Radiation Detection with a Multi-Channel Analyzer [MCA]
Abstract
Things to do:
1. References
2. Proofread
Radiation is emitted by both natural and manmade sources (Figure 1). Radiation detectors
determine the amount of radiation emitted by a source [5]. These detectors convert radiation into an
electric current for signal processing and display. Gamma rays can be detected by a microcomputerbased Multi-Channel Analyzer (MCA) with a photomultiplier tube, and a Sodium Iodide scintillator.
With the use of the MCA the energy spectra of many sources and background radiation can be
determined and examined [6].
Figure 1: Sources of Canadian radiation exposure[5].
Radioactivity
Radioactivity is the decay of unstable nuclides to produce other nuclides by emitting particles, and
electromagnetic radiation. Of the 2500 known nuclides only 300 are stable, and the rest are
radioactive. The particles that are emitted are usually alpha particles (α), beta particles (β), or
gamma rays (γ) and all have different penetration characteristics. Gamma rays are produced when a
particle decays form an excited state to the ground state emitting one or more photon called a
gamma ray [1].The time it takes for these decay processes to occur range from a fraction of a
microsecond to billions of years.
Radiation interacts with matter when it pass through it by losing energy, breaking molecular bonds,
and creating ions by dislodging electrons from atoms and molecules [3]. For example, high energy
electromagnetic radiation (x-rays) can cause damage to DNA by causing single and double strand
brakes [2].
There are many sources of radiation both natural and manmade (Figure 1). Natural sources of
radiation include, cosmic radiation, terrestrial radiation (in the earth’s crust), and internal sources (in
the human body such ascarbon-14 and potassium-40) [1, 4]. Manmade sources of radiation include
medical radiation, consumer products, atmospheric testing of nuclear weapons, industrial uses, and
nuclear power. On average Canadians are exposed to 3.5 mSc of radiation per year, 80% from
natural sources and 20% from manmade sources [5]. Together manmade and natural radiation
sources make up the background radiation that is in the environment.
A very common naturally occurring source in our environment is potassium-40 (40K). Potassium is an
element that is essential for all life and is found in the muscle tissue of humans, in plant fertilizer, and
in certain foods such as bananas, brazil nuts, and salt replacer. Not all potassium is radioactive.
Potassium-40 is a radioactive isotope of potassium, found in concentrations of 1-part-in-10,000
within natural potassium. It has a very long half-life of 1.28 x 109 years [6]. Humans have about 40
milligrams of 40K present in their bodies which accounts for about half of our yearly exposure to all
sources of radiation [4, 5]. Potassium-40 emits alpha particles, beta particles, and gamma rays. Its
gamma rays have an energy value of 1416 keV, and its beta particles have an energy value of 511
keV [6].
Detector
Radiation detectors are devices that convert radiation into an electric current or voltage for signal
processing and display. These detectors can consist of single radiation sensing elements or of
several small elements arranges in one or two-dimensional arrays [7]. A commonly used detector
used for work in the ultraviolet to visible light rage is the photomultiplier tube (PMT). In the PMT
electrons are ejected form a metal or scintillator (a material that turns the lost ionizing radiation
energy into light), when it is exposed to radiation. This is known as the photoelectric effect. There
are three experimental characteristics of the photoelectric effect; electrons are only ejected once the
frequency exceeds a threshold value characteristic of the metal, the kinetic energy of the ejected
electron is independent of the intensity of the radiation but increases linearly with the frequency of
the incident radiation, if the frequency is above the threshold electrons are ejected straightaway
even at low light intensities [7]. In the PMT an electron is ejected when it is involved in a collision with
a particle-like projectile that carries enough energy to eject the electron form the metal. The PMT
then converts each photon into a small current, which is combined with the current produced by the
other electrons arriving at the PMT at about the same time, to produce a large current pulse that is
then converted into a voltage pulse. The size of the voltage pulse is in proportion to the gamma ray’s
energy. The PMT is connected to an analyzer that can determine the gamma ray’s energy form the
voltage pulse.
An example of a detector with a photomultiplier tube is a 3.8 cm (diameter) x 2.5 cm (thick) Sodium
Iodide (NaI(Tl)) scintillator with a microcomputer-based Multi-Channel Analyzer (MCA). For this
detector, a gamma ray interacts with the sodium iodide crystal undergoing the photoelectric effect by
giving all its energy to an atomic electron. The electron bounces around in the crystal, colliding with
many of the crystals atoms, and converting its energy into photons of light. The greater the gamma
rays energy the more photons of light are produced. Once the PMT converts the current pulse into a
voltage pulse, the pulse is amplified and measured by an Analog to Digital Conversion (ADC)
process. The 10-bit ADC process produces an integer between 0 and 1023. Where 0 is the measure
for a voltage pule less than a hundredth of a volt and 1023 is the measure for the largest voltage
pulse accepted by the ADC, which is larger than approximately 8 volts. The pulses between these
values, 0 to 8 volts, are proportionately assigned an integer measure (channel number) between 0
and 1023. The computer then records and displays the number of gamma rays observed for each
channel number, and produces a graph of the number of gamma rays as a function of the channel
number. To correlate the channel number with its gamma energy calibration first needs to be
complete. By calibrating with known sources channels are assigned energy values. For example a
three point calibration can be achieved by first collecting the data for the two known sources,
Sodium-22 (22Na) and Mangniese-54 (54Mn). This produces an image with three photopeaks. These
photopeaks are then assigned the known energy values, and the computer uses these three values
to fill in the energy values for the rest of the channels to produces a calibration spectrum (Figure 2).
Now the channel number referrers to gamma energy and the calibration spectrum graph represents
gamma frequency (counts) as a function of a gamma ray energy (channel number) [6].
The calibration spectrum in Figure 2 has four photopeaks, two for 22Na (511 keV and 1275 kev), one
for 54Mn (835 keV) and a sum peak. The sum peak is the energy of two gammas due to their arrival
at the detector at the same time. When different gammas interacts with the crystal at the same time
they produce pulses simultaneously which combine and are interpreted by the computer as one
gamma energy. In Figure 2 the sum peak is 1786 keV due to the two gamma energies of 22Na
arriving at same time. Another interesting structure of Figure 2 is the small peak or cliff prior to each
photopeak which is called a Compton edge. This edge is a result of Compton scattering by
electrons. Compton scattering occurs when a gamma’s photon collides with an atomic electron and
relativistic mass-energy and momentum are conserved. The gamma’s loss of energy results in it
having a lower frequency wave. For Figure 2 the Compton scattering occurred when the gamma
enters the detector’s crystal and scatters off an electron. This lower frequency gamma leaves the
detector and the electron, with its lower kinetic energy given by the gamma ray, to be detected.
Since, the greater the gamma rays energy the more photons of light are produced, theses lower
energy electrons produce a lower energy the Compton edge or bump before the photopeak[6].
Figure 2: Gamma Calibration Spectrum with a 3.8 cm x 2.5 cm NaI(Tl) scintillation detector.
References
1.
2.
3.
4.
University Physics with modern Physics 11th edition Young & Freedman
Biochemistry
General chemistry
http://www.cna.ca/curriculum/cna_general_res/fact19-eng.asp?bc=Facts&pid=Facts,
retrieved February 15, 2012
5. http://www.nuclearfaq.ca/cnf_sectionD.htm#communities
6. Lab manual
7. Physical chemistry