A Primer on Radiochemistry

Michelle Huelskamp

Technical Intern

International Security Initiatives

Cooperative Monitoring Center

Sandia National Laboratories

Albuquerque, NM

 

 

ABSTRACT

                This report gives a introduction to radiochemistry.  It covers definitions of radioactive particles and radionuclides, as well as methods of detecting and analyzing these nuclides in soil and water samples.

 

BACKGROUND INFORMATION

Alpha Particles (a)

An alpha particle consists of two neutrons and two protons.  This gives the alpha particle a positive charge of two and a relatively large mass.  The alpha particle has a very short range of about 1 to 2 inches in air.  A single piece of paper or even the outer layer of your skin is enough shielding to stop an alpha particle.  Therefore, external alpha particles are not considered to be dangerous.  There are, however, serious health hazards if inhaled, ingested, or absorption through an open wound.  Some alpha sources include uranium-238, radium-226, and thorium-232. 

 

Beta Particles (b)

Beta particles have a charge of negative one and are very small in size, 1/3600 the mass of a proton or neutron.  Negative beta particles are also known as electrons.  Compared to an alpha particle, a beta particle’s range is much greater.  The range a beta particle can travel depends upon the energy level of the specific beta emitter.  Beta particles can penetrate soft tissues like your eyes and the outer layer of skin.  Generally, beta particles can be shielded with plastic, glass, aluminum, or wood.  Beta-emitting sources include tritium, lead-214, cesium-137, strontium-90 and potassium-40. 

 

Tritium (H3)

Tritium is a radioactive isotope of hydrogen.  It consists of one proton and two neutrons, and has a half-life of 12.3 years.  Beta radiation from H3 has a very low energy and can travel just a few inches in air.  However, because tritium is a pure beta emitter, it can easily be absorbed through a person’s skin.  Proper shielding, such as glass or wood, should be used to prevent this absorption.

 

Gamma-Emitting Radionuclides:

Gamma radiation originates in the nucleus of the atom and travels by way of electromagnetic waves, similar to light.  Gamma rays have neither charge nor mass.  Gamma rays are produced succeeding spontaneous decay of radioactive substances such as cobalt-60 or cesium-137.  A gamma ray can travel through the air at long ranges, as much as several hundred feet.  Furthermore, gamma rays such as cobalt-60 can penetrate deep into the human body and can therefore be used as the radiation in cancer treatment.  Thick shielding is needed for gamma ray protection, for example: lead, steel, or concrete.

 

·         Uranium-238  (U-238)

Naturally occurring uranium generally consists of 99.3% uranium-238.  All isotopes of uranium are radioactive.  U-238 decays by way of alpha emission and has a very long half-life of 4.5 billion years.  This extremely long half-life has been useful in estimating the age of igneous rocks.  The most important aspect of uranium is its potential as a nuclear fuel.  U-238 itself is not useful, but it can be converted into Plutonium-239, which is fissionable by means of a breeder reactor.  Daughters of U-238 include Th-234, Ra-226, Pb-214, and Bi-214.

 

·         Uranium-235  (U-235)

                Naturally occurring Uranium-235 is only about 0.7%.  U-235 has a half-life of approximately one billion years and it decays by alpha and gamma emission.  U-235’s significance comes from its ability to easily fission.  U-235 is used to fuel nuclear power reactors for the generation of electricity.  The average human intake of uranium is due primarily to food ingestion.

 

·         Radium-226  (Ra-226)

                Radium is known to have sixteen isotopes.  Radium-226 is the most common of these isotopes.  Radium-226 is found in the uranium series, which has a decay chain that begins with U-238 and ends with Pb-206.  Ra-226 has a half-life of 1,602 years and decays by way of alpha and gamma emission. 

 

·         BI-214

                Bismuth-214 is also a daughter nuclide of RA-226.  The half-life of BI-214 is approximately 20 minutes.  BI-214 decays by beta emission at 1000 keV (23%), 1510 keV (40%), and 3260 keV (19%).  Three main gamma energy peaks for BI-214 are at 609 keV, 1120 keV, and 1764 keV.

