CINCH NucWik

Jon Petter Omtvedt (j.p.omtvedt[at]kjemi.uio.no)
Nuclear Chemistry, Institute of Chemistry
Faculty of Mathematics and Natural Sciences
University of Oslo

Understand the principles of Neutron Activation
Have basic skills in measuring nuclear radiation ("counting")
Deconvolution of a decay curve with two components

Neutron Source
For this lab exercise a neutron source is needed. The RoboLab setup at the University of Oslo use 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 referred 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 Capture Reaction
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.

Neutron Capture with Silver
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.

Transport Track
The Pu/Be source is placed inside paraffin blocks (for slowing down the neutrons to thermal energies). A slide with a holder for the silver plate is pushed back and forth along a track that is 3 m long. One end of the track is inside the paraffin blocks with the n-source. The other end of the track is positioned above a NaI(Tl) detector that measures gamma radiation. A one meter thick concrete wall is separating the n-source and the NaI(Tl) detector to protect the detector (and it's operator) from the neutrons. The track goes through this wall at an angle in such a way that there the radiation around the n-source are not able to escape through the wall.

Shielding
Concrete is not very good at shielding against neutrons, but will reduce gamma radiation fields significantly. We therefore must absorb the neutrons before they enter the concrete shielding wall. The absorption typically will utilize a (n,gamma) reaction. I.e. the neutrons are captured and energetic gamma-radiation is emitted. The paraffin moderator can be used for this, but not very effectively. For efficient conversion boron is commonly used since it has a very high likelihood to react with (absorb) neutrons.

A practical way to put boron around the n-source is to dispersed e.g. boron acid in molten paraffin and cast large shielding blocks. The combination of paraffin and boron will thermalize and capture the neutrons efficiently. These blocks is placed between the n-source and the concrete wall.

The advantage of using borate paraffin, is that about 20% of the natural boron is ¹⁰B, which has several orders of magnitude higher cross section for the capture of thermal neutrons than hydrogen (in the paraffin), which can reduce the overall thickness of the shielding. The second advantage is that the process of thermal neutron capture on boron will produce a 0.478 MeV gamma-radiation which would need less lead for attenuation than the 2.225 MeV gamma rays from capture in hydrogen.

The picture below shows the borate paraffin blocks that is arranged around the n-source. The borate paraffin is more whitish than the pure paraffin block (which are more of a yellow color). The track exits the concrete wall and is protruding into the middle of the borated paraffin shielding structure where the n-source is placed close to the track.

The tube closest to the track (left in the picture) is a tube for lowering the n-source into place close to the track. The source can also be positioned lower down and is then used to do n-irradiation of samples that can be inserted into the other tubes in the picture. This is not in use for the this RoboLab exercise.

On the other side of the one meter thick concrete wall, the track exits and ends in a lead shielding tower for the NaI(Tl) detector. This is shown in the picture below.

As you can see, there is some extra lead-shielding outside the concrete wall close to the detector. This helps reduce the gamma-radiation field even more that what is done by the concrete wall. The mirror in the image is for the webcam that feeds the video stream you watch during the experiments. This is easier to see in the picture below:

In this picture you can also see the vacuum control valves that is pushing the silver disc back and forth during your experiments. Through the mirror you also get a glimpse of the red slide for the silver disk. The webcam is positioned to providing a better picture of this and you enable you to see when the silver disk is in the detector position.

You should perform at least five irradiations. The following durations are suggested: 12, 24, 48, 72, 144 s. You can add measurements if you have time. For each irradiation you will measure gamma radiation between 500 and 750 keV as a function of time. The measurement is provided as number of counts per a preset time interval. The system will automatically measure a sequence of preset intervals. This gives you a measure sequence reflecting the disintegration of the silver isotopes induced by the n-irradiation.

Your web page to control the RoboLab should look something like this:

(click on the picture to see a larger version.) You must preset the duration for all the measurement periods (rows in the table) before you perform irradiations. In the beginning you want short intervals to follow the rapid decay, then you switch to longer intervals. We suggest 5x 20 sec followed by 100 sec intervals for the remaining time periods (this will be the defaults when you start up RoboLab).

Background Measurement
For each irradiation you do, you should continue measuring until the number of counts fluctuate around the background radiation level in the lab. Therefore, before you start irradiations you should perform a background measurement. The easiest way to do this is to select e.g. a 300 sec preset duration for the first row in the table and start counting. Once the system finishes with the first measurement and start measuring counts for the second row in the table you can stop and write down the number of counts obtained for the first row. You then divide the counts by the duration to obtain your background count rate in cps (counts per second).

Performing irradiations
Make sure you have reset the counting duration after the background measurement to whatever you have selected (probably 20 sec). You can now start performing irradiations. Select a preset time in the irradiation control box (to the left) and push the start irradiation button. In the video feed you will see that the slide with the silver disc disappears - it is sent to the neutron irradiation position on the other side of the concrete wall. There is a sensor inside that keeps track of the actual irradiation time. There is also an irradiation indicator light on the control panel. As soon as you see the slide back on top of the detector you start counting by clicking the start counting button (make sure you have cleared the counters before you started the irradiation). The system will now automatically fill in counts in the table and update the start time for each individual measurement interval. Let it run until you are certain you are only measuring background. Write down all the results before clearing the counter for the next irradiation.

Repeat the procedure above for all your irradiation times. This will conclude the experimental part of your experiment. Remember that if you start an irradiation that somehow did not work out as expected (e.g. wrong preset time) you must wait for the induced radioactivity in the silver disk to die out before making a new attempt. Otherwise the measured activity is not representative for the selected irradiation time.

Use a high-quality data plotting and fitting program (e.g. Origin) to plot and analyze the data. If you use the plotting program to also "fit" the data, i.e. find parameters for a two-component decay curve that matches your measurement points in the best possible way, the fitting algorithm must take the uncertainty into account (do not use Excel unless you know how to use the "solver" add-on to do this correctly), otherwise you will get wrong results.

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".

For each irradiation interval plot your data as follows:

• For each data point calculate the net count (gross count minus background count), the uncertainty of the net count (based on uncertainty of both the gross count and the background count).
• Enter your data in a table ("worksheet" if you use Origin) or whatever your plotting program uses: Include measurement time (relative to end of irradiation) as x-value, the net count as y-value, and the uncertainty as y-error.
• Plot the data - does it look OK? If not, find the error. Your measurement points should lie on a line that gradually decay in a smooth way (within statistical uncertainty).

Notice: The steps indicated below is not very detailed. We assume that you have a teacher physically present that can help you use whatever software and method he/she has prepared for this exercise. How your teaching institution use the data measured with this RoboLab will vary. They might also have provided a more detailed description than what is provided below.

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 your plotting programs fitting functionality to determine the measured half-life of both decay components simultaneously. It is also possible to fit the background level automatically. If so, it should not deviate much from the background you measured (this typically happen if you have terminated your decay measurement too early).

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

• Plot the R₀ values as a function of irradiation time (use Origin or similar software).
• 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.
• Use the weight of the silver plate to determine the number of target atoms (silver atoms).
• Now calculate the theoretical points for the nine other R₀ points.
• How does your theoretical and experimentally measured points agree?

Notice: Again, the above description is not very detailed. Your teacher will instruct how your are supposed to do this for your particular exercise with the RoboLab system.