Nuclear Spectroscopy

E X P E R I M E N T 1
Energy Calibration
INTRODUCTION
When a gamma ray (that is emitted during a change
in an atom’s nucleus) interacts with your sodium
iodide crystal, NaI(Tl), the gamma will frequently
give all its energy to an atomic electron via the photoelectric
effect (photoeffect). This electron travels a
short, erratic path in the crystal, converting its energy
into photons of light by colliding with many atoms in
the crystal. The more energy the gamma ray has, the
more photons of light that are created.
The photomultiplier tube (PMT) converts each photon
into a small current, and since the photons arrive
at the PMT at about the same time, the individual
currents combine to produce a larger current pulse.
This pulse is converted into a voltage pulse whose size
is in proportion to the gamma ray’s energy.
The voltage pulse is amplified and measured by an
Analog to Digital Conversion (ADC) process. The
result of this measurement is an integer between 0
and 1023 for a 10-bit ADC. Zero is the measure for a
voltage pulse less than a hundredth of a volt, and
1023 is the measure for a pulse larger than about 8
volts (or the largest voltage pulse accepted by your
ADC). Pulses between 0 V and 8 V are proportionately
given an integer measure between 0 and 1023.
This measure is called the channel number. The
analog to digital conversion process is performed by
your interface board. Your computer records and
displays these measurements as the number of gamma
rays observed for each integer measure, or channel
number. Your screen displays the number of gamma
rays as a function of the channel number.
Calibration with sources of known energies allows
you to correlate the channel number with the gamma
energy. The result is a graph of the gamma frequency
(counts) as a function of gamma-ray energy (channel
number). Figure 1.1 is a calibrated spectrum. The
photopeaks are marked; other structures in the data
will be discussed in later experiments.
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Sum
Figure 1.1 Gamma Spectrum of 22 Na source with a 3.8 cm x 2.5 cm NaI(Tl) scintillation detector.
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