radiation

Radiation

Chemical properties are determined by electron distributions and are only indirectly influenced by atomic nuclei. While nuclear reactions involve changes in the composition of nuclei. These extraordinary processes are often accompanied by the release of tremendous amounts of energy and by transmutations of elements. Some differences between nuclear reactions and ordinary chemical reactions are mentioned below.

THE NUCLEUS

The neutrons and protons together constitute the nucleus, with the electrons occupying essentially
empty space around the nucleus. The nucleus is only a minute fraction of the total volume of an atom, yet nearly all the mass of an atom resides in the nucleus. Thus, nuclei are extremely dense. It has been shown experimentally that nuclei of all elements have approximately the same density, 2.4 1014 g/cm3.



From an electrostatic point of view, it is amazing that positively charged protons can
be packed so closely together. Yet many nuclei do not spontaneously decompose, so they
must be stable.

NEUTRONPROTON RATIO AND NUCLEAR STABILITY

The term nuclide is used to refer to different atomic forms of all elements. The term isotope applies only to different atomic forms of the same element. Most naturally occurring nuclides have even numbers of protons and even numbers of neutrons; 157 nuclides fall into this category. Nuclides with odd numbers of both protons and neutrons are least common (there are only four), and those with oddeven combinations are intermediate in abundance





Furthermore, nuclides with certain magic numbers of protons and neutrons seem to be especially stable. Nuclides with a number of protons or a number of neutrons or a sum of the two equal to 2, 8, 20, 28, 50, 82, or 126 have unusual stability. Examples are 42 He, 16 8O, 42 20Ca,
88 38Sr, and 208 82Pb. This suggests an energy level (shell) model for the nucleus similar to the shell model of electron configurations.




The plot above represents the number of neutrons (N) versus number of protons (Z) for the stable nuclides (the band of stability). For low atomic numbers, the most stable nuclides have equal numbers of protons and neutrons (N =Z). Above atomic number 20, the most stable nuclides have more neutrons than protons.

The nuclide symbol for an element is

where E is the chemical symbol for the element, Z is its atomic number, and A is its mass number.

NUCLEAR STABILITY AND BINDING ENERGY

Experimentally, we observe that the masses of atoms other than 11H are always less than
the sum of the masses of their constituent particles. We now know why this mass deficiency
occurs. We also know that the mass deficiency is in the nucleus of the atom and has nothing
to do with the electrons.
The mass deficiency,  QUOTE

m, for a nucleus is the difference between the sum of the
masses of electrons, protons, and neutrons in the atom (calculated mass) and the actual
measured mass of the atom.




EXAMPLE : Mass Deficiency
Calculate the mass deficiency for chlorine-35 atoms in amu/atom and in g/mol atoms. The actual mass of a chlorine-35 atom is 34.9689 amu.
Plan
We first find the numbers of protons, electrons, and neutrons in one atom. Then we determine the calculated mass as the sum of the masses of these particles. The mass deficiency is the actual mass subtracted from the calculated mass. This deficiency is commonly expressed either as mass per atom or as mass per mole of atoms.

Solution

Each atom of 3517Cl contains 17 protons, 17 electrons, and (35 _ 17) = 18 neutrons. First we sum the masses of these particles.

protons: 17 1.0073 amu = 17.124 amu

electrons: 17 0.00054858 amu = 0.0093 amu
neutrons: 18 1.0087 amu = 18.157 amu
..
 sum = 35.290 amu calculated mass



The amu can be expressed in g/mole, then




In 1905, Einstein set forth the Theory of Relativity. He stated that matter and energy are equivalent. An obvious corollary is that matter can be transformed into energy and energy into matter. The transformation of matter into energy occurs in the sun and other stars. It happened
on earth when controlled nuclear fission was achieved in 1939Einsteins equation, which is
E = mc2
E represents the amount of energy released, m the mass of matter transformed into energy, and c the speed of light in a vacuum, 2.997925 108 m/s (usually rounded off to 3.00 108 m/s).
A mass deficiency represents the amount of matter that would be converted into energy
and released if the nucleus were formed from initially separate protons and neutrons. This
energy is the nuclear binding energy, BE. It provides the powerful short-range force
that holds the nuclear particles (protons and neutrons) together in a very small volume.
We can rewrite the Einstein relationship as


