INTRODUCTION The detection of ?-rays was carried out

INTRODUCTION

Radioactive decay
involves particles or radiation being emitted from unstable nuclei by
transitioning from a high energy state to a lower one. The emitted
radiation/particles may involve gamma rays, alpha particles, beta particles
along with a neutrino or only a neutrino or an electron in the cases of
electron capture and internal conversion. Radiation emitted from radioactive
material can be ionising, and when these emitted particles meet organic
material, such as human tissue, they can cause cellular mutations by ionising
atoms and breaking molecular bonds. 1 The ionising effect can be
either direct- with charged particle radiation, or indirect- with neutral
radiation. Therefore, radioactive sources are considered hazardous. Over the
years, physicists have built various types of detectors to detect the presence
of these kinds of radiation, which include, scintillation detectors,
semi-conductor based detectors, gaseous ionisation and Cherenkov detectors.

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For the purpose of this
experiment, the only radioactive sources used were those which mainly radiated
gamma, ?-rays and how these ?-rays were attenuated by various materials was
investigated. The detection of ?-rays was carried out using thallium doped
sodium-iodide, NaI (TI) scintillation detector. The energy spectra of
radioactive sources of ?- rays were obtained and detailed analysis of these
spectra allowed the identity of the ?- rays to be determined. 2
When gamma radiation passes through matter, it is not completely absorbed, but
only reduced in intensity. 3 They are the most penetrative out of
the three main types of radiation, which is why they are the most difficult to
avoid. The fraction of ?- rays which pass through matter without interacting is
what defines the attenuation of these ?- rays.4 The attenuation
coefficient of the material ‘absorbing’ the ?- rays is in proportion of the
interaction probability, commonly known as the ‘cross-section’.3 The
relationship between the intensity of the radiation and the thickness of the
material was also investigated in this experiment.

Safety and Rules 4

Care must be taken not to
lose or damage sources. If a source appears to be damaged, a member of the
staff is to be notified immediately. Sitting near the source for long periods
should be avoided. Care must be taken not to overload the bench when using lead
bricks to shield the apparatus. High voltage supplies should be turned off
before disconnecting cables or any internal examination of the detector. The
mains power supply should never be turned off to any apparatus supplying a HV
to a detector without first reducing the HV supply setting to zero. Failure to
do so may result in the electronics being destroyed.

THEORY OF THE DETECTOR

Majority of the detectors
tend to be either semi-conductor based or are scintillation detectors, which in
turn are typically NaI based. For this experiment, a NaI(TI) scintillation
detector was used. The two principle advantages of using this type of detector
include; its capacity to be produced in large crystals, hence yielding a good
efficiency and producing intense bursts of light in comparison to other
spectroscopic scintillators.2

Scintillation detectors
tend to be sensitive to one type of radiation. As for the case of this
experiment the NaI detector detected only gamma radiation, however some
detectors can have a higher sensitivity and even detect some high energy beta
particles. Scintillators are materials that produce light when ionising
radiation passes through them. Scintillation detectors work by detecting and
measuring ionising radiation by using the excitation effect of incident
radiation on the scintillator material and by detecting the resultant light
pulses. The common structure and layout of the scintillation detector is shown
below in figure 1.

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