Neutron Activation 

and

Activation Analysis

11/26/09
General
General

Many nuclear reactions produce radioactive produThe most common of these reactions involve neutr

Neutron + Target Nuclide → Activation Produ
Important Applications/Issues Associate

Neutron Activation1. Neutron Activation Analysis (NAA)This is an extraordinarily powerful technique for ideGeneraland quantifying various elements (and nuclides) in sample.2. Neutron Fluence Rate (Flux) MeasurementsNeutron fluence rates in reactors or other neutron can be measured by exposing targets (e.g., metal the neutrons and measuring the induced activity.
Important Applications/Issues Associate

Neutron Activation3. Dosimetry Following Criticality AccidentsThe induced activity in objects or individuals followGeneralcriticality accident can be used to estimate the dosthese individuals.4. Hazards from Induced ActivityInduced radioactivity in the vicinity of intense neutrsources can constitute an exposure hazard. Examsuch sources include reactors, accelerators and, onuclear explosions.
Neutron Capture

The most important reaction is neutron capture:GeneralThermal neutrons are most likely to be captured.The target nuclide is usually, but not necessarily stIf the product is radioactive, it is likely a beta emitteThe gamma ray, referred to as a prompt gamma or capture gamma, is typically of high energy. 
Neutron Capture Example:

This is an exception to the generalization that the aproduct is a beta emitter. C

r-51 deca

ys b

y electron 

Generalp yy The major prompt gamma rays:

749 keV produced 11.0% of the time 

8512.1 keV produced 6.16% of the time 

8484.0 keV produced 4.54% of the time 
Neutron-Proton Reaction

Another potentially important reaction is the n-p reaGeneralThe n-p reaction is most likely for fast neutrons and nuclides with low atomic numbers.

Example: 
Factors Affecting Activatio
General

The following factors need to be considered:

1. Stability of the reaction product2. Half-life of the activation product

Factors Affecting Activation

3. Radiation emitted by the activation product4. Cross section of the target nuclide5. Abundance of the target nuclide6. Neutron fluence rate (and mass of target ele

The following section will consider each of these in using the element hydrogen as an example. 
1. Stability of the Activation Product

Factors Affecting Activation

If H-1 absorbs a neutron, it simply becomes H-2 wstable.However, when H-2 (deuterium) absorbs a neutrobecomes H-3 (tritium) which is radioactive. This is reaction of importance. Examples would be the pof tritium at heavy water reactors and tritium in the pumps at accelerators. 
2. Half-life of the Activation Product

If the exposure of the target to neutrons is continuoactivation product activity increases and eventually a maximum equilibrium activity when the rate of prequals the rate of decay. As an approximation, thequilibrium is achieved after the neutron exposure continued for five half lives of the activation producFactors Affecting Activationcontinued for five half-lives of the activation produc

Activity of 

Activation 

Product (A)

Time 

Approx. 5 half-lives 
For situations when the exposure is brief, the short

half-life of the activation product, the greater its actthe end of activation.2. Half-life of the Activation ProductFactors Affecting ActivationThe long half-live of tritium (12.3 years) means thaexposures to neutrons will produce almost no activ
The radiation emitted by the activation product will 

the hazard associated with its production and the mby which it can be detected.3. Radiation Emitted by Activation ProduFactors Affecting ActivationThe beta particles emitted by tritium are of such a energy (18.6 keV max) that they cannot be detecteconventional survey instruments. In addition, there gamma ray emitted when tritium decays. 
The chart of the nuclides does not tell us this, but a 

MeV prompt gamma is produced every time H-1 abneutron to become deuterium.3. Radiation Emitted by Activation ProduFactors Affecting ActivationPrompt gamma rays like this tend to be of high enetherefore hard to shield.At the same time, the detection of 2.2 MeV gamma an indication that there is a neutron source nearby
The cross section of a reaction reflects the probab

the reaction. The latter depends on the energy of tneutrons and the identity of the target nuclide.For neutron reactions, the cross section is symboli4. Cross Section of the Target NuclideFactors Affecting Activationthe Greek letter sigma (

F). 

The subscript indicates the specific reaction, e.g.,:

F

( is the cross section for neutron capture

Fp is the cross section for the n-p reaction
Cross sections are expressed in cm

2 or barns.