Specifically, if 1 mole of 35Cl nuclei were to be formed from 17 moles of protons and 18 moles of neutrons, the resulting mole of nuclei would weigh 0.321 gram less than the original collection of protons and neutrons Nuclear binding energies may be expressed in many kilojoules/ mole of atoms,


Nuclear Binding Energy


Calculate the nuclear binding energy 35Cl in (a) kilojoules per mole of Cl atoms, (b) kilojoules per gram of Cl atoms
Plan
The mass deficiency that we calculate is related to the binding energy by the Einstein equation.


(b) the actual mass of 35Cl is



The nuclear binding energy of a mole of 35Cl nuclei, 2.89 1013 J/mol, is an enormous amount of energy—enough to heat 6.9 107 kg (≈76,000 tons) of water from 0C to 100C! Stated differently, this is also the amount of energy that would be required to separate 1 mole of 35Cl nuclei into 17 moles of protons and 18 moles of neutrons. This has never been done.

RADIOACTIVE DECAY

Nuclei whose neutron-to-proton ratios lie outside the stable region undergo spontaneous radioactive decay by emitting one or more particles or electromagnetic rays or both.
The type of decay that occurs usually depends on whether the nucleus is above, below, or to
the right of the band of stability . Common types of radiation emitted in decay processes are summarized as follow



The energy of the gamma ray (h υ) is equal to the energy difference between the ground and excited nuclear states. This is like the emission of lower energy electromagnetic radiation that occurs as an atom in its excited electronic state returns to its ground state
Studies of gamma ray energies strongly suggest that nuclear energy levels are quantized just as are electronic energy levels.
The penetrating abilities of the particles and rays are proportional to their energies. Beta particles and positrons are about 100 times more penetrating than the heavier and slower-moving alpha particles. They can be stopped by a 1/8 inch thick (0.3 cm) aluminum plate. They can burn skin severely but cannot reach internal organs. Alpha particles have low penetrating ability and cannot damage or penetrate skin. They can damage sensitive internal tissue if inhaled, however. The high-energy gamma rays have great penetrating power and severely damage both skin and internal organs. They travel at the speed of light and can be stopped by thick layers of concrete or lead.

DETECTION OF RADIATION

1-Photographic Detection
Emanations from radioactive substances affect photographic plates just as ordinary visible light does. Becquerels discovery of radioactivity resulted from the unexpected exposure of such a plate, wrapped in black paper, by a nearby enclosed sample of a uranium containing compound, potassium uranyl sulfate. After a photographic plate has been developed and fixed, the intensity of the exposed spot is related to the amount of radiation that struck the plate. Quantitative detection of radiation by this method is difficult

2-Detection by Fluorescence

Fluorescent substances can absorb high-energy radiation such as gamma rays and subsequently
emit visible light. As the radiation is absorbed, the absorbing atoms jump to excited electronic states. The excited electrons return to their ground states through a series of transitions, some of which emit visible light. This method may be used for the quantitative detection of radiation, using an instrument called a scintillation counter.

3-Gas Ionization Counters

A common gas ionization counter is the GeigerMller counter . Radiation enters the tube through a thin window. Windows of different stopping powers can be used to admit only radiation of certain penetrating powers.


The principle of operation of a gas ionization counter. The center wire is positively charged, and the shell of the tube is negatively charged. When radiation enters through the window, it ionizes one or more gas atoms. The electrons are attracted to the central wire, and the positive ions are drawn to the shell. This constitutes a pulse of electric current, which is amplified and displayed on the meter or other readout.

