Re: Neutron Activation Analysis and gamma rays

Question: Neutron Activation Analysis and gamma rays
From: Winnie
Grade: 10-12

Hello,
I would like to know if it is always the gamma rays which is 
emitted from the neutron activation analysis process and how can the rays 
be 'measured'?

Hi Winnie,

Thanks for the question.  
There are several good introductions to it on the web. Here is a list of
the sites I looked at:


An Overview of Neutron Activation Analysis

by Michael D. Glascock, Missouri University Research Reactor



Neutron Activation Analysis

A general description of the method by William D. James



List of Possible Uses  


While I have never used neutron activation analysis, I am familiar with
most of the ideas on which it is based. Here are my comments on the
subject.

Neutrons are used to activate the nuclei of the various elements that
make up a sample.  Because they are electrically neutral, the neutrons
slip past the electron clouds of the atoms/molecules in the sample and
are also unaffected by the positive charges of the protons in the
nuclei. A standard method of activating a sample is to place it within a 
nuclear fission reactor for a short period of time. The activated sample is 
then removed from the reactor and emitted gamma rays are measured for 
period of time that could be hours or days.

Many nuclei can readily accept an extra neutron.  Often, a neutron
"falls" into a nucleus so that the neutron binds to the nucleus
with a simultaneous release or "freeing up" of energy.

You ask if gamma rays are always emitted after neutron activation.  The
answer is no, gamma rays are just one of several channels that an
excited nucleus can use to release energy.  Other possibilities include an 
electron or beta ray, or an alpha particle. 

While a gamma ray is just one of several possible emissions produced by an 
excited nucleus, it is the most useful for analysis.  Gamma rays are more 
penetrating than beta or alpha particles.  This means that it is much more 
likely a gamma ray emitted inside a sample will escape the sample 
unchanged.  This allows an accurate analysis of all the material in the 
sample.  If beta or alpha rays were used for the analysis, only the surface 
of the sample could be studied. Betas or alphas released by nuclei inside 
the sample would be absorbed or scattered by the sample before they could 
reach the detectors.

The same penetrating power that lets a gamma ray escape a sample with 
useful information also works against us when we try to measure its 
strength.  This relates to the second part of your question on how gamma 
rays are detected.  

Neutron activation analysis relies on being able to see the energies of the 
emitted gamma rays clearly, with high resolution.  High resolution means 
that the gamma rays cluster in sharp peaks on a histogram of counts versus 
energy.  These peaks are the fingerprints for each element in the sample. 
Sharp peaks make it easier to identify elements and also easier to see 
small peaks that might otherwise hide in the shoulder of a nearby large 
peak. 

To get high resolution, the gamma rays must be completely absorbed within 
the active region of the detector.  This means that a gamma ray will 
interact many times within the detector without escaping it.  Each 
interaction takes away some of the gamma ray energy, allowing it to be 
measured. If the gamma ray doesn't leave the detector, then we know it has 
deposited all of its energy where it can be measured.  The detector sums 
all of the bits of energy transferred while the gamma ray is being 
absorbed, giving us the total energy of the gamma.

There are three ways that a gamma ray can lose energy within a detector.  
One is the photoelectric effect, where the gamma ray interacts with an atom 
or molecule and ionizes it, releasing an electron from its orbital.  
Another mechanism is Compton scattering, where the gamma ray interacts 
directly with a free or nearly free electron. The electron recoils and the 
gamma loses energy. The final mechanism is pair production, where a 
powerful gamma ray interacts with an electron or nucleus, and produces an 
electron-positron pair. All three of these mechanisms contribute to 
capturing and measuring the energy of a gamma ray in a typical detector.

There are  two general types of gamma ray detectors.  These are 
scintillators and solid state detectors.  With scintillators, the gamma 
rays enter a clear material that gives off light in flashes as gamma rays 
are captured.  The amount of light given off is proportionate to the energy 
of the gamma ray.  The light is measured with a photomultiplier tube (PMT). 
Scintillators are good because they can be made from elements with high Z, 
highly charged nuclei, that increase the interaction rate with gamma rays. 
Sodium Iodide (Tl) is a typical scintillator.

Solid state detectors are diodes with a high voltage applied across the 
device.  Without gammma rays, only a small electrical current flows through 
the detector.  This current can be reduced significantly by cooling the 
device, often with liquid nitrogen.  When a gamma ray interacts with the 
detector, it produces a large number of electrons and ions within the 
detector.  These electrons and ions are pushed by the high voltage and 
produce a current pulse that is amplified and then measured. Solid state 
detectors are good because they provide much higher resolution than 
scintillators, giving sharp peaks in histograms that show counts versus 
energy. 

Solid state detectors have much higher resolution than scintillators.  
However, they are typically made from silicon or germanium, elements that 
do not have the highest Z. Also, they are difficult to make in a large 
size.  This means that for a typical solid state detector, many gamma rays 
will escape and not be accurately measured.  To avoid the inaccurate 
measurements, solid state detectors and scintillators are often combined.  
The solid state detector is surrounded by scintillators.  If a gamma ray 
escapes the central high resolution detector, the scintillators will see 
the escape and their signal will veto the signal from the central detector. 
This keeps inaccurate gamma ray measurements from being included in the 
data.

Here are some other links you may want to check out:

Neutron Cross Sections at:
NIST


Application of Gamma Ray Spectroscopy: NEAR spacecraft on EROS:
The radiation in the space environment causes the elements of the asteroid 
to emit characteristic gamma rays.  The detector on NEAR measures the gamma 
rays coming from the asteroid and these measurements allow us to deduce the 
elemental composition of the surface and near surface of EROS.
NEAR Gamma Rays


Here are some related articles from our MadSci archives:

Why do neutrons have to be slow to cause fission?
1

Can radioactive decay be 'stimulated' by particle beams?
2


Can we force a nucleus to be unstable?
3

Are there elements/compounds that accept neutrons and then release 
them?
4

Regards,

Everett Rubel