 

·         Thorium-232  (Th-232)

                Thorium is thought to be even more abundant than uranium.  Twelve isotopes of thorium are known.  Thorium-232 occurs naturally.  It has a half-life of 141 billion years and decays by alpha and gamma emission.  Thorium-232 is at the top of the thorium series, which ends with the stable isotope Pb-208.  The mass numbers of the isotopes in the thorium series are exactly divisible by 4.  This implies that all members of that series decay by alpha emission.

 

·         Beryllium-7  (Be-7)

                Beryllium is one of the lightest of all metals.  It is utilized as an alloying agent in generating beryllium copper.  Beryllium itself is toxic and should be handled accordingly.  Be-7 is a radionuclide that comes from natural background radiation that is created in the upper atmosphere, mostly in the stratosphere, by cosmic rays spoliation of carbon-12, nitrogen-14, and oxygen-16.  Therefore, it will show up in most all samples analyzed by the Gamma Spectroscopy System.  Be-7 has a half-life of 53.44 days and decays by electron capture. 

 

·         Cesium-137  (Cs-137)

                The metal cesium is used in photoelectric cells.  The detection of the radioactive isotope Cs-137 in samples is due to past nuclear activity.  This nuclear activity is primarily due to nuclear fall-out and testing. Over time, however, the Cs-137 levels have returned to “normal” levels.  Cs-137 has a half-life of 30 years and is a beta emitter.  

 

·         Potassium-40  (K-40)

                 Potassium is the seventh most abundant metal.  It makes up about 2.4% of the earth’s crust.  K-40, a radioactive isotope, occurs naturally but “presents no appreciable hazard” (Weast B-25.)  Potassium is found in most soils and is essential for plant growth.  The half-life of K-40 is 1.27 billion years.  K-40 decays by way of beta and gamma emission, which accounts for the positive correlation between high K-40 activity levels and high beta activity levels in most every soil sample.

 

Lead  (Pb)

                Radioactive isotopes of lead include Pb-212 and Pb-214, which decay both by beta emission and gamma emission.  Lead isotopes in general have relatively short half-lives compared to other radioactive isotopes.  The half-life of Pb-214 for example is about 27 minutes.  Stable lead isotopes are the end products of each of the series of radioactive decay processes.

 

METHODS OF ANALYSIS

 

Liquid Scintillation Counter  (LSC)

                The LSC counts each sample for gross alpha, gross beta, and tritium activity levels.  Typical efficiencies are 99% for gross alpha, 81% for gross beta, and 13.5% for tritium.  If any of the activity levels of an individual sample are higher than twice the minimum detectable activity (MDA) level of the instrument the sample is considered radioactive waste.

 

Soil:

                The liquid scintillation cocktail is the detector in this case.  The cocktail is made up of primarily three components: solvent, fluor, and an emulsifier.  The solvent absorbs the kinetic energy of the decay particle and tends to lose this energy in the form of heat.  The fluor is the molecule to which the solvent’s excited energy is transferred.  The fluor molecule returns to the ground state by emitting radiation in the form of light photons.  The emulsifier is a detergent like molecule, which aids in the proper mixing of samples in the cocktail.

 

                The light emission from the sample vial is aimed into two photomultiplier tubes.  These tubes then convert the light into a measurable electrical pulse.  The height of the pulse is proportional to the energy of the particle.  The liquid scintillation counter is equipped with a multi-channel analyzer, which can display sample results as a spectrum.  It then computes count rates by method of applied spectrum analysis.  Problems such as beta backscattering and selection of a proper efficiency are completely avoided by counting with the LSC.  The intimate mixture of the sample with the detector allows the LSC to be especially suited for low energy beta emitters such as tritium and carbon-14.

