CINCH NucWik

Developed by
Department of Chemistry
Radiochemistry
University of Helsinki

Students will learn how to perform alpha spectroscopic measurements, how to process alpha measurement data, execute energy calibration and resolution determination by measuring standard samples. Finally, the students will know via demonstration, how the medium affects on the path of an alpha particle from the sample to the detector, and further to the quality of the alpha spectrum.

In an α decay heavy nucleus decreases its mass and total energy by emitting an α particle, which has a certain kinetic energy. α particle is the nucleus of the He atom and its mass number is A = 4 and charge q = +2. Due to its high electric charge α particle interacts strongly with the electrons in the surrounding material. As an α particle passes through a medium it loses its energy constantly, at a rate depending on the energy of the α particle and the characteristics of the medium. Owing to the high mass of α particle, the electrons interacting with the α particle do not have an impact on the direction of the particle, from which follows that the track of the α particle is straight until it stops.

The strength of the alpha particle's interaction is well described by its range in different materials. The range depends on the energy of the alpha particle and the surrounding material. For example, the range of a 5 MeV alpha particle in air is about 3.6 cm, in water and human tissue ~35 μm, and in the detector material silicon ~24 µm. When measuring alpha radiation, the short range has both advantages and disadvantages. All alpha particles that hit the detector stop even in a thin detector and give up their energy completely, so that the energy of the alpha particle can be measured perfectly and the measurement efficiency is good. On the other hand, the sample to be measured must be almost massless and there must be no layers of material between the sample and the detector, in which the energy of the alpha radiation could be partially or completely absorbed before reaching the detector.

Resolution, as FWHM (Full Width at Half Maximum, see the next figure) of an alpha peak, is used for describing the performance of the detector. Resolution is typically expressed as energy unit keV. The smaller the keV value is, the better the alpha peaks are resolved from each other in the spectrum. Commonly the alpha peak of ²⁴¹Am at 5486 keV is used in reporting resolution of the detectors. Detector manufacturers give a nominal resolution for their products and the nominal resolution of a PIPS (passivated implanted planar silicon) detector, the type of which is used in this exercise, is typically about 20 keV, determined for FWHM of ²⁴¹Am alpha peak.

Definition of FWHM. σ denotes here a standard deviation.

In this work, we will measure samples containing alpha emitters with a silicon semiconductor detector. We will observe the effect of media on the measured alpha spectrum.

Measurement system
The alpha measurement system is presented schematically in the next figure. The detector and the sample have been located inside a chamber, where air can be pumped off for obtaining a vacuum. A connector goes through the ceiling of the vacuum chamber. High voltage goes through this connector to the detector. Electric pulses, caused by alpha particles, go to the opposite direction in the connector, from the detector to a pre-amplifier. High voltage is transferred to the detector via a pre-amplifier unit. Electric pulses from the detector are converted to voltage pulses in the pre-amplifier and they are amplified to strong enough for transportation further in the system. Voltage pulse goes from the pre-amplifier to a linear amplifier, where the pulse is further amplified and modified to shorter. From the linear amplifier, the pulse continues to an analogy-digital converter (ADC) that changes the pulse height information from voltage value to a binary number series. This number series corresponds some channel number in a multichannel analyzer. After a pulse arrives, the content of this channel is increased by one. The multichannel analyzer is connected to a computer that includes a spectral analysis program, which can read the content of the analyzer, meaning that how many pulses have been observed in each channel. Alpha measurement setup. Open arrows mark the transfer of electric pulse, caused by radiation, in the system, while filled arrows indicate routes of working voltages (solid line – high voltage of the detector, dashed line – working voltage of the pre-amplifier).

The basic principle of an alpha spectrometer is uniform. However, besides the setup presented in this exercise instruction, where all components are separate and voltages and vacuum are adjusted manually, there are other type of alpha spectrometer setups, where, e.g., the vacuum and voltages are controlled automatically by the computer program, and all components except the oil pump are placed inside a single computer controlled box. It is fine to use other types of alpha spectrometers as well in this exercise, because the main point to be demonstrated, interaction of alpha particles with matter, will be the same, regardless of the used alpha spectrometer setup.

Energy calibration
The content of the multichannel analyzer is the measured alpha spectrum. It is a chart where x axis represents channel of an analyzer and y axis number of observed pulses in a channel. In order to get useful information about this chart and finding out the alpha decay energies of the peaks in the spectrum, an energy calibration has to be performed first. In energy calibration, a relation between each channel and energy value is formed. This can be done in practice by measuring a (standard) sample with known radionuclides and alpha decay energies.

Exploring the interaction of an alpha particle with media

Interaction of an alpha particle with media is investigated by measuring first the spectrum of an alpha sample in optimal conditions. The sample is almost massless and the measurement is performed in a vacuum chamber. After this, a mylar film with thickness of 5 µm is added between the sample and the detector. The sample measurement is repeated with the film and from the measured alpha spectra, effects from the film on the spectra can be evaluated.