One barn = 10-24 cm

2

In general, one barn is a fairly large cross section.4. Cross Section of the Target NuclideFactors Affecting ActivationUnless indicated otherwise, the cross sections on tof the nuclides are in barns. 
4. Cross Section of the Target Nuclide

Factors Affecting Activation

The capture cross section for H-1 is fairly large (0.but that for H-2 is small (0.52 millibarns).As such, neutron capture by an atom of H-2 to protritium has a low probability.The greater absorption of neutrons by H-1 than H-

heavy water can be considered a superior modera
4. Cross Section of the Target Nuclide

Factors Affecting Activation

The chart of the nuclides indicates two cross sectieach reaction. The first is the cross section for theneutrons (0.333 barns for H-1) – this is the most imcross section. The second (0.150 barns for H-1) is resonance capture cross section.
4. Cross Section of the Target Nuclide

Factors Affecting Activation

The above figure shows the neutron capture cross for vanadium-50 as a function of neutron energy. at the higher energies are referred to as resonancoccurs when the neutron energies match up with elevels of the target nucleus. 
5. Abundance of the Target Nuclide

Factors Affecting Activation

Towards the top of the box of a stable nuclide, the indicates the percent abundance – the percent of element’s mass that consists of the nuclide.The above shows that only 0.015 percent of naturhydrogen (by mass) is H-2. This means that there little H-2 in hydrogen that would serve as a target production. 
This is somewhat self evident, but the greater the n

fluence rate, the greater the induced activity of the activation product.6. Neutron Fluence Rate and the Mass of Target ElementFactors Affecting ActivationIn addition, the greater the mass of the target elemthe number of target atoms), the greater the induceactivity of the activation product.
Neutron Activation Analysis
Neutron Activation Analysis (NAA)

GeneralNeutron activation analysis is a powerful technique identifying and quantifying elements (and nuclides)advantages include the fact that NAA is:

• A multi A multi

-element technique element technique -- many elements 

many elements 

analyzed simultaneously.

• Sensitive. Most elements can be analyzed to 

and many can be analyzed many orders of mbelow that.

• Fast. The only processing of the sample rou

employed is drying and mixing. 
General

Its advantages include the fact that NAA:

• Nondestructive. Aside from the production of 

short lived activity, the sample is chemically a

ph sicall nchanged after the anal sis

Neutron Activation Analysis (NAA)

physically unchanged after the analysis.

• Especially effective with large solid samples (

that are difficult to analyze by other means.
Neutron Activation Analysis (NAA)

How NAA WorksNAA typically works as follows:

1. The sample is exposed to neutrons2. The activated sample is analyzed by gamma 

spectroscopy; the activation products are idenand “quantified”.

3. The identity and quantity of the target 

nuclides/elements are deduced.
Neutron Activation Analysis (NAA)

How NAA Works – ExampleExample of using NAA to identify a nuclide (and elea sample:The sample in question is exposed to neutrons for of time. After the irradiation, the activated sample is analyzgamma spectroscopy. Two peaks show up on the spectrum: one indicating a gamma ray at 1173 keV other a gamma ray at 1332 keV. What is the target nuclide?
Neutron Activation Analysis (NAA)

Co-59 + n 

→ Co-60

(stable target) (activation product)How NAA Works – Example

1173 keV gamma1332 keV gamma

Ni-

The target nuclide is Co-59, the only stable nuclide 
Predicting the Sensitivity of Neutron Act

Analysis (NAA)Neutron activation analysis does not identify and qall nuclides or elements equally well. 

f 

Neutron Activation Analysis (NAA)The 

following discussion will consider the sensitivit

neutron activation analysis for two elements: iron manganese.In any sample containing iron, all the stable nuclideiron will be present.
Predicting the Sensitivity of NAA - I

Beginning with Fe-54 serving as the target nuclide, irradiation 

produces Fe-55 

( p) the activation 

product

). 

