Radionuclidic purity control
Radionuclidic purity is the ration, expressed as a percentage, of the radioactivity of the
radionuclide concerned to the total radioactivity of the radiopharmaceutical preparation.
The relevent radionuclidic impurities are listed with their limits in the individual
monographs.
During the radionuclidic purity control, the ratio of the total radioactivity of the
radiopharmaceutical preparation corresponding to the concerned radionuclide is
determined and also the presence of undesirable radionuclide is detected according to
their half-life, type of the decay and energy.
In most cases, to state the radionuclidic purity of a radiopharmaceutical
preparation, the identity of every radionuclide present and their radioactivity must be
known.
The most generally useful method for examination of radionuclidic purity is that
of gamma spectrometry. Gamma and RTG spectrum of a radionuclide is characteristic for
each radionuclide. It is characterised by the energies and by number of photons of certain
energies emitted during the transition from one energetic level to another. In praxis, the
spectrum of radiopharmaceutical preparation is compared with spectrum of a
standardised preparation. The individual monographs prescribe the radionuclidic purity
required and may set limits for specific radionuclidic impurities (for example cobalt-60 in
cobalt-57).
The intensity of emitted energy (height of photopeak) in the spectrum of
radiopharmaceutical preparation decreases with the half-life of the radionuclide. If there
is a presence of radionuclidic impurity in the radiopharmaceutical preparation –
radionuclide with different half-life, it is possible to detect it: by identification of the
characteristic peak or peaks, whose heights decrease by different velocity than is
expected from required radionuclide in the radiopharmaceutical preparation.
Determination of half-life of these extra peaks by repeated measurements of the sample
can help in identification of impurities.
Identification of beta emitter is base either on measurement of the spectrum or on
measurement of absorption of beta radiation in metal filters (e.g. aluminium) with various
thickness – attenuation curve.
Radionuclidic purity in the radiopharmaceutical preparation with short half-life is
possible to be determined by the measurement of dependecy of radiopharmaceutical
preparation´s activity on time (radionuclides have different half-lives). The radionuclidic
purity is determined when the activity of concerned radionuclide decreases to the
minimum and the radionuclidic impurities with long half-lives are identified.
Determination of the nature and energy of the radiation
Identification of radionuclides by measuring their gamma
spectrum (γ-spectrometry)
Gamma spectrometry is a radiochemistry measurement method that determines
the energy and count rate of gamma rays emitted by radioactive substances. A detailed
analysis of the gamma ray energy spectrum is used to determine the identity and quantity
of gamma emitters present in a material. Measuring γ-spectrum of unknown radionuclide
is used to identify radionuclides by the energy and intensity of their gamma rays and Xrays. In case of mixed radiation of various radionuclides, measurement of spectrum
enables the determination of energy and activity of individual parts of mixed gamma
radiation spectrum.
The equipment used in gamma spectroscopy includes a scintillation detector and
amplitude gamma spectrometer. Scintillation detector is composed of the scintillator
(usually NaI/Tl crystal), photomultiplier tube and pre-amplifier. Amplitude spectrometer
is composed of amplifier, one or multichannel analyzer and counter of impulses counts or
other data readout devices.
Scintillation spectroscopy is based on the dependence between the intensity of
scintillation in scintillator and the energy of the particle, which created this scintillation.
The scintillation is lineary amplified by the photomultiplier tube, so the height or size of
the impulse leaving the photomultiplier corresponds to energy of the incoming particle.
The γ-spectrum consist of all interaction types of γ-quanta with scintillator
(photoelectric effect, Compton scattering, pair-production). During the photoelectric
effect, photo looses all its energy in the scintillator and that results in scintillation
proportional to the the enregy of the photon. There is a appearance of expressive
maximum on the spectrum line called photopeak (with an ideal radiation detector, this
would produce a single narrow line) with the shape of gaussian curvature. The location
and the size of the peak inform about energy and intensity of photons. Photopeaks
dominate the γ-spectrum. In Compton scattering, only a part of the γ-ray energy is
transferred to the electron. Compton electron is stopped in the scintillator and scattered γradiation can be absorbed by photoelectric effect or leave the crystal. In the first case, all
energy of photon is transferred to an electron in the photoelectric effect. If there is not
another interaction, the spectrum is fomer only by Compton electrons, which energy
varies from 0 to maximum value: Ec = 4 E2/ 4 E + 1 where Ec is the energy of Compton
electrons, A is the energy of original γ-radiation.