Performing the alpha measurements

• in the beginning of the exercise, get familiar with the components of the measurement system (see the previous figure) and its connection to an oil pump
• in case that the used alpha spectrometer system deviates from the scheme presented in the previous figure, then the following measurement instructions will be applied in a corresponding way
• write down the technical information of the used detector: type, window thickness, surface area, nominal resolution and working voltage
• before starting measurement, check that the high voltage of the detector is off and that pumping valve from the measurement chamber (containing detector) to the vacuum line (oil pump) is closed
• open the venting valve of the measurement chamber, so that there is a normal air pressure inside the chamber
• open the measurement chamber and, by using tweezers, set the standard sample containing three alpha emitters, onto the sample rack. Avoid touching the centre parts of the sample, where the radionuclides exist
• close the venting valve and open the pumping valve. Pump a vacuum of 1 ∙ 10^-1 mbar or less into the measurement chamber
• close the pumping valve and raise the high voltage of the detector slowly to suitable working voltage value, typical for the detector used, e.g., +40 V
• start the measurement with spectral analysis program, the measurement is continued as long as there is preferably about 10 000 pulses in each of three peaks
• stop the measurement, write down the measurement time and save the spectrum
• set the high voltage off and vent the measurement chamber
• remove the standard sample from the measurement rack with tweezers
• next, an alpha sample of good quality (for example, ²⁴¹Am electroplated on a metal disc, or some other thin alpha source) will be measured, in a similar way that the standard sample. Write down the measurement time and save the spectrum
• finally, place a mylar foil with thickness of few μm onto the sample in the measurement rack and repeat the measurement. Write down the measurement time and save the spectrum
• set the high voltage off and vent the measurement chamber
• remove the sample and mylar foil from the measurement rack with tweezers

Processing of measurement results

Energy calibration
Open the spectrum of the calibration sample and draw a chart, e.g., with Excel, Origin or other suitable program. Track the peak top channels, i.e., the channel that contains the highest number of counts in each peak. When all the peak top channels are known, the energy calibration line can be plotted, having the top channel of the peak in x axis and literature value for the peak alpha energy in y axis. Fit a line through the three data points and the line equation will be used for energy calibration in the next phase, for the measured alpha spectrum of an alpha sample. Remember to save the energy calibration spectrum and the plot fitting chart with the calibration equation.

Analysis of the measured sample spectra
Open the spectrum measured without the foil. Search the top channel of the peak and the amount of pulses in that channel. Determine the full width at half maximum (FWHM) as channels. Use the energy calibration equation for calculating the central channel and FWHM in energy unit. After this, determine the peak area and its uncertainty.

After this, open the spectrum measured with the mylar foil. Perform the same analysis as with the first sample spectrum (central channel of the peak, FWHM and peak area with uncertainty).

Drawing graphs for the work report: At least four graphs are needed for the work report, they are, measured energy calibration spectrum, determination of energy calibration line with line equation, and alpha spectra of the sample with and without a foil. Pay attention to channel area and scaling of the sample spectrum. The channel range (x axis) should be the same for both spectra and axis scaling should be optimised for good visibility of the alpha peaks.

• what was energy and resolution of the investigated peak in the sample spectrum measured without a foil? Is there a difference between this experimentally determined resolution to the nominal resolution of the detector? If yes, what could be the reason(s) for this difference?
• what happened to the investigated peak in the alpha spectra when a mylar foil was set between the sample and the detector?
• from the literature [1] we know, that the energy loss of an alpha particle in mylar foil with alpha energy of ²⁴¹Am (5.49 MeV) is about 115 keV/µm. (For other alpha emitters, see the database [1] and find corresponding energy loss values.) Does this value give similar result with your experimentally determined result?
• consider factors causing the change is resolution
• compare the amount of observed alpha particles per unit of time (taking into account the uncertainties of the measurement) with and without the mylar foil on the sample. Find reasons for change/stability.
• expand this result: what would happen to the peak in spectrum (or to energy of an alpha particle) if the sample would not be “massless”. Imagine the sample being prepared as a mixture of radionuclide and foil material, so that part of the alpha emitting particles would exit the sample from the sample surface, part of the particles would exit from the bottom of the sample and rest of the particles from between these levels.
• continuing from the previous question, think what kind of spectrum would be obtained, if the sample would be thicker than the range of the exiting alpha particle (still assuming that the alpha emitters would be evenly distributed in the sample)

[1] www.srim.org – The Stopping and Range of Ions in Matter – computer program and database.

Report of the exercise should include the following sections and attachments

• a brief introduction defining alpha radiation and its interaction with the surrounding media
• the purpose of the performed exercise
• description of the used instrumental setup
• information about the used radioactive sources (radionuclides, energies)
• if relevant, presentation of mathematical formulas used in the work
• describe the executed measurements and present the obtained results in a logical order
• answers to the ”Questions for Students”
• previously mentioned at least four graphs about the results

Alpha (and gamma) radiation from calibration source and the sample will not cause significant radiation exposure when they are handled appropriately, i.e. with tweezers. Otherwise, there are no specific safety concerns regarding the exercise.

• alpha spectrometer with an oil pump, apply the previous instructions according to the available setup type

• tweezers
• piece of mylar foil

• a calibration source containing three alpha emitting radionuclides
• a sample containing e.g. ²⁴¹Am or other alpha emitter having activity level high enough (resulting a peak area of thousands of counts in a measurement of 10-60 minutes)

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