Nuclear Experiments - Gamma-Ray Detection and Coincidence Measurements
In this experiment high energy photons - gamma-rays - are detected using
scintillation detectors read out by photomultipliers. The gamma-ray energy
spectra of various radioactive sources is measured and the linearity of
the detection apparatus is determined. The absolute activity of the source
is found. The use of coincidence timing is explored in an experiment employing
two scintillation detectors to record the simultaneous occurrence of two
gammas from the decay of positronium.
We follow the procedure in ORTEC Application Note 34: "Experiments
in Nuclear Science." The required experiments are:
Exp. 1 Basic Identifications in Electronic Measurement Systems
Exp. 3 Gamma-Ray Spectroscopy Using NaI(T1)
1.1 Observing the direct and attenuated outputs of the pulser
1.2 Using the pulser as the linear input to a typical counting system
1.3 Using a single-channel analyzer
Note: Draw the signals observed on the oscilloscope at: the output of the
pulser, the output of the preamplifier, and the output of the amplifier.
Exp. 9 Time Coincidence Techniques and Absolute Activity Measurements
3.1 Energy calibration
Note: Use 22Na as well as 60Co and 137Cs
for the energy calibration
3.2 Energy analysis of an unknown gamma source
3.3 Spectrum analysis of 60Co and 137Cs
3.4 Energy resolution
3.6 Activity of a gamma emitter (absolute method)
3.7 Mass absorption coefficient
Exp. 13 Gamma-Gamma Coincidence
9.1 Simple fast coincidence experiment
It is strongly urged that, after completing an experiment and before carrying
on to the next experiment, the student perform a rudimentary analysis in
order to insure that the data is good.
13.1 Overlap coincidence method for measuring gamma-gamma coincidence of
13.2 Linear gate method for measuring gamma-gamma coincidence of 22Na
Each of the four experiments should have a separate write-up. Keep them
short and to the point. Each report should include:
Do not include a detailed description of the experimental set-up, but rather
a simple drawing of the equipment. In experiment 1 the student should observe
the signal after each stage of amplification and pulse shaping and draw
its shape, height (in volts), and length (in seconds).
the raw data (photocopied from your notebook, if necessary
plots of the gamma ray spectra recorded by the multichannel analyzer
any plots you are asked to make (use graph paper)
answers to the exercises
It is important for the student to do an error analysis with every measured
number having an associated estimated error. There are many good texts
which explain how to treat errors.
A simple introductory text is:
A more advanced text (which includes FORTRAN programs) is:
Hugh D. Young, "Statistical Treatment of Experimental Data".
Nuclear and particle physics data almost invariably follow a Poisson probability
distribution which is the Binomial distribution in the case where the number
of events N is large and the probability p of any one event, small. The
Poisson distribution has the particularly appealing property that the error
in the mean (a = Np) is simply the square root of a.
Philip R. Bevington, "Data Reduction and Error Analysis for the Physical
Besides the ORTEC application note 34 and the various manuals describing
the equipment used, we recommend the following books which can be found
on the reserve shelf.
W.R. Leo, "Techniques for Nuclear and Particle Physics Experiments"
This recent, well-written book fills a long-standing need for a practical
introduction to the techniques of experimental nuclear and particle physics.
It does so admirably.
Glenn F. Knoll, "Radiation Detection and Measurement".
Another excellent text, albeit more advanced than Leo.
A. Melissinos, "Experiments in Modern Physics"
A classic text, although dated. Skip over the description of the electronics
- the remainder is well done.
Nuclear Decay Processes
Nuclei can undergo a variety of processes which result in the emission
of radiation. The three most important of these processes are: alpha decay,
beta decay, and gamma emission. We briefly describe these processes below.
The probability that a nucleus will emit radiation is random and depends
on the number of nuclei. Hence the mean number of decays as a function
of time is given by the exponential:
Alpha decay: This is the emission of an alpha particle (Helium nucleus)
from the nucleus: (A,A)-> (Z-2,A-4) + a The
emitted alpha particles are monoenergetic, their energy in the range of
a few MeV. The alpha interacts strongly and has a very short range - a
few cm in air.
Beta decay: This is the decay of a neutron into a proton and electron
and a neutrino: n -> p + e- + n.