Although radioactive, Fe-55 has a long half life (2.7For significant production of Fe-55, very long exposperiods would be required. Even if it could be produced in quantity, Fe-55 decaexclusively by electron capture (

ε); no gamma rays 

emitted with which we could detect and identify its p
Predicting the Sensitivity of NAA - I

The next stable nuclide of iron that we might be abldetect is Fe-56. Unfortunately, the Fe-56 in the sample is simply coanother stable nuclide, Fe-57. Similarly, any Fe-57 present in the sample will be cinto a stable product, Fe-58.
Predicting the Sensitivity of NAA - I

If NAA is going to detect the presence of iron with aof sensitivit

y yg , it must be done b

y convertin

g Fe-58 in

The half life of Fe-59 (44.51 days) is a bit long but ionly option. Fe-59 emits gamma rays at 1099.2 and 1291.6 keV good intensities, 56% and 43% respectively. The cross section of the target, Fe-58, is decent: 1
Predicting the Sensitivity of NAA - I

The problem lies in the low abundance of the targeAs such, the detection of trace quantities of iron reqneutron fluence rates.
Predicting the Sensitivity of NAA - Manga

We now consider the case of manganese. The target abundance is great. Manganese has onstable nuclide, Mn-55, with an abundance of 100%Mn-55’s cross section for neutron capture is high:13Mn-56, the activation product, has an ideal half-life hours and emits a gamma ray at 846.8 keV. The latintensity is 99%. As a result, manganese is extraordinarily easy to de
Quantifying an Element via NAA

There are two general approaches to quantitative nactivation analysis:1. Use a standard (also referred to as a comparat

monitor)

Neutron Activation Analysis (NAA)

. 

2. Without a standard. Use equations to estimate 

of the target element based on the measured athe activation product. Since this approach uses relatively simple mathto describe a complex situation, it has a numbeproblems that are avoided when a standard is u
Quantifying an Element via NAA by Usin

Standard1. A standard containing a known mass of the ele

interest (m

S) is irradiated with neutrons for time 

2 After activation the sample is counted for timeNeutron Activation Analysis (NAA)

. After activation, the sample is counted for time

gamma spec system. The time between the enactivation and the start of the count is t

w.

3. The net counts (C

S) are determined for the mai

photopeak produced by the activation produc

4. A calibration factor (F) is determined relating th

element mass in the standard to the photopeak 
Quantifying an Element via NAA by Usin

Standard - Example:A standard soil sample is obtained from the Nationof Standards and Technology (NIST) at great expeAccording to the calibration sheet it contains:Neutron Activation Analysis (NAA)

10.0 ± 0.4 mg Mn

The standard is exposed to neutrons for 6 minutesThe activated sample is counted 0.5 minutes after The count is for 2 minutes using a germanium dete
Quantifying an Element via NAA by Usin

Standard - Example:A photopeak appears for the Mn-56 847 keV gammNeutron Activation Analysis (NAA)

A region of interest is set up about the peak and the net count is 1000.
Quantifying an Element via NAA by Usin

Standard - Example:The calibration factor is calculated as follows:Neutron Activation Analysis (NAA)
Now an unknown sample is analyzed. While not e

is best to handle the unknown and standard the sa

The unknown is activated for 6 minutes

Quantifying an Element via NAA by UsinStandard - Example:Neutron Activation Analysis (NAA)

The unknown is activated for

6 minutes.

The time between activation and counting is 0.5 The count time is 2 minutes. 

If enough manganese is present to exceed the lowdetection, a discernible photopeak will show up at Assuming that such a peak yields a net count of 50
If such a peak yields a net count of 500, what is th

the manganese in the unknown?Quantifying an Element via NAA by UsinStandard - Example:Neutron Activation Analysis (NAA)
The mass of the manganese in the unknown (m

U) 

calculated as follows:

Quantifying an Element via NAA by UsinStandard - Example:Neutron Activation Analysis (NAA)

It couldn’t be easier!
The following equation gives the activity (A) of the 

product at the start of the count:Quantifying an Element via NAA Without StandardNeutron Activation Analysis (NAA)N is the number of target atoms (e.g., Mn-55)F is the thermal neutron capture cross section of t

nuclide (cm

2) 

N is the thermal neutron fluence rate (n/cm

2/s) 

8 is the decay constant of the activation product (t

a is the activation time (e.g., min)

t

w is the time between the end of activation and th

the count (e.g., min) 
In neutron activation, we are primarily interested in 

determining the amount of the target nuclide in the As such, the appro

priate form of the 

previous e

qua

Quantifying an Element via NAA Without StandardNeutron Activation Analysis (NAA)

, pp p p q

A is the activity of the activation product at the starcount (Bq). It is usually determined by gamma spe
The mass of the target nuclide (e.g., Mn-55) in the 

