Half-life of 212Po
Radioactive isotope 212Po is produced as a part of thorium decay series. Thorium and its daughter products undergo many radioactive transmutations ending at the stable 208Pb lead isotope. The isotope 212Po is the daughter nucleus with the shortest half-life (t1/2 ~ 300ns). The purpose of this lab is to measure this half-life. The experimental setup is similar to the setup described by Keith Ruddick in an article published in the American Journal of Physics, 63 (1995) 658.
A small piece of scintillator attached to a photomultiplier is illuminated by radiation emitted from a thorium source. This radiation consists mainly of alpha particles which are very easily stopped even by a very thin absorber (for instance a few centimeters of air). Thus the thorium source must be placed directly on the surface of uncovered scintillator crystal. A convenient thorium source is an older type of gas lantern mantle which contains a significant amount of thorium. Signals from the scintillator counter are shaped in NIM electronics and fed to a Time to Amplitude Coverter with the output range set to 50 microseconds per 10 Volts. The inverted TAC output is sent to a CAMAC Amplitude Digital Converter (range 0 - 2 Volts) gated by the TAC "end of conversion" signal and read by a LINUX workstation. The measurements are displayed and evaluated using CERN Physics Analysis Workstation software.
Raw distribution of polonium decay times (in channel number) collected by Sofia Cunha-Vasconcelos and Jason Kamin in November 2002.
Same as in the figure above but with additional panels showing expanded views of low and high channel regions
The data were evaluated using previously performend time calibration of the CAMAC analog-to-digital converter. It was done using precise time calibrated digitizing scope and a variable delay generator feeding start and stop inputs of the time-to-amplitude converter. The calibration yielded 0.0063552 microseconds per the ADC channel. To increase statistics in each channel the data were compressed to 194 channels that were in turn converted to time units. The data were fitted with two exponential functions plus constant background (initially assumed one exponential function yielded a very poor fit). The five parameter fit gave value of the 212Po half-life (0.313 +- 0.004) microseconds plus some unidentified yet activity with half-life of (1.46 +- 0.02) microseconds. The final data with the fitted line are presented in the figure below.
The compressed data and exponential decay lines fitted to the data.