PHYSICAL CHEMISTRY OF MILK

Lesson 29. HALIF LIFE PERIOD OF RADIO ISOTOPES AND MEASUREMENT OF RADIO ACTIVITY

Module 11. Molecular spectroscopy

Lesson 29
HALIF LIFE PERIOD OF RADIO ISOTOPES AND MEASUREMENT OF RADIO ACTIVITY

29.1 Introduction

The radioactive materials are of use in several research activities especially to monitor the movement of substances across the body and as a diagnostic tool for some of the abnormalities in the body. These radio isotopes are also of some use in the dairy field, because of the fact that they are often found in milk in traces.

Radioactive material will lose its radio activity by emitting the radiation into the atmosphere. Consequently the ability to emit radiation by a given radioactive material progressively gets reduced. The time taken for half of the radionuclide's atoms to decay is commonly referred as half life. This property is related to decay constant. The radioactivity of any radionuclide is based on its half-life. A radio nuclide is said to be highly radioactive if the half life is short while a radionuclide having longer half-life is considered to radioactively weak. There is wide variation in half-life of known radionuclides which ranges from 1019 years to 10 -23 seconds.

29.2 Measurement of Radioactivity

29.2.1 Radiation detection

The oldest principle for detecting radiation is by darkening photographic emulsion. This principle is used in the personnel dosimetry. The film badge is most popular and cost-effective for personnel monitoring and gives reasonably accurate readings of exposures from beta, gamma and x-ray radiations. The film badge consists of a radiation sensitive film held in a plastic holder. Filters of copper and lead are attached to the holder to differentiate exposure from different types and energies of radiation.

29.2.2 Thermoluminiscence

Thermoluminiscence is another principle for detecting the radiation. Several inorganic crystals (e.g. LiF) can accumulate radiation energy and hold it. If the crystal is heated from 300 to 400 °C, it emits light in amounts proportional to the absorbed energy. Thermoluminiscent dosimeters, so called TLD, are mostly used as finger dosimeters, so inorganic crystals are held in a plastic holders and plastic rings. It gives an accurate exposure reading and can be reused.

29.2.3 Converting the energy of radiation to electric current

Converting the energy of radiation to electric current is also used for detecting the radiation. There are two basic principles which are based on ionization and excitation. First principles are based on ionization of gas molecules, while the second is based on excitation and ionization of solid, liquid or plastic material in a Scintillator, which emits photons of light after absorbing radiation. Light is then converted to the electric current by means of photomultiplier tube, and is measured.

29.3 Gas-Filled Detections

In gas-filled detectors voltage is applied between two electrodes and the ion pairs formed due to excitation are collected as a current. The measured current is proportional to the applied voltage and the amount of radiation.

In ionization chambers lower voltage ranging from 50 to 300 V is applied resulting in the formation of primary ion pairs by the initial radiation. Gas-filled detectors are cylindrical chambers with a central wire filled with air or different gases. These detectors are primarily used for measuring high intensity radiation. Dose calibrators and pocket dosimeters are the common ionization chambers used in nuclear medicine. The dose calibrator is one of the most essential instruments in nuclear medicine for measuring the activity of radionuclides and radiopharmaceuticals. It must be regularly checked for constancy, accuracy, linearity and geometry.

At higher voltages from 1000 to 1200 V, the current becomes identical regardless of how many ion pairs are produced by the incident radiation. Geiger-Müller counters operate in this region. They are used to monitor the radiation level in different work areas and they are called area monitors or survey meters. They are more sensitive than ionization chambers but cannot discriminate between energies. They are almost 100% efficient for counting alpha and beta particles but have only 1 to 2% efficiency for counting gamma and x rays.

29.4 Scintillation Detection

The detectors of this type consist of scintilator emitting flashes of light after absorbing gamma or x-radiations. The light photons produced are then converted to an electrical pulse by means of a photomultiplier tube. The pulse is amplified by a linear amplifier, sorted by a pulse-height analyzer and then registered as a count. Different solid or liquid scintillators are used for different types of radiation. In nuclear medicine, sodium iodide solid crystals with a trace of Thallium sodium iodide (NaI (Tl)) are used for gamma and x -ray detection.