Quantifying an Element via NAA Without StandardNeutron Activation Analysis (NAA)

Mi is the isotopic mass of the target nuclide (gram6.02 x 1023 is Avogadro's number (atoms per mol
The mass of the target element (e.g., manganese) 

sample is:Quantifying an Element via NAA Without StandardNeutron Activation Analysis (NAA)f is the fractional abundance of the target nuclide (percent abundance), e.g., the fraction of the mass manganese that is Mn-55.
The preceding equations are reasonably straightfo

the neutrons are all of the same energy and the crosection for neutrons of that energy is known.In man sit ations thermal ne trons are so predomQuantifying an Element via NAA Without Standard - ProblemNeutron Activation Analysis (NAA)In many sit

uations, thermal ne

utrons are so predom

that they are the only ones that need to be consideFurthermore, capture cross sections are often mucfor thermal neutrons than neutrons of higher energAnother potential problem is a non-uniform fluence throughout the sample volume. If the cross sectionlarge, the fluence rate can be lower in the center ophysically large sample than near its surface.
Measurement of Neutron 

Fl R t uence 

R

a

t

e
Measurement of Neutron Fluence Ra

GeneralThe following equation can be used to calculate the fluence rate if the neutrons are predominantly of the of interest (e.g. thermal) and if the capture cross secthe other neutron energies are small.For example, a certain mass of vanadium (i.e., a knnumber of V-51 atoms) is irradiated for a known per

specified time after irradiation (t

w), the activity of V-5

foil (A) is determined by gamma spectrometry and tequation used to predict the neutron fluence rate. 
Criticality Accident 

Di t os me

try
Criticality Accident Dosimetry 

Initial Screening

The neutrons emitted during a criticality accident min sufficient activation of the body's sodium that it cadetected with a GM or NaI detector:

nth + Na-23 

→ Na-24

There is quite a bit of the target nuclide, Na-23, in tand it has a large capture cross section for thermal The activation product, Na-24 has a half life of 14.9and emits high energy gammas. Immediately after the criticality, approximately 50% induced activity is Cl-38. After 2 and 4 hours, the contribution drops to 10% and 1% respectively. 
A quick sort of the victims can be accomplished by p

side window GM or NaI against the victim's abdomehaving them bend over the probe to improve the cougeometry. Another approach is to place the detectothe armpitsCriticality Accident Dosimetry Initial Screeningthe armpits.

Any increase in count rate is indicative of an exposuSeveral different rules of thumb have been proposeevaluating the neutron dose. Some of these are ind

the following slide. 
As a very crude quantitative estimate, an increase o

mR/hr (0.10 uSv/h) above background indicates a faneutron exposure of 1 rad (0.01 Gy). If the instrument reads out in mR/h, the neutron dosCriticality Accident Dosimetry Initial Screeningestimated by using the following relationship:

Dose (rad) = 8000 X/body weight in poun

where X is the exposure rate in mR/h

“ 70 counts per minute above background equals onneutron dose.” (Ricks et al 2002). 
By measuring the 

sodium-24 in the blood, backcalculating the activity to the time

Criticality Accident Dosimetry More Detailed Analysis

activity to the time

of the accident, and using a graph like that to the right, it is possible to estimate the neutron dose.
Hazards Associated with 

N t A ti ti Neu

tron 

A

ctivation 
Hazards Associated with Neutron Activat

GeneralAfter the source of neutrons has been removed, the associated with the activation products can persist.For example, following a reactor shut-down it is necwait before entering containment to permit the short wait before entering containment to permit the short

activation products to decay. The activation produca particular problem because it has a long half-life ahigh energy (penetrating) gammas. For the most paproduced by activation of the cobalt present in valvecoolant lines. Approximately 50% of the radiation exat a nuclear power plant are due to Co-60. Other acproducts of importance in reactors include Cr-51, Co54, Fe-59, and H-3.
Hazards Associated with Neutron Activat

GeneralHigh energy (fast) neutrons are often produced in laquantities in the vicinity of accelerator targets. Although the induced activity decreases by one halfirst 10 minutes after shut-down, long-lived activatioproducts may persist products may persist. 

Common activation products near the target includeNa-24, Cu-62 and Al-28. Tritium, which is an internal hazard, can also be proa large quantities, especially in pump oil, e.g. ion puN-16 can be produced in the air near the target and cooling water supply by the activation of 0-16.