[L | t+ | ★★★★] | keywords: neutron activation, induced radioactivity, thermalization of neutrons, neutron capture, cross sections, isotopes, half life, exponential decay, beta decay

What it shows:
One of the more important discoveries in modern physics is the production of isotopes (both radioactive and stable) by the capture of neutrons. ¹   In this experiment the bombardment of silver by thermalized neutrons produces short lived radioactive isotopes of silver whose half lives can readily be measured. It can also be shown that bombardment by fast neutrons does not induce radioactivity because of the extremely low neutron cross sections involved. Using a Geiger counter in conjunction with a multichannel analyzer in the MCS (multichannel scaling) mode, the exponential decays of the radioactive isotopes of silver are monitored and displayed as a function of time.

How it works:
Thermal neutrons, which are neutrons in thermal equilibrium with their surroundings, can be produced by passing neutrons through materials containing a high concentration of light nuclei. A high-energy neutron making a collision with a light nucleus will lose a good fraction of its initial kinetic energy to the recoil nucleus of the target. In this experiment, a water bath is used as a neutron moderator. Water is rich in hydrogen and thus the neutron loses roughly half of its initial energy in each collision. Our neutrons originate from a Ra-Be source which is surrounded by a thick silver foil positioned in the center of the moderator. The fast neutrons (1-13 MeV) pass through the silver foil, become thermalized (to about 0.02 eV), and subsequently are captured by the silver. After neutron irradiation, the foil is removed from the neutron flux and the induced activity (beta decay) is measured using a thin-walled Geiger counter.

The activated silver foil is wrapped around the Geiger tube (to maximize counting efficiency) and the ensemble is placed inside a small "house" of lead bricks to minimize background counts. The output of the Geiger counter goes to the MCS which plots out (in real time) a histogram of the number of counts (beta decays detected) versus time. If the silver foil has been irradiated for a sufficiently long period of time (5 to 10 minutes), the statistical variations will be small and the resulting histogram will look quite exponential. Switching the MCS over to a semi-log scale will give a straight line display!

The half lives for ¹¹⁰Ag and ¹⁰⁸Ag are 24.6 seconds and 2.42 minutes, respectively. The basic decay processes are n → p + e^- + ν° plus a delayed γ, the two silver isotopes decay to the stable elements ¹¹⁰Cd and ¹⁰⁸Cd. A chart summarizing the information about the neutron source and silver isotopes is included at the end of this write-up and can be used for an OHP transparency.

Setting it up:
The experimental arrangement is very simple and is best set up on one strong cart with good casters (the many lead bricks used for shielding weigh 25 lbs each). The cart should be positioned as far away from the audience as is possible to minimize radiation exposure. The large lead pig containing the neutron source ²   should be situated between the cart and the blackboard ... practice opening and closing the lead pig.

The moderator is water-filled glass container ³   fitted with a wooden lid. The lid supports (through a hole in the middle) a plastic tube that hangs down inside the glass container. The silver foil, shaped into a cylinder, is dropped down the plastic tube. A half cylindrical wall of lead bricks should be built in front of the moderator to shield the audience from the gamma radiation.

A small house of lead bricks should be built next to the moderator on the cart for the purpose of shielding the Geiger tube. The Geiger detector counter/bias supply ⁴   is operated at a voltage around 950 volts and should be adjusted using a strontium-90 beta source. The output is taken from the rear of the chassis (labeled SCOPE) and is a negative 0.6 volt pulse.

For the MCS, we're presently using a Canberra (Series 20) Multichannel Analyzer which conveniently has a video output so that the screen information can be viewed on a large monitor or video projector by the audience. The following settings will give you a quick start in set-up and can be fine-tuned:

amplifier input: negative     amp gain: 200     shaping: slow
SCA:       LLD: 3%     ULD: 110%
preset: 1 sweep     4 second dwell time
display: automatic VFS
expand: 64 channels (will show approx. 1 3/4 half lives)
memory: 1/8 (128 channels is equivalent to about 8 1/2 min of time, or 3 1/2 half lives)

The demonstration procedure is as follows. Show the audience the glass container and then place it behind the wall of lead bricks. Fill the container with water (obviously, if you wish to perform the experiment first with fast neutrons, you would skip this step). Drop the silver foil into the plastic tube (sample holder) and put the lid/sample holder in place. Open the lead pig and pull out the neutron source. It is secured by a short string to the end of a meter stick, fishing pole style. "Fish" it out and, using a special long "grabber' to guide it, drop it into the plastic tube.

For best results, irradiate the sample 5 to 10 minutes ⁵   so plan to continue lecturing as there's nothing else to do for a while. The next steps should be done as quickly and smoothly as possible; so practice! Fish the neutron source out of the moderator and back into the lead pig--close it. ⁶   Pull the plastic tube out of the moderator, dump the irradiated foil out and quickly position it around the Geiger tube, taping it in place. Put the silver sample and detector in the lead house. Start the MCS counting by pushing the COLLECT button. Whew!

Comments:
This is a good demonstration experiment and one of the few of its kind. It does take some time (at least 15 minutes) plus some planning so that the lecture continues smoothly. The half-life can be quickly deduced during the lecture or the data can be used for quantitative analysis as a homework problem. Note that the two decay curves are superimposed upon each other. This is only evident on a semi-log scale which shows two intersecting straight lines of different slope.

A word of caution concerning radiation safety: observe the three rules. (1) Shielding--use the lead bricks for protection. (2) Time--keep exposure time to a minimum. (3) Distance--use the 1/r² law to your advantage.

Neutron Activation:
Neutron Source:
Ra-Be ≈ 20.7mCi = 7.66x10⁸ Bq ⁷
energies: ≈ 1 - 13 MeV fast neutrons (5 MeV average)
yield: ≈ 3.2x10⁵ neutrons/second

²²⁶Ra (t_1/2=1600 yr) → ²²²Rn (Rn) + ⁴α (4.78 MeV) + γ (.186 MeV)
²²²Rn (t_1/2=3.8 day) → ²¹⁸Po (RaA) + ⁴α (5.49 MeV) + γ (.510 MeV)
²¹⁸Po (t_1/2=3.1 min) → ²¹⁴Pb (RaB) + ⁴α (6.00 MeV)
→ (etc. by α and β decays)

⁴α + ⁹Be → ¹²C + ¹n

1 The first half of the book by R. Rhodes, The Making of the Atomic Bomb, (Simon & Schuster, N.Y., 1986) gives a wonderfully readable historical account.
2 The Ra-Be neutron source is stored in a large lead pig on wheels in the (locked) radiation closet of the P191/247 laboratory (room 103b). It is a 20.7mCi source with a flux of 3.2 x 10⁵ neutrons/second. The fast neutrons range in energy from 1-13 MeV with an average of 5 MeV.
3 23 cm diameter by 30 cm tall holds about 12 liters of water (a bucket)
4 Nuclear Chicago model 1613A
5 The number of radioactive atoms builds up exponentially and reaches about 80% of its asymptotic value after about 2.3 half-lives (5 1/2 minutes); the shorter-lived isotope will have reached almost 100% saturation in this time.
6 The neutron source, moderator, and Geiger counter should be placed at convenient distances so that the silver sample can be transferred quickly from moderator to counter. However, the neutron source should be well shielded from the counter, and at a sufficient distance so that the background count is not raised appreciably by gamma rays from the radium.
7 1 curie (Ci) = 3.7 x 10¹⁰ decays/s is defined as the activity of 1 gm of radium; the SI unit for activity is the becquerel (Bq) equal to one decay per second.

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