Laboratory work 2.3
Spectrum registration of scintillation detector with the NaI crystal
Objectives:

– to study configuration and principle of operation of the
scintillation detector with the NaI crystal,
– to get and study the amplitude spectrum from the scintillation
detector.

Instrumentation:

– Gamma source
– Scintillation detector with the NaI crystal
– USB oscilloscope
– Amplitude-to-digital converter (ADC)
– Personal computer (PC)
– Cables and accessories
Study the scheme of the experimental setup.
The scintillation gamma detector consists of:

– an inorganic scintillator (NaI crystal), in which ionizing particles cause a burst of luminescence;
– a photomultiplier tube (PMT), that converts a light flash into the pulse of electric current;
– a resistor divider for the PMT, that allows to supply the proper operating high voltage distribution to the PMT;
– an amplifier, that amplifies the electric pulse from the PMT.
Study the interface of the USB oscilloscope. The vertical scale shows the amplitude of a signal in millivolts (1 mV = 10-3 V). The horizontal scale shows the time in microseconds (1μs = 10-6 s) and nanoseconds (1ns = 10-9 s). Using the corresponding controls you can change the vertical or horizontal scale. The trigger level sets an internal voltage which is compared to the voltage of the input signal. When the input signal voltage equals the trigger voltage, the scope triggers. If you get an image that seems to be a superposition of many waves, turn the level knob back and forth slowly until you get a stable image.
Connect the USB oscilloscope with the scintillation detector and with the personal computer. Using the oscilloscope interface settings find the signal from the detector.
Now let’s change the USB oscilloscope to the amplitude-to-digital converter (ADC). The ADC converts the input analogue signal into the discrete code (digital signal). Study the scheme of the experimental setup.
Connect the ADC with the scintillation detector and with the personal computer. Study how the picture from the oscilloscope corresponds with the amplitude spectrum collection.
There is the spectrum from the gamma source. Find centers of gravity of 3 designated peaks. Move the blue and the red cursors to limit the area of the peak you wish to study and see «Mean value». Fill the table with calculated values (round values to 1 decimal place).
Mean value 125.8 514 keV 898 keV 1836 keV Mean value, channel Energy, keV 514 898 1836
Build the graph of the obtained dependency.
The obtained slope and intercept are the coefficients of the linear calibration of the energy scale of the PIN diode. Mean value, channel Energy, keV 514 898 1836
Let’s remove the gamma source from our setup and see the spectrum from the scintillation detector. We see that we still have one peak in our spectrum. We had the same peak in the spectrum obtained from the removed gamma source. It means that our detector has some impurity that also emits gamma quanta.
Using the obtained calibration formula and the table with energies of gamma emitters define the origin of the impurity (choose the gamma emitter in the table).
Recall that:
Mean value 125.8 Gamma emitter Energy, keV Bismuth-205 (Bi-205) 1 764 Lanthanum-140 (La-140) 1 596 Potassium-40 (K-40) 1 460 Cobalt-60 (Co-60) 1 333 Calcium-47 (Ca-47) 1 297 Sodium-22 (Na-22) 1 275 Something wrong in your calculations! Right answer: potassium-40.