counting distribution of beta particles using the GM tube follows
(A) Normal distribution (B) Binomial distribution
(C) Poisson distribution (D) Rayleigh’s distribution
Answers
Answer:
bjective: To become acquainted with the operation and characteristics of the Geiger-Müller
(GM) counter. To determine the best operating voltage and the resolving time of a Geiger
counter. The resolving or dead time is used to correct for coincidence losses in the counter.
Experimental Apparatus: A typical Geiger-Müller counter consists of a cylindrical gas filled
tube, a high voltage supply, a counter and timer. A large potential difference is applied between
the tube body which acts as a cathode (negative potential) and a wire down the tube axis which
acts as an anode (positive potential). The sensitivity of the instrument is such that any particle
capable of ionizing a single gas molecule in the GM tube (thus producing an electron-ion pair)
will initiate a discharge in the tube.
What happens next depends on the voltage across the gas-filled tube. For the lowest applied
voltages, only the ions created by direct interaction with the incoming radiation are collected. In
this mode, the detector is called an ion chamber. For higher voltages, the ions created are
accelerated by the potential difference gaining sufficient energy to create more ion pairs. This
results in a localized avalanche of ions reaching the wire. This is the proportional region. The
pulse height (or voltage of the signal) is proportional to the number of initial ion pairs created by
the incoming radiation. This in turn is proportional to the energy of the incoming radiation. For
even higher voltages, the new ions can create additional photons which move out of the local
region and further down the tube; essentially the discharge propagates an avalanche of ionization
throughout the entire tube, which results in a voltage pulse--typically a volt in amplitude. Since
the discharge is an avalanche and not a pulse proportional to the energy deposited, the output
pulse amplitude is independent of the energy of the initiating particle and, therefore, gives no
information as to the nature of the particle. This is the Geiger-Müller region. In spite of the fact
that the GM counter is not a proportional device, it is an extremely versatile instrument in that it
may be used for counting alpha particles, beta particles, and gamma rays. Such a large output
signal obviates the need for more than a single stage of amplification in the associated electronic
counter.
Geiger-Mueller tubes exhibit dead time effects due to the recombination time of the internal gas
ions after the occurrence of an ionizing event. The actual dead time depends on several factors
including the active volume and shape of the detector and can range from a few microseconds for
miniature tubes, to over 1000 microseconds for large volume devices. When making absolute
measurements it is important to compensate for dead time losses at higher counting rates.