29:30 Physics IV Experiment 4

Experiment 4 : The NaI(Tl) Scintillation Spectrometer

Purpose:

To investigate some properties of the NaI(Tl) scintillation counter.
To study the use of a single channel analyzer (SCA).
To measure several gamma-ray spectra.
To verify the need for relativistic kinematics to treat the 180° scattering of photons.

Discussion:

In this experiment you will use a single-channel analyzer to measure various gamma-ray spectra as detected by a sodium iodide crystal activated with thallium, or NaI(Tl). Such a crystal detects gamma rays by the conversion of all or part of the photon's energy into the kinetic energy of charged particles. These charged particles in turn lose their energy by exciting atomic electrons in the NaI lattice as they travel through the crystal. The de-excitation of these excited states produces photons in the visible range. These photons then are reflected around from the highly reflective surface which coats the crystal until they impinge on the photocathode of the photo- multiplier tube which is optically coupled to the crystal. At this surface photoelectrons are ejected at the rate of about three photoelectrons per keV of energy of the recoiling charged particles. The photomultiplier multiplies this number by something like 10e+8, the whole process taking about 0.25 µsec. You might verify that this means that a recoiling electron with an energy of 5 MeV will produce an output current pulse of almost 1 amp. Pulses from the photomultiplier are amplified and routed to a single channel analyzer (SCA). The SCA is a precision device that produces an output pulse only if the input pulse-height is in a selected range. The lower limit of this range is set by the "E" dial on the SCA. The range itself is set by the DE or

E dial. Thus output pulses will occur only for input pulses which have amplitudes between E and E +

E. By observing the count rate of output pulses as the E dial is varied, a pulse height spectrum can be obtained.

Procedure:

Connect the photomultiplier tube to the high voltage supply and the preamplifier. Connect the preamp output to the main amplifier input and connect the amplifier output to the SCA. In some setups the preamplifier is not used and the phototube output is connected directly to the amplifier input. Have your setup checked and turn on the equipment. Set the high voltage to 1000 volts. Start with a Cs-137 source which emits a gamma ray with an energy of 662 keV. Locate the photopeak by slowly varying the E dial with the

E dial set at 10%. Adjust the amplifier gain so that the peak occurs at about 8 volts. Then adjust the position of the source so the maximum count rate in the peak is about 1000 counts per second. For the data runs, reduce the

E dial to 2% and count for 10 seconds. Take enough data points (count rate versus E-dial setting) to map out the photopeak (about 10 points), the Compton edge (about 10 points) and 10 to 20 lower points. Then place a scatterer directly behind the source and map out the backscatter peak, which should appear at about 1/3 the E-setting of the photopeak. Remove the scatterer and measure the count rates at the same E-settings so the enhance- ment of the backscatter peak will be evident. Repeat the above using a Na-22 source. This source emits positrons which annihilate with electrons to produce photons with energies of 511 keV.

Report:

Construct graphs of count rate versus pulse height for both sources. For both graphs include data taken with and without the scatterer on the same graph. Identify the various characteristic features of each spectrum. Find the peak positions of the photopeaks of each spectrum and plot these positions versus the corresponding energies. Connect the two points by a straight line, which should go very nearly through the origin. The resulting calibration curve will allow you to determine the energy of any point on the pulse height spectrum. Determine the energy of the backscatter peaks and compare with the value predicted by the Compton formula. (See Eq. 2, Experiment 6). In addition, calculate the photon energy expected for scattering at 180° assuming the classical relation between momentum and energy: E = p²/2m. Are your experimental results accurate enough to definitely prove the necessity of relativistic kinematics for this experiment?