclear medicine
Region III measuring factor example
Your current device reads 5,000,000 and the factor used for this proportional counter is 5000. If we are detecting beta particles, how many do we actually have?
5,000,000/5000=1000 beta particles
Region IV is called what region
Nonproportional which ranges from 800-1000 volts
In the fourth region what happens
More ions are produced from the primary ion pairs the amount of radiation entering the chamber is not proportional to to the current measuring device and the actual amount of radiation entering the chamber can not be determined and no equipment is used in this region
Voltage curve Region V or 5 is the what region?
The Geiger muller region ranges from 1000-1500
Region 5 or the Geiger muller range what happens
Max gas amplification the most amplification that can occurs which increasing voltage doesn’t increase amplification we have reached saturation point this region can’t tell the difference between alpha,beta and gamma
What is quenching?
Starting in the GM region you can have continues discharge of e-
The positive gas molecule is accelerated toward the cathode but when it reaches the cathode it combines with an e- due to the higher velocity more energy is released on the combination of the e- and gas molecule the energy will act in the form of an x-ray and the X-ray acts as radiation and more ionization and this produces more avalanches which can result in a false high result
Quenching can prevent x-rays from causing ionizations in the
GM tube
