Alpha-particle spectroscopy
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Alpha spectrometry (also known as alpha(-particle) spectroscopy) is the quantitative study of the energy of
alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
emitted by a radioactive nuclide that is an
alpha emitter Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...
. As emitted alpha particles are mono-energetic (i.e. not emitted with a spectrum of energies, such as
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
) with energies often distinct to the decay they can be used to identify which radionuclide they originated from.


Experimental methods


Counting with a source deposited onto a metal disk

It is common to place a drop of the test solution on a
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample. If the thickness of the layer formed on the disk is too thick then the lines of the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...
are broadened to lower energies. This is because some of the
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
of the
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
s is lost during their movement through the layer of active material.


Liquid scintillation

An alternative method is to use liquid scintillation counting (LSC), where the sample is directly mixed with a
scintillation cocktail Liquid scintillation counting is the measurement of radioactive activity of a sample material which uses the technique of mixing the active material with a liquid scintillator (e.g. zinc sulfide), and counting the resultant photon emissions. The pu ...
. When the individual
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
emission events are counted, the LSC instrument records the amount of light energy per
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
event. The alpha spectra obtained by liquid scintillation counting are broaden because of the two main intrinsic limitations of the LSC method: (1) because the random quenching reduces the number of
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s emitted per radioactive decay, and (2) because the emitted
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s can be absorbed by cloudy or coloured samples ( Lambert-Beer law). The liquid scintillation spectra are subject to Gaussian broadening, rather than to the distortion caused by the absorption of alpha-particles by the sample when the layer of active material deposited onto a disk is too thick.


Alpha spectra

From left to right the peaks are due to 209Po, 239Pu, 210Po and 241Am. The fact that
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s such as 239Pu and 241Am have more than one alpha line indicates that the (daughter)
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
can be in different discrete
energy level A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The t ...
s. Calibration: MCA does not work on energy, it works on voltage. To relate the energy to voltage one must calibrate the detection system. Here different alpha emitting sources of known energy were placed under the detector and the full energy peak is recorded. Measurement of thickness of thin foils: Energies of alpha particles from radioactive sources are measured before and after passing through the thin films. By measuring difference and using SRIM we can measure the thickness of thin foils.


Kinematics of alpha decay

The decay energy, Q (also called the ''Q-value of the reaction''), corresponds to a disappearance of mass. For the alpha decay nuclear reaction: ^_P -> ^_D + \alpha , (where P is the parent nuclide and D the daughter). Q = (m_P - m_D - m_\alpha) \ c^2, or to put in the more commonly used units: ''Q'' (M eV) = -931.5 Δ''M'' ( Da), (where Δ''M = ΣMproducts - ΣMreactants''). When the daughter nuclide and alpha particle formed are in their ground states (common for alpha decay), the total decay energy is divided between the two in kinetic energy (T): Q_\alpha = T_\alpha + T_D The size of T is dependent on the ratio of masses of the products and due to the conservation of momentum (the parent's momentum = 0 at the moment of decay) this can be calculated: p_\alpha + p_D = 0 T = 0.5mv^2 and p = mv, \therefore p = \sqrt \begin \sqrt &= -\sqrt \\ pt2m_\alpha T_\alpha &= 2m_D T_D \\ pt\fracT_\alpha &= T_D \end \begin Q_\alpha &= T_\alpha + \fracT_\alpha \\ pt&= T_\alpha\bigg(1 + \frac\bigg) \\ pt&= T_\alpha\bigg(\frac+\frac\bigg) \\ pt&= T_\alpha\bigg(\frac\bigg) \\ pt\end \therefore T_\alpha = \fracQ_\alpha The alpha particle, or 4He nucleus, is an especially strongly bound particle. This combined with the fact that the binding energy per nucleon has a maximum value near A=56 and systematically decreases for heavier nuclei, creates the situation that nuclei with A>150 have positive Qα-values for the emission of alpha particles. For example, one of the heaviest naturally occurring isotopes, ^238U -> ^234Th + ^4He (ignoring charges): : Qα = -931.5 (234.043 601 + 4.002 603 254 13 - 238.050 788 2) = 4.2699 MeV Note that the decay energy will be divided between the alpha-particle and the heavy recoiling daughter so that the kinetic energy of the alpha particle (Tα) will be slightly less: Tα = (234.043 601 / 238.050 788 2) 4.2699 = 4.198 MeV, (note this is for the 238gU to 238gTh reaction, which in this case has the
branching ratio In particle physics and nuclear physics, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to eithe ...
of 79%). The kinetic energy of the recoiling 234Th daughter nucleus is TD = (mα / mP) Qα = (4.002 603 254 13 / 238.050 788 2) 4.2699 = 0.0718 MeV or 71.8 keV, which whilst much smaller is still substantially bigger than that of chemical bonds (<10 eV) meaning the daughter nuclide will break away from whatever chemical environment the parent had been in. The recoil energy is also the reason that alpha spectrometers, whilst run under reduced pressure, are not operated at too low a pressure so that the air helps stop the recoiling daughter from moving completely out of the original alpha-source and cause serious contamination problems if the daughters are themselves radioactive. The Qα‐values generally increase with increasing atomic number but the variation in the mass surface due to shell effects can overwhelm the systematic increase. The sharp peaks near A = 214 are due to the effects of the N = 126 shell.


References

{{DEFAULTSORT:Alpha-Particle Spectroscopy Spectroscopy