Nuclear Spectroscopy

Na I (Tl)
D2
Electrons
Anode
θ
θ
θ
etc.
θ
Photocathode
D1
D3
Figure B1. A schematic diagram of a photomultiplier tube assembly.
High voltage
to dynodes
collisions with electrons in the crystal. Recombination
of these electron-hole pairs can result in light emission
by radiative transitions or heating in the form of lattice
vibrations. The presence of thallium at the 10 −3 molar
fraction level significantly increases the light emission
of the crystal, and is referred to as an activator for the
crystal.
Experiments have shown that luminescence from the
activated NaI(Tl) crystals is characteristic of an excited
state decay in thallium ions (Tl + ), even though
the energy of the gamma (and the electrons that it
produced) is dissipated almost entirely in the NaI
host! The diffusion rates of the electrons and the
holes (they are not the same) result in thallium emission
that decays nearly exponentially after the first few
microseconds. Almost two thirds of all the luminescence
is emitted in the first quarter of a microsecond.
This time is important because to be useful as a
detector of individual gamma photons, the flashes of
light (scintillations) must be distinguishable in time.
The production of electron-hole pairs from a gamma
interaction usually takes less than a nanosecond and
does not affect this result.
The result of the Thallium activation of NaI is a crystal
that converts about 11% of the incident gamma energy
into photons with an average energy of 3.0 eV.
The burst of photons has a characteristic (although
not purely exponential) time of 0.23 µsec that has a
maximum emission at a wavelength of 415 nm. A 1,000
keV gamma will produce, on average, 4.3 x 10 4 photons.
Statistical variations in this number for each
1,000 keV gamma observed is one of the contributing
factors in the observed width (energy resolution) of
the photopeaks.
Output
PHOTOMULTIPLIERS
Rutherford and his graduate students used their unaided
eyes to detect the individual scintillations from
their ZnS phosphor. Distinctions between variations
of the light intensity of individual pulses is very difficult
if not impossible. In the late 1930’s a tube was
invented that converted light into a electron current
that was amplified by many orders of magnitude. This
tube eventually became the television camera of the
1960’s and 1970’s and became the photomultiplier for
observing the scintillations from NaI(Tl) and detection
of low intensities of light in general. A typical
photomultiplier tube is shown in Figure B1.
The photomultiplier has a semitransparent photocathode
at its detecting end which produces with some
probability, one electron for each photoelectric event,
for photons between designed photon wavelength
ranges. For NaI(Tl) scintillations, the wavelength range
is centered near 415 nm. The photoelectron is accelerated
between the photocathode and the first dynode,
striking the dynode with a kinetic energy between
a few tens and a hundred eV. The collision at the
surface of the first dynode produces a few electrons for
each incident electron. These electrons are accelerated
to the second dynode where the process is repeated
for each succeeding dynode until the last
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