The result is the wide Compton continuum in γ-spectrum, energetically lower than
photopeak. Compton continuum ends near the photopeak by Compton edge.
The electron pair production which occurs with E > 1.02 MeV, can participate in
the formation of γ-spectrum in 2 ways: with energy of created photon and with energy of
photoelectron created from photon. This is the formation of electron pairs peak.
N
Photopeak
N – number of counts
E - energy
E (eV)
Material:
Scintillation detector, single channel analyzer, standard 241Am, E = 60 keV, sample.
Method:
Switch on the measuring devices. Set the working voltage of scintillation detector and set
the width of measured energetic interval (99 channels).
Put the standard under the detector an measure the counts per 10 second on channel
number 1. Switch the channel with regulator and measure counts (from 0 – 99). Put the
results into the table.
Results:
Make a graph (x ax – number of channel, y ax - counts rate). In the spectrum of standard,
look for the maximum counts that belong to photopeak. Its energy is in the tables (for
241
Am it is 60 keV). This energy is in the spectrum is given by the number of channel.
The sample:
Measure the counts of the sample. Find the channel corresponding to highest counts
(maximum of energy). Calculate the energy of the sample radionuclide. Identify this
radionuclide using tables.
Table
Number
channel
1
2
3
4
5…..
of Counts rate (imp./10s) - Number
standard
channel
of Counts rate (imp./10s) standard
Determination of the nature and energy of the radiation
The nature and anergy of the radiation emitted may be determined by several procedures
including the construction of an attenuation curve and the use of spectrometry. The
atenuation curve and beta spectrometry can be used for identification of radionuclide
beta; gamma spectrometry is used for identification of radionuclides emitting gamma and
X-rays.
Identification of radionuclide by the construction of an
attenuation curve (beta absorption in aluminium)
Radinuclide bta emits polyenergetic beta radiation. This radiation contains the
electrons with different velocities (energies). During the transition of beta particles
through the metal filter, part of the total electron flow is absorbed and part is slowed
down but pass through. With increasing thickness of the absorbing metal layer, the
number of unabsorbed particles decreases (faster concenring slow particles, slower
concerning fast particles).
Attenuation curve is drawn for pure electron emitters when no spectrometer for
beta rays is available or for beta/gamma emitters when no spectrometer for gamma rays is
available. This method of estimating the maximum energy of beta radiation gives only an
approximate value. The source, suitably mounted to give constant geometrical conditions,
is placed in front of the thin window of a Geiger-Müller counter or a proportional
counter. The count rate of the source is then measured. Between the source and the
counter are placed, in succession, at least six aluminium screeens of increasing mass per
unit area within such limits that with a pure beta emitter this count rate is not affected by
the addition of further screens. The screens are inserted in such a manner that constant
geometrical conditions are maintained. A graph is drawn showing, as the abscissa, the
mass per unit area of the screen expressed in milligrams per square centimetre and, as the
ordinate, the logarithm of the count rate for each screen examined. A graph is drawn in
the same manner for a standardised preparation. The mass attenuation coefficinets are
calculated from the median parts of the curves, which are practically rectilinear.
The mass attenuation coefficient µm, expressed in square centimetres per
milligram, depends on the nergy spectrum of the beta radiation and on the nature and the
physical properties of the screen. It therefore allows beta emitters to be identified. It is
calculated using the equation:
µm = lnA1 – lnA2
m2 – m1
m1 = mass per unit area of the lightest screen
m2 = mass per unit area of the heaviest screen, m 1 and m2 being within rectilinear
part of the curve
A1 = count rate for mass per unit area m1
A2 = count rate for mass per unit area m2
The mass attenuation coefficient thus calculated does not differ by more than 10
% from the coefficient obtained under identical conditions using a standardised
preparation of the same radionuclide.
