CINCH NucWik

Section of Radiochemistry
Institute of Chemistry
Faculty of Mathematics and Natural Sciences
University of Oslo

Understand the principles of Neutron Activation
Deconvolution of a decay curve with two components

Neutron Activation of Ag with a Pu/Be n-source

Neutron Source
For this lab exercise a neutron source is needed. If you are using the RoboLab remote experiment facility the neutron source in Oslo is a Pu/Be source. This source consist of powdered beryllium metal mixed with 350 GBq (9,6 Ci) of ²³⁸Pu, an α-emitter. When the α-particles hit the Be-nuclei the following nuclear reactions take place:

⁹Be(α,n)¹²C

The source emits 2.8*10⁷ neutrons per second. Most often we want to know the number of neutrons that hits our target, the neutron flux. That is, the number of emitted neutrons per unit area and time at the location of the material we irradiate. The neutrons emitted from the source have an average energy of 4-5 MeV and are called fast neutrons.

Thermal Neutrons
Thermal neutrons have an average energy of 0.025 eV and an energy distribution comparable to that of gas molecules at room temperature. At this low energy the probability of neutron capture is much higher for most elements compared to using high-energy neutrons. Therefore we want to reduce the fast neutrons from our source to thermal energies. Since neutrons are neutral they do not lose their energy in electrostatic interactions. Rather they lose energy in collisions with other nuclei. The most efficient transfer of energy is when the colliding neutron and nucleus have the same mass. So, materials containing a lot of hydrogen are good materials for moderating fast neutrons, two examples are paraffin and water. When used for this purpose, the paraffin or water is usually refered to as "the moderator". Thermal neutrons will move in all directions, because of the collisions with the nuclei in the moderator. Thus, they can then be considered a gas, filling the moderator, where the density decreases with the distance from the detector. There will therefore be an optimal distance between the n-source and the sample, where the neutrons have been efficiently moderated but the "n-gas" is not too diluted. The type of moderator, n-source, geometrical construction of the irradiating facility, etc. will influence this optimal sample position, but in most cases it will be between 3-7 cm away from the source.

Neutron Activation following a flux φ of thermal neutrons of an isotope M of a given element is typically:

^MA (n, γ) ^M+1A

The new isotope is of the same element, but with the mass number increased by one. We refer to this as n-capture, since the new isotope has "captured" an extra neutron. Since the mass number changes, the isotope produced in the reaction may be radioactive - which is the whole point of n-activation. In this way the amount of the element in the sample can accurately be determined even at at very low concentrations, we then would call it Neutron Activation Analysis (NAA). It is also a common way to produce radionuclides for use as e.g. tracers.

If you look in your nuclear chart, you will find two stable isotopes of silver: ¹⁰⁷Ag and ¹⁰⁹Ag. For each, both a metastable state and the ground state of the daughter will be produced (as you can see from the cross section - it is given as the sum of two numbers, indicating the cross section for forming the metastable state (first number) and ground state (last number).

I.e. for n-activiation of natural silver we will get: ^108mAg, ¹⁰⁸Ag, ^110mAg, and ¹¹⁰Ag.

Neutron Activation of Ag

The Pu/Be source is placed inside paraffin blocks (for slowing down the neutrons and shielding). A slide with a holder for the silver plate will push the plate inside the paraffin block close to the source. When the slide is withdrawn the silver plate will fall out of the holder (into your waiting hand). Your supervise will operate the slide and you should catch the silver plate after the required irradiation time and at the same time start your stopwatch. You then run as fast as possible (STRAFAP - STudents Running As Fast As Possible) but carefully to your detector with the source. Perform five irradiations with the following times: 12, 24, 48, 72, 144 s and measure the resulting decay curves of the two silver isotopes. It is important that you let the silver decay before you perform the subsequent irradiations.

How to measure the Decay of n-activated Ag

Introduction
For this part of the exercise, you will use a NaI detector connected to a Multi-Channel Analyzer (MCA) to determine the disintegration rate of the n-activated silver.

Principle
This description assumes you have the Maestro MCA software from ORTEC. If you are using an alternative system, you will have to consult the manual to figure out how to use it. The procedure should not be very different, though.
We want to make successive 20-s measurements followed by 120-s ones to determine the half-life curve of the silver isotopes. This can be done manually by successive starting-waiting-stopping-saving-clearing operations.
However, with a modern system this tiresome procedure can be automated: In Maestro jargon you do this by preparing a job-description file (it would be called a script file or batch file in most other software). This file contain all the instructions you would have to execute, but can be simplified by using the built-in loop structure. Furthermore, once running, it will execute the correct commands at exactly the right time.
Since the commands execute very rapidly, you will also be able to spend practically all the time actually counting, something witch is not possible if you are doing everything manually.