The proton does not have enough energy to escape the nucleous but both
the electron and neutrino do. The electron has a continuous energy spectrum
because of the kinematics of a three body decay. A related process is the
emission of a positron: p -> n + e+ + n.
Neither the electron or the positron is very penetrating.
Gamma emission: The nucleus has discrete energy levels, like those
of the electrons in an atom. The nuclear force, however, is much stronger
than the electromagnetic and hence transitions from one state to the other
are characterized by the emission of photons of much larger energy - from
a hundred keV to a few MeV. Such photons are called gamma-rays and are
N(t) = N(0) e-t/t
where N(0) is the number of nuclei at t=0. The number t
is known as the mean lifetime - the time it takes the sample to
decay to 1/e of its initial activity. The half-life is the time it takes
the sample to decay to one-half of its initial activity.
Handling Radioactive Sources
The activity of a source is the number of decays which can occur in
a given time. It is usually measured in Curies which are defined as:
1 Curie = 3.7 x 1010 disintegrations/s
This is a very large unit (originally defined as the activity of one
gram of Radium). One usually deals with sources which have activities on
the order of a microCurie (µCi).
The sources we use in this experiment have low activity. For example,
a 100 µCi 22Na source produces an exposure rate of 4.47
mrem/hr. For comparison, one x-ray produces a dose of 100-200 mrem. A rem
is defined as an energy of 100 erg deposited in one gram of material multiplied
by a quality factor for the type of radiation. For gammas the quality factor
One must be careful, however, with sources at the µCi level that
one does not ingest them. Small amounts of radioactive sources in the body
can be very harmful. The sources we have in this lab are normally sealed
and hence ingestion is improbable. Nevertheless one should always obey
the following two rules when handling radioactive sources:
- Never eat or smoke in the laboratory.
- Wash your hands after handling radioactive material.
A Short ( click for long version
) Description of the Electronics Used in the Experiment
Two distinct measurements are to be made. First, the energy of the gamma-rays
is to be determined; secondly, their time of occurrence (for coincidence
The two measurements are closely related to two types of electronic
pulses you will observe; namely, linear pulses and logic pulses. The first
is defined as those pulses in which the signal amplitude is proportional
to the parameter of interest (energy in our case). The cataloging of such
signals according to amplitude is accomplished by the use of a multichannel
pulse-height analyzer. By contrast, logic signals have a fixed shape and
amplitude, and they convey information by their presence, absence, or relation
of time. In this experiment logic pulses are used to determine time relationships
Preamplifier - The preamp converts the pulse from the photomultiplier
anode - a charge pulse - to a voltage pulse using a variable capacitance.
The rise time of the pulse (important for timing measurements) is dependent
upon the scintillation decay time and on the collection and transit time
characteristics of the photomultiplier tube.
Amplifier - Besides amplifying the pulse this unit shapes the
pulse to obtain either optimum energy resolution of time resolution. Observe
the effect of the different pulse shape controls (differentiation and integration
switches). Note that the decay time of the pulse is much stronger than
after the preamp. This is done to prevent overlap (pile-up) of pulses in
a high count rate experiment.
Single Channel Analyzer - This instrument produces a logic output
pulse indicating the presence of a linear input pulse within the range
determined by the "E" and "E+DE" settings (differential
mode) or merely exceeding the "E" setting (Integral mode). Also, the logic output
pulse bears a definite time relationship to the linear pulse causing it. Thus this
module converts linear signals to logic signals used in the time coincidence
Overlap Coincidence Module - This unit (also called an "and-or
gate") produces a logic output pulse when two or more logic input pulses
"overlap" in time of arrival (coincidence mode, also called "and gate")
or when they do not overlap at all (anticoincidence mode). The coincidence
resolving time of this unit is limited to the sum of the widths of the
Fast Coincidence Module - This unit also performs an overlap
coincidence, but only after inspecting the input pulses to more accurately
determine their time of origin.
Linear Gate - This instrument allows the passage of a linear
signal only with the simultaneous arrival at the unit of a logic signal
(which "opens" the gate). The latter signal is called an enable signal.
Thus, the unit performs a coincidence function, its resolving time being
limited by the gate width (the length of time the gate is open after the
arrival of the enable signal).
Multi-Channel Analyzer - This versatile instrument gives a plot
of the pulse height spectrum of all of the pulses input to it. It takes
each pulse, converts its pulse height into a digital number and increments
the bin count that number falls within.