 

Water:

                The samples are compared to de-ionized water blank, which was also counted for 200 minutes.  The LSC counts each sample for gross alpha, gross beta, and tritium activity levels.  The LSC Efficiency is approximately 24% for tritium, 99% for gross alpha, and 89% for gross beta.  If any of the activity levels of a sample were higher than twice the MDA, then the sample was considered to be radioactive waste.  The activity levels are reported in picocuries per liter (pCi/L).

 

High Purity Germanium Gamma Spectroscopy System  (Gamma-Spec)

                The detector is made of a germanium crystal, from which the highly purified germanium originates.  The gamma rays interact with this crystal in mainly two manners; they are known as the photoelectric effect and Compton scattering.  The purpose of the Gamma-Spec is to identify and quantify the gamma emitting radionuclides in the soil and water samples. The gamma-ray energy pulse first goes through a preamplifier before reaching an amplifier, where the current pulse is converted into a voltage pulse.  This is done by performing an energy calibration resulting in the pulse height being proportional to the gamma-ray energy.  Next the pulse travels through an Analog to Digital Converter (ADC) where the voltage pulse is transformed into a number.  Finally, the number is reported using a spectrum by the Multi-Channel Analyzer (MCA). 

 

By performing an energy calibration, a relationship between channel number and gamma ray energy is determined.  Each radionuclide has one or more identifying energy levels.  Each peak produced and measured by the Gamma-Spec analyzer is matched to a radionuclide energy level.  The detected nuclides activity levels are reported in picocuries per milli-liter (pCi/mL).

               

Soil:

The gamma emitting radionuclides specifically recorded were Uranium-238, Radium-226, Thorium-232, Uranium-235, Beryllium-7, Cesium-137, and Potassium-40.

 

Water:

                The radionuclides of concern are Radium-226, Lead-214, Bismuth-214, and Potassium-40.

 

Kinetic Phosphorescence Analyzer  (KPA)

                The Kinetic Phosphorescence Analyzer was designed to strictly assess the total uranium concentration in dissolved soil samples through phosphorescence emission.  This is done by determining the relationship of phosphorescence intensity to time.

 

Soil:

KPA measurement is fast and accurate, however, the sample preparation does take slightly longer than other RPSD analysis techniques.  In general, using a hydrofluoric/nitric acid digestion dissolves an aliquot of 2 grams of soil.  From this liquid solution, 0.1 gram is pipetted, dried and heated to remove any interference such as chlorine.  This dissolution and drying process takes about 3 days.   The solution is placed in the KPA where a nitrogen-pumped dye laser is admitted and the phosphorescence decay is measured.  The sample is analyzed three times to determine error.  The analysis is recorded in micrograms per gram (mg/g), with a minimum detectable concentration (MDC) level of 0.1 mg/g.

 

Water:

                The Kinetic Phosphorescence Analyzer directly detects Total Uranium content in samples.  This measurement is fast and accurate; however, the sample preparation takes longer than the other sample techniques.  The Total Uranium content is reported in micro-grams per liter (mg/L).

 

Atomic Absorption (AA) Spectrometer

                The Atomic Absorption Spectrometer measures concentration and absorbance of lead or other metals by way of flame emission.  This method only accounts for elemental lead; it does not identify specific isotopes.  Flame emission requires aspiration of the liquid sample.  The liquid sample is acquired by the same dissolution method as the KPA.  The temperature must be watched very carefully while dissolving the sample because lead can be very volatile causing complete evaporation. 

 

Soil:

The sample is aspirated into a flame that is hot enough to atomize the vapor.  A beam of light that is set to specifically detect the lead atoms passes through a narrow slit to reduce interference, and then passes through the flame measuring the concentration of lead atoms that are present.  The final results are given in micrograms per gram (mg/g), with a MDC at approximately 5.2 mg/g.

 

 

Reference:

 

 

Goetz, Jennifer L.  Background Radiation in Soil Samples.  Sandia National Laboratories.  Albuquerque,

NM.  1997. 

 

Goetz, Jennifer L.  Detecting Background Radiation in Drinking Water.  Sandia National Laboratories. 

Albuquerque, NM.  1996.