Job-description file:

set_preset_real 20\\
loop 7\\
clear\\
start\\
wait\\
save m:\spectra\KJM5911_D130_A???.chn\\
end_loop\\

set_preset_real 120\\
loop 7\\
clear\\
start\\
wait\\
save m:\spectra\KJM5911_D130_B???.chn\\
end_loop\\

set_preset_real 300\\
clear\\
start\\
wait\\
save m:\spectra\KJM5911_D130_Background.chn\\

This job-file will perform 8 20-s measurement, then 8 120-s measurements and finally a 5-min background measurement.

Procedure
The MCA will save spectra containing counts vs. energy. The two interesting gamma-rays from the n-activated silver will overlap and you will not be able to differentiate between them in the NaI spectra. Thus, we will simply use the gross counts and subtract the background as if we had used a simple counter.
The procedure for measuring each irradiated silver disk is as follows:

1. Measure a background spectrum for as long as possible if you have not already done this.
2. Get the irradiated silver disk and put it as quickly as possible on top of the detector.
3. Start the job-file and note down the time between end-of-irradiation and starting the job-file.
4. Now, sit back and relax! Alternatively (better), if the job-file is saving spectra to a network disk, you can analyze the spectra as they are produced (using another pc which can read the same disk).
5. When the job-file finishes, repeat the measurement for the different irradiation times (irradiation times = 12, 24, 48, 72, and 144 s). (Remember to rename or move your spectra, otherwise they will be deleted or the job-file stops.)

From the spectra you should get the following data (by opening each spectrum in Maestro): The measurement start time and the gross count (total number of counts in the spectrum). Use the "sum" command to get the total number of counts (the spectrum must contain no region-of-interest markings).

Alternative procedure: Select the relevant spectrum region with the photo-peaks and only use the integrals under this (double-)peak for analyzing the data.

Analyzing a two-component Decay curve

Use a high-quality data plotting and fitting program (e.g. Origin) to analyze the data. The fitting must take the uncertainty into account (do not use Excel), otherwise you will get the wrong result. Notice that you always shall use the 1/3 of the time into each measurement as the "middle time point". This is due to decay - after 1/3 of the time you will have equally many counts before and after the 1/3 point (i.e. it is the "middle point".

1. For each data point calculate the net count (gross count - background count), the uncertainty of the net count (based on uncertainty of both the gross count and the background count). You might want to use e.g. MS Excel or similar for doing this.
2. Enter your data in a table ("worksheet" in Origin jargon): Include measurement time (relative to end of irradiation) as x-value, the net count as y-value, and the uncertainty as y-error.
3. Plot the data - does it look OK?

Manual Method

1. Look at the decay curve and identify the part where you only have the longest living component. Make a new curve where you only include this part of the data.
2. Fit the slow component and note down the parameters describing the decay.
3. Use the fitted parameters from the slow component to calculate the amount this component contributed to the total for each of the measured data points.
4. Now plot the new data set, which only should include the fast component.
5. Fit the fast component. Does it look right?
6. Calculate the R₀ (count rate if you had measured exactly at the end of of the irradiation) for each component.

Automated Method
Use the Origin data-fitting functionality to determine the measured half-life of both components simultaneously. "Alternative/Extra": Plot the gross counts instead of the net counts and ask Origin to fit both the background and the two components at the same time.

Analyzing the Production Curve of n-activated Ag

From analyzing the decay curves for the different irradiation times (12, 24, 48, 72, and 144 s) you should have five R₀ values for each of the isotopes:

1. Plot the R₀ values as a function of irradiation time (use Origin or similar software).
2. Assume that the R₀ for the ¹¹⁰Ag irradiation is exact. Use this value to determine the product of the detector-efficiency and neutron flux. In the following, use this value as "true" whenever you need the product.
3. Use the weight of the silver plate to determine the number of target atoms (silver atoms).
4. Now calculate the theoretical points for the nine other R₀ points.
5. How does your theoretical and experimentally measured points agree?

Use the cross-sections from the nuclear chart, the half life and a thermal neutron-flux of 2*103 n/cm²s to estimate the relative amount you will produce for a 144 s irradiation. Which nuclei will be dominant? Determine what kind of gamma radiation to expect from the silver isotopes produced in the irradiation and their associated relative intensity (e.g. from Berkeley/Lund database).

Need silver disk and a neutron source, or if using RoboLab an internet connection.

It is possible to perform this exercise with RoboLab.