7/31/2019 Sodium Iodide Detector

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Nuclear Counting Laboratory

Sodium Iodide NaI(Tl) Scintillation Detector

By: E.S. Krage

Department of Physics, South Dakota State University, Brookings, SD 57007.

Radioactivity is a process where an unstable nucleus will

adjust itself until it is stable either by ejecting portions of its

nucleus or by emitting energy in the form of photons

(gamma rays).

Sodium Iodide (NaI) is a type of scintillation detector that is

able to detect these forms of radioactive decay.

The scintillation detector was developed by Sir Samuel

Curran in 1944 while working on the Manhattan Project.

These detectors are used every day in many places such as nuclear

power plants national labs and any other place that is concerned with

radiation effects.

Some highlights of what they are being used for today

environmental assay of the Fukushima power plant and how

radionuclides are being deposited throughout the world.

Scintillation detectors are used as a form of spectroscopy to analyze

elemental composition through neutron activation analysis.

Carbon dating can use scintillation detectors to age archeological

artifacts.

A combination of multiple detectors can be used to create medical

images.

BACKGROUND

ANALYSIS

EFFICENCIES

APPLICATIONS

CONTACT

BASIC PRINCIPAL CONTINUED

BASIC PRINCIPAL

The Sodium Iodide detector is a form of scintillation

detector. The basic principal is the use of a crystal (Sodium

Iodide) which will emit photons (scintillates) when

interacting with radiation.

While the Sodium Iodide is the most common type of

material used there are many other forms available.

In general the light produced from the scintillating material

travels through a window where it gets transferred into adevice called a photomultiplier tube (pmt). A photomultiplier

tube is made up of components called photocathode,

dynodes and an anode.

The photons strike the photocathode and in turn it produces

electrons. These electrons are then pulled to a series of

dynodes through the amplification of a positive high voltage.

When the electrons from the photocathode hit the first

dynode, several electrons are produced for each incident

electron.

These many electrons are now pulled to another dynode and

the process repeats until it reaches anode. At the time it

reaches the anode the volume of electrons is much greater

than initially incident on the detector. When the electrons are

connected they form a pulse this pulse is further analyzed to

determine needed information.

PMT

Incident

Radiation

NaIThallium Excited Atom

Electron Given

off

Photon

From Tl

PMT

DynodesCathode

Anode

Solid Scintillation

In solid scintillation an incident beam of radiation in this case gamma rays comes in and interacts with the Tl doped NaI

and ionizes the Tl.

Then in the process of de-excitation a photon will be produced that can be measured by the pmt.

Summarizing the mechanisms of photon emission subsequent to electron-hole production in the NaI(Tl) crystal can occur

in the following two sequences electron hole trapping and radioactive recombination mechanisms.

Electron hole reaction is the majority of the mechanism by which we measure scintillation.

The Tl is normally found in the crystal lattice as Ti+ ions. Electrons, e-, are trapped by the Tl+ to form T10 by +Tl+Tl

Tl0 and h++ Tl+ Tl++

The second method is the recombination method where holes are tryapped by the Tl+ to form Tl++. When added to the

high voltage we have e - + Tl++ Tl+ + (a photon between 335 to 420nm) this photon is then captured by the pmt.

From the pmt the signal will be analyzed using a multi channel analyzer which automatically separates the counts based on

the amount of energy contained in the incident photon.

The accuracy of separating the different energies is dependent on the type and efficiency of detector. Figure 2 shows the ideal

peaks that we would desire for a scintillation detector and what we actually get using a NaI(Tl) detector.

Imperfections in the crystal and circuitry lead to the blurring of the photopeak.

The energy the photopeak occurs at can be matched up to a specific nuclide and

identified.

In the case of Cs137 it has a gamma () decay of 661.65 KeV in the spectrum

below we can analyze and find what elements we have present.

Energy

C

o

u

n

t

s

661.7 Kev

Cs137

1.17 MeV 1.33 MeV

Co60 Peaks

Compton

Valley

Compton

Peak

Compton

Plateau

Figure 2: The difference between the ideal photopeak and the true photopeak.

Figure 1: The incident radiation excites the hole and then it gives off

an electron and a photon from the Tl which can be then analyzed by

the pmt base.

Pmt Base

Figure 3: Screen shot using Cs137 and Co60 test sources using NaI(Tl)

detector serial no. 895

In Figure 3. we observe the effects of Compton peak, Compton

peak, and Compton valley.

In Compton scattering the incident photon transfers part of its

energy to an outer shell or a free electron, ejecting it from

the atom. Upon ejection this electron is called a Compton

electron.

If both the Compton electron and deflected photon are

detected their total energy will equal that of the incident

photon and the event will register in the photopeak.

Often the photon escapes detection, so that the event deposits

only the energy of the Compton electron producing a reduced

peak value.

The Compton valley sum energy of multiple Compton

electrons created by one incident photon.

The Compton plateau refers to the energies that are less than

the Compton peak.

The fundamental meaning of efficiency in nuclear counting

is the amount observed as a fraction of the amount

expected. The overall efficiency of a detector can be

considered in terms of intrinsic and geometric efficiency.

Geometric efficiency is the measure of the number of

photons striking the face of the detector compared to

partial interactions of the lack of interactions due to

radioactivity being emminated in all directions.

The intrinsic efficiency can be calculated if a known

source emitting a known number of photons is placed

directly against the face of the detector. In this

arrangement geometric factors can be ignored.

There are two different types of detectors flat and well

detectors.

The flat detector in Figure 4 is considered to be a 1-Defficiency only capturing incident radiation in on

direction.

The well detector in Figure 5 is a 3-D detector because of

its ability to capture incident photons in 3 directions.

Figure 5: Flat NaI Detector Figure 6: Well NaI Detector

Dr. Robert McTaggartAssociate Professor of Physics

Coordinator of Nuclear Education

South Dakota State University

President, North Central Chapter of the Health Physics Society

(605) 688-6306