Measurement of radiation energies and thickness of 252Cf source backing
Objectives:

– study the spectrum of fission fragments from californium-225,
– study the spectrum of alpha particles from radium-226 passing through the backing of the californium-252 source,
– calculate the energy loss of alpha particles passing through the backing of the californium-252 source,
– calculate the thickness of the backing of the californium-252 source.

Instrumentation:

– Radium-226 source
– Californium-252 source
– PIN-diode detector
– Crate CAMAC
– Intelligent controller
– High voltage power supply (HV)
– Amplifier
– Charge-to-digital converter (QDC)
– Vacuum station
– Vacuum gauge
– Computer
– Cables and accessories
Study the scheme of the experimental setup.
Set the same amplification that you used for the PIN diode calibration in the previous laboratory work.
Compare the energy spectrum of alpha particles from radium-226, obtained in the previous laboratory work, with the energy spectrum of alpha particles from radium-226 passed through the californium-252 source. Alpha particles lose their energy when passing through the backing of the californium source, that is why the second spectrum should be shifted to the left relative to the first spectrum. At the same time in the second spectrum we can distinguish one additive peak which corresponds to the alpha particles from californium-252 with the energy of 6.112 MeV.

To compare peak values, use the blue vertical guideline (move it with the mouse).
Energy spectrum from Ra-226 Energy spectrum from Ra-226 + Cf-252 Set the same scale as for the first spectrum from radium-226 source
Define the centers of gravity of:
– the alpha particle peak from californium-252,
– the alpha particle peak from radium-226 with the highest energy.

To measure the center of gravity of a peak, select it using the bounding dashed lines and read the data from the cell “Mean value” (write down the value with an accuracy of 3 decimal places).
Mean value 125.8 Name Mean value, channel Alpha particles of californium-252 Alpha particles of radium-226 with the highest energy
Determine the centers of gravity of the peaks of fission fragments of californium-252 (write down the value with an accuracy of 3 decimal places). For better peak separation, it is recommended to select a linear scale of the vertical axis (y).

Keep in mind that light fragments have more kinetic energy. This follows from the relation arising from the law of conservation of momentum:

ЕLF · mLF = ЕHF · mHF ,

where
ЕLF — kinetic energy of a light fission fragment,
ЕHF — kinetic energy of a heavy fission fragment,
mLF — mass of a light fission fragment,
mHF — mass of a heavy fission fragment.
Mean value 125.8 Name Mean value, channel Light fragment (LF) Heavy fragment (HF)
Convert the obtained mean values of the centers of gravity into megaelectron-volts (round values to 3 decimal places) by using the calibration dependence obtained in the previous laboratory work:

Energy = 0.35288 + 0.02682·Channel number .

Name Mean value, channel Energy, MeV Alpha particles of californium-252 Alpha particles of radium-226 with the highest energy Light fragment (LF) Heavy fragment (HF)
Calculate the difference between experimental and reference values of energy of fission fragments of californium-252 (round values to 3 decimal places).
Name Experimental energy , MeV Reference value of energy, MeV Difference, MeV Light fragment (LF) Heavy fragment (HF) 102.540 78.680
The difference between experimental and reference values of fission fragment energy doesn’t indicate a mistake. In the process of detection of fission fragments (heavy ions) by semiconductor detectors (in our case a PIN diode) we should take into account the “amplitude defect”. The amplitude defect is associated with three components:

– energy losses in the input window and the dead layer of the silicon detector;
– non-ionizing energy losses during elastic collisions of a moving heavy ion with atoms of the detector substance;
– loss of part of the electron-hole pairs generated by the ion as a result of their recombination in the plasma column of the track.

Today parametrization of this phenomenon is the topical problem of nuclear physics.
Calculate the difference between the reference value of energy of alpha particles with the highest energy from radium-226 and the calculated energy of the same alpha particles that passed through the californium source in our experiment.
Name Experimental energy , MeV Reference value of energy, MeV Difference, MeV Alpha-particles of radium-226 with the highest energy 7.687 The difference you have found corresponds to the energy that alpha particles from radium lose passing through the californium source. Attention!!! Remember this value. You will need it for your following calculations.
After estimation of the energy loss of alpha particles of radium in the process of passing through the californium source, we can define a thickness of this source. The californium source consists of a thin backing of aluminum oxide (Al2O3) with a drop of radioactive californium. The source backing is much thicker than the layer of the radioactive californium. That is why the energy losses in the layer of the radioactive californium can be neglected.

Define the thickness of the source tacking into account that energy losses occur only in the aluminum oxide backing. Below there is a dependence of alpha particle energy losses from thickness of aluminum oxide layer obtained by using the SRIM program (The Stopping and Range of Ions in Matter).
The thickness of the 252 Cf-source is                    nm.