Why it is important to take account long half life of silver to determine neutron flux in neutron howitzer ?
Answers
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.
neutron activation of silver
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.
neutron activation of silver
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)