4. Gamma energy Flashcards
What is the main purpose of the detection of gamma radiation?
to determine the photon energy and activity of gamma radiation
What is mono-energetic gamma radiation?
gamma radiation containing photons of equal energy.
The gamma radiation to be measured is usually ___
What is the consequence?
Mono-energetic
→ during the decay of the isotope photons of equal energy are emitted
→ theoretical energy distribution of the radiation is a narrow line
How can a SCINTILLATION COUNTER work?
Each gamma-photon absorbed in the detector crystal (e.g., NaI(Tl)) causes a light flash (scintillation).
A photomultiplier tube connected to the crystal converts the scintillations into electronic pulses.
Pulses are counted and sorted according to their amplitude.
What is a SCINTILLATION COUNTER?
A device for measuring gamma radiation
In the scintillation counter used in this experiment, each gamma photon absorbed in the NaI(Tl) detector crystal results ___ which are liberated from the atoms of the crystal (photo- and Compton electrons).
primary electrons
In the scintillation counter used in this experiment,
→ what is the relationship between voltage pulses of amplitude and and energy of primary electrons?
Voltage pulses of amplitude propor- tional to the energy of the primary electrons are produced on the output of the photomultiplier tube
Voltage pulses of amplitude propor- tional to the energy of the primary electrons are produced on the output of the photomultiplier tube
→ What happen if photoeffect occurs?
HINT
- Total energy of gamma photon can be converted into?
- Consequence of 1 - the pulse amplitudes
- the total energy of the γ-photon is converted into the energy of the primary photoelectron
- The pulse amplitudes of the output are proportional to the photon energy of the γ-radiation → can be used for the determination of photon energy (red pulses).
Voltage pulses of amplitude propor- tional to the energy of the primary electrons are produced on the output of the photomultiplier tube
→ Why are pulses that originate in Compton effect have different amplitudes
because the energy of the incident γ- photon is converted only partially into the energy of the primary Compton electron.
→ they cannot be used for photon energy determination (blue pulses).
Voltage pulses of amplitude propor- tional to the energy of the primary electrons are produced on the output of the photomultiplier tube
→ Do noise pulses contain any information about gamma energy? Why?
Useless noise pulses of mostly small amplitude are also produced during detection (black pulses).
→ These do not contain information about the number or the energy of the detected γ-photons.
What does this figure show? Which pulses are counted?
the cumulative pulse rate distribution curve acquired by using the integral discriminator
→ Only pulses with amplitudes exceeding the discriminator level (Ud) are counted (N).
What is The proper setting of the discriminator level (Ud)
- most of the noise pulses are blocked
- most of the Compton pulses and all of the pulses originating from photoeffect are counted
=> thereby optimizing the signal-to-noise ratio
Fig. 1d shows the cumulative pulse rate distribution curve acquired by using the integral discriminator
Only pulses with amplitudes exceeding the discriminator level (Ud) are counted (N)
Is this method suitable for determination of gamma energy? Why?
This method is suitable for estimating the activity of isotopes with known photon energy (thus the measured pulse rate is proportional to the activity), but the determination of the γ-photon energy is difficult.
What is the differential discriminator (DD)?
a signal-selection logic that collects pulses with amplitude between two DD voltage levels that define a channel, but discards those with amplitudes outside this channel.
The position of the channel (central discriminator level) and its width are user adjustable.
The DD is used to measure the size distribution of pulses.
→ enables us to separate pulses which originate from the photopeak and the Compton region
When using the differential discriminator, only those pulses are counted which (1)____ that fall within a narrow range, between___
- correspond to photon energies
- E and E+ΔE.
When using the differential discriminator, only those pulses are counted which correspond to photon energies that fall within a narrow range, between E and E+ΔE.
What is the consequence for voltage pulse?
Voltage pulses that fall in the ΔU-wide channel between discriminator levelUdand voltageUd +ΔUare counted (ΔN/ΔU), corresponding to the photon energies as mentioned/
What is the relationship between photopeak and gamma energy?
The photopeak is proportional to the gamma energy
Why is the total pulse rate decreased?
by the noise (ID level = Ud, Fig. 2.) (area under the pulse-rate distribution curve)
When using the differential discriminator,
- The measured pulse rate distribution curve corresponds to the energy spectrum of the radiation as detected by the detector.
- The photopeak, which is proportional to the gamma energy, (1)____ and (2)___ can be separated and the position (Uphoto) of the (3)___ can be determined from this curve.
- Compton region
- noise pulses
- photopeak
When using the differential discriminator,
The measured pulse rate distribution curve corresponds to the energy spectrum of the radiation as detected by the detector.
What happen as a result of Compton scattering?
There is a Compton region of nearly uniform distribution of pulse amplitudes
When using the differential discriminator,
The measured pulse rate distribution curve corresponds to the energy spectrum of the radiation as detected by the detector.
The pulse rate distribution at smaller energies is distorted by the (1)__generated in the detector (Fig. 1b), while large-amplitude noise pulses are rare.
noise pulses
What does the total pulse rate decreased by the noise depend on?
the total number of detected γ-photons, which is related to the activity of the measured isotope.
What does The position of the photopeak (Uphoto) depend on?
the anode voltage of the photomultiplier tube
Compare Semiconductor radiation detectors to scintillation detectors
Semiconductor radiation detectors (e.g., Ge(Li): germanium doped with lithium) have greater energy resolution (Fig. 4, the photopeak is very narrow), but much lower sensitivity than scintillation detectors.
What is PHOTOPEAK?
a distinct peak in the energy spectrum of gamma radiation.
How to find photopeak?
A gamma photon that is absorbed in the scintillation crystal via photoeffect causes a light flash (scintillation) with an intensity proportional to the photon energy.
→ The amplitude of the corresponding voltage pulse generated by the photomultiplier tube is therefore also proportional to the photon energy.
→ When sorted by using the differential discriminator, these pulses form a histogram with a narrow peak called the photopeak.
What is Compton region?
a distinct region in the energy spectrum of gamma radiation
How to find Compton region?
Gamma photons absorbed in the scintillation crystal via Compton effect cause scintillations with a wide range of intensities.
→ the amplitudes of the voltage pulses on the output of the photomultiplier tube are not proportional to the original photon energies.
→ When sorted by using the differential discriminator → these pulses form a flat or slightly decreasing histogram called the Compton region.
Determination of gamma energy based on photopeak measurements.
Measure the pulse amplitude spectrum of both isotopes separately
changing the discriminator level from 100 in steps of 50 (BASELINE).
Continue the measurement until the photopeak has been passed and the pulse rate decreases to zero.
Plot the two pulse-amplitude spectra and the superimposed spectrum in the same graph.
Determine the positions of the photopeaks
Calculate the photon energy (ECr) of the gamma radiation emitted by the isotope with unknown photon energy from this formula
