Since the experiment involves the use of a radioactive source, a radiation safety course was attended. This involved many topics which are concerned with using radioactive material both safely and legally. Such as the biological effects of radiation, working with isotopes safely and the legislations, namely The Radioactive Substances Act, 1993 (RSA93) and The Ionising Radiations Regulations, 1999 (IRR99).
The discriminator is also connected to a Lab jack which is then connected to a computer which collects the data.
A Bit About the Devices Involved:
The Inside of a Photomultiplier:
How The Experiment Works
Caesium is the radioactive source for this experiment. It emits high energy Gamma ray photons randomly during radioactive decay. Radioactive decay is a set of various processes by which unstable atomic nuclei emit subatomic particles (radiation). Decay is said to occur in the parent nucleus and produces a daughter nucleus. This is a truly random process, i.e. it is impossible to predict when an atomic nucleus will decay. Photons are emitted in all directions, but those that travel in the direction of the photomultiplier hit the scintillation crystal which in this case is a Sodium Iodide crystal. The scintillation crystal converts each gamma photon into a large number of lower energy photons. These photons are called the optical photons and have a lower energy due to their decreased frequency (although they are named optical we still cant see them). The photomultiplier tube is sensitive to optical photons and produces electrons from them by the photoelectric effect. The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation x-ray. Hence the electrons leaving the photocathode have the energy of the incoming photon.
The photomultiplier is mostly made up of empty space but it does contain electrodes which are used to amplify the electrons. There are 10 electrodes, each coated with dynodes. Each dynode has a different voltage across it. The difference across all the voltages is 1000V. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode and move towards the first dynode where they are accelerated by the electric field and arrive with much greater energy. After striking the first dynode, more lower energy electrons are emitted and these are accelerated towards the second dynode. The dynodes cause a cascade of electrons which keep increasing at each dynode. Finally they reach the anode where the accumulation of charge results in a sharp but small current pulse. This current pulse leaves the photomultiplier and goes through the amplifier in the NIM before it is directed to the discriminator.
The discriminator has a threshold voltage which can be set. The threshold voltage is set to a value which is halfway up the incoming pulse. For example if the pulse height was 4V the threshold voltage is set at 2V. Then, whenever a signal crosses the threshold voltage, the discriminator produces a standard flat top pulse. The Lab jack then counts the number of these standard pulses per second which is a random number since radioactive decay is random. This is the number we will be getting the results of our lottery from.
How The Results Are Going To Be Generated
The experiment will be run and the number of events observed in a second will be determined from the average of many periods (50 or more). We will then, having set this base line, determine the number of gamma emissions in eight one second time bins. If the number of emissions lies above the long term average then we will assign the number 1 and each count which is below the average will be assigned the number 0. Thus whoever has the correct combination of 1s and 0s in the correct order is the winner. As an example a trial experiment was run. The results were as follows:
The Lab jack has a timer which has been set in the past and can not be reset therefore the time does not start at 0 seconds as one would expect. However it is still measuring at one second time intervals so this makes no difference what so ever. According to the trial experiment, the winning combination is 1,0,0,0,1,1,1,0. The winner will be the participant who has chosen this exact combination.