29.5 Half Life of Radio Active Elements

The formula useful for calculating the half life of radio isotopes is

Nt = No x (0.5)number of half-lives

Where:

Nt = amount of radioisotope remaining
No = original amount of radioisotope

number of half-lives = time/half-life

Table.29.1 Calculated half life of radio active isotopes

29.5.1 Half life of radioactive elements

The half life of some of the commonly used isotopes is being shown here under:

Please note that some of the longer half-lives are written in scientific notation (i.e., 7.2E1 is equal to 7.2 x 10, or 72.)

Barium: Ba137m - 2.552 minutes, Ba 139 - 82.7 minutes, Ba-140 - 12.74 days, Ba-141

Carbon: C-11 - 20.38 minutes, C-14 - 5730 years

Cesium: Cs-134 - 2.062 Years, Cs-134m - 2.90 Hours, Cs-135 - 2.3E6 Years, Cs-136 - 13.1 Days, Cs-137 - 30.0 years Cs-138 - 32.2 minutes

Iodine: I-123 - 13.2 hours ,I-125 - 60.14 days ,I-129 - 1.57E7 years ,I-130 - 12.36 hours I-131 - 8.04 days ,I-132 - 2.30 hours ,I-133 - 20.8 hours ,I-134 - 52.6 minutes ,I-135 - 6.61 hours

Plutonium: Pu-238 - 87.74 Years, Pu-239 - 24065 years, Pu-240 - 6537 years, Pu-241 - 14.4 years, Pu-242 - 3.76E5 years, Pu-243 - 4.956 hours, Pu-244 - 8.26E7 years

Potassium: K-40 - 1.27E9 Years, K-42 - 12.36 Hours, K-43 - 22.6 hours,

Radium: Ra-223 - 11.434 Days, Ra-224 - 3.66 Days, Ra-225 - 14.8 days, Ra-226 - 1600 years, Ra-228 - 5.75 years

Rubidium: Rb-86 - 18.66 Days, Rb-87 - 4.7E10 Years, Rb-88 - 17.8 Minutes, Rb-89 - 15.2 minutes

Selenium: Se-75 - 119.78 Days, Se-79 - 65000 years

Sodium: Na-22 - 2.602 Years, Na-24 - 15.00 hours

Strontium: Sr-85 - 64.84 Days, Sr-87m - 2.81 Hours, Sr-89 - 50.5 Days, Sr-90 - 29.12 years Sr-91 - 9.5 Hours, Sr-92 - 2.71 hours,

Sulfur: S-35 - 87.44 days

Thallium: Tl-201 - 73.06 hours, Tl-207 - 4.77 minutes, Tl-208 - 3.07 minutes, Tl-209 - 2.20 minutes

Thorium: Th-227 - 18.718 Days, Th-228 - 1.913 Years, Th-229 - 7340 years ,Th-230 - 7.7E4 years ,Th-231 - 25.52 hours ,Th-232 - 1.41E10 years ,Th-234 - 24.10 days

Tin: Sn-119m - 293.1 days, Sn-123 - 129.2 days, Sn-125 - 9.64 days, Sn-126 - 1.0E5 years

Tungsten: W-181 - 121.2 days, W-185 - 75.1 days, W-187 - 23.9 hours

Uranium: U-232 - 72 Years, U-233 - 1.59E5 years, U-234 - 2.445E5 years, U-235 - 7.03E8 years, U-236 - 2.34E7 years, U-237 - 6.75 days, U-238 - 4.47E9 years,

Vanadium: V-48 - 16.238 days

Xenon: Xe-131m - 11.9 days, Xe-133 - 5.245 days, Xe-133m - 2.188 days, Xe-135 - 9.09 hours, Xe-135m - 15.29 minutes, Xe-138 - 14.17 minutes

Zinc: Zn-65 - 243.9 days, Zn-69 - 57 minutes,