The range of beta particles is a further parameter which ca be used for the
determination of the bta energy. It is obtained from the graph described above as the mass
per unit area corresponding to the intersection of the extrapolations of the descending
rectilinear part of the attenuation curve and the horizontal line of the background
radioactivity.
Material:
Aluminium screens with different thickness,GM-counter, single-channel counting device,
pincers, holder for the screens, lead shield, standards, sample.
Method:
Between the GM-counter and a sample place, in succession, at least 10 aluminium
screeens of increasing mass per unit area. Measure counts per 60 s. In the same way,
measure the count rate for the standards. Put the results into the table.
Results:
Draw a graph of an attenuation curve: as the abscissa - the mass per unit area of the
screen expressed in milligrams per square centimetre and, as the ordinate - the logarithm
of the count rate for each screen examined. Calculate the mass attenuation coefficient µm.
Identify the sample.
Table 1: Standard No.1
Thickness of the Count
rate Count
rate Count
screen (mg/cm2) (imp./min)
(imp./min)
(imp./min)
No.1
No.2
average - x
1
2...
Table 2: Standard No.2
Thickness of the Count
rate Count
rate Count
2
screen (mg/cm ) (imp./min)
(imp./min)
(imp./min)
No.1
No.2
average - x
1
2...
Table 3: Sample
Thickness of the Count
rate Count
rate Count
2
screen (mg/cm ) (imp./min)
(imp./min)
(imp./min)
No.1
No.2
average - x
1
2...
rate logarithm of an
average (lnx)
rate logarithm of an
average (lnx)
rate logarithm of an
average (lnx)
Identification of radionuclides by measuring beta spectrum
The beta decays are the transformations of the nucleus during which occurs the
mutual transformations of nucleons and emission of the particles. They are divided in
emission of the negatrons (negatively charged electrons β-), emission of positrons,
(positively charged electrons β+) and electron capture. During the negatron and positron
transformation, accompanied by emission of a negatron and positron, the constant
amount of energy is released and carried away with particles emitted from the nucleus in
form of their kinetic energy. Along with β- and β+ particles, antineutrino and neutrino are
emitted from the nucleus.
Spectrum of emitted beta radiation is polyenergetic, continuous. A particle βcarries away only a part of the energy released during decay (difference between energy
of parent and daughter nucleus), resting energy is carried by antineutrino. The sum of
energies of these two particles emitted during one decay, is called maximal energy Emax.
Emax = E + E
Maximal energy is characteristic for the decay mode of radionuclide - identify the
radionuclide. In determining this energy, it is possible to to identify the radionuclide or to
determine the purity of the radionuclide.
Spectrum of emitted negatrons energies is continuous and there are the negatrons
with energy from zero energy to the maximum energy. Number of negatrons with
minimum (zero) energy and maximum energy approaches zero. Average energy of
negatrons is called mean beta radiation energy and it is equal approximately to one-third
of the maximum energy.

E = Emax/3
It is possible do determine the maximum energy and the spectrum with
spectrometer.
Devices: Scintillation detector, single-channel analyzer, standards, sample
Method: Switch on the measuring technique, set the high voltage to the working
voltage of scintillation detector. Set the channel 1. Place the standard under the detector
and measure the counts per time 10 s. Change channels with the regulator and measure
the counts. Write the results (channel and counts) in the table. Measure the sample.
Results: Use the registered results in table to make a graph (ax x – number of
channel, ax y counts rate). Find the channels corresponding to maximum energy (the
counts are minimal). Compare these spectrums. Calculate the maximal energy of samples
(thanks to the maximum energy of standard) and use it for identifying the radionuclide.
Number of channel
1
2
3
…
Counts rate (imp/10 s)
standard
sample