RATES OF DECAY AND HALF-LIFE

Radionuclides have different stabilities and decay at different rates. Some decay nearly completely in a fraction of a second and others only after millions of years. The rates of all radioactive decays are independent of temperature and obey first-order kinetics. The rate of a first-order process is proportional only to the concentration of one substance. The rate law and the integrated rate equation for a first order process




Here A represents the amount of decaying radionuclide of interest remaining after some time t, and A0 is the amount present at the beginning of the observation. The k is the rate constant, which is different for each radionuclide. Each atom decays independently of the others, so the stoichiometric coefficient a is always 1 for radioactive decay. We can therefore drop it from the calculations and write the integrated rate equation as





Because A0/A is a ratio, A0 and A can represent either molar concentrations of a reactant or masses of a reactant. The rate of radioactive disintegrations follows first-order kinetics, so it is proportional to the amount of A present; we can write the integrated rate equation in terms of N, the number of disintegrations per unit time:


In nuclear chemistry, the decay rate is usually expressed in terms of the half-life, t1/2, of the process. This is the amount of time required for half of the original sample to react. For a first-order process, t1/2 is given by the equation




The isotope strontium-90 was introduced into the atmosphere by the atmospheric testing of nuclear weapons. Because of the chemical similarity of strontium to calcium, it now occurs with Ca in measurable quantities in milk, bones, and teeth as a result of its presence in food and water supplies. It is a radionuclide that undergoes beta emission with a half-life of 28 years. It may cause leukemia, bone cancer, and other related disorders. If we begin with a 16-g sample of 9038Sr, 8 g will remain after one half-life of 28 years. After 56 years, 4 g will remain; after 84 years, 2 g; and so on
Similar plots for other radionuclides all show the same shape of exponential decay curve . About ten half-lives (280 years for 9038Sr) must pass for radionuclides to lose 99.9% of their radioactivity.

EXAMPLE :Rate of Radioactive Decay

The cobalt treatments used in medicine to arrest certain types of cancer rely on the ability of gamma rays to destroy cancerous tissues. Cobalt-60 decays with the emission of beta particles
and gamma rays, with a half-life of 5.27 years.


How much of a 3.42 g sample of cobalt-60 remains after 30.0 years?
Plan
We determine the value of the specific rate constant, k, from the given half-life. This value is then used in the first-order integrated rate equation to calculate the amount of cobalt-60 remaining after the specified time.
Solution
We first determine the value of the specific rate constant.


This value can now be used to determine the ratio of A0 to A after 30.0 years.


Taking the inverse ln of both sides, A0 /A = 51

Radioactive Dating

The ages of articles of organic origin can be estimated by radiocarbon dating. The radioisotope carbon-14 is produced continuously in the upper atmosphere as nitrogen atoms capture cosmic-ray neutrons.


The carbon-14 atoms react with oxygen molecules to form 14CO2. This process continually
supplies the atmosphere with radioactive 14CO2, which is removed from the atmosphere by photosynthesis. The intensity of cosmic rays is related to the suns activity. As long as this remains constant, the amount of 14-CO2 in the atmosphere remains constant. 14-CO2 is incorporated into living organisms just as ordinary 12-CO2 is, so a certain fraction
of all carbon atoms in living substances is carbon-14. This decays with a half-life of
5730 years.


After death, the plant no longer carries out photosynthesis, so it no longer takes up 14CO2. Other organisms that consume plants for food stop doing so at death. The emissions from the 14C in dead tissue then decrease with the passage of time. The activity per gram of carbon is a measure of the length of time elapsed since death. Comparison of ages of ancient trees calculated from 14C activity with those determined by counting rings indicates that cosmic ray intensity has varied somewhat throughout history. The calculated ages can be corrected for these variations. The carbon-14 technique is useful only for dating objects less than 50,000 years old. Older objects have too little activity to be dated accurately.

EXAMPLE : Radiocarbon Dating

A piece of wood taken from a cave north Iraq is found to have a carbon-14 activity (per gram of carbon) only 0.636 times that of wood cut today from Najaf city. Estimate the age of the wood. The half-life of carbon-14 is 5730 years.
Plan
Determine the specific rate constant k from the known half life. The time required to reach the present fraction of the original activity is then calculated from the first-order decay equation.
Solution
First we find the first-order specific rate constant for 14C.





Nuclear Equations
A radioactive nuclide is spontaneously converted to another nuclide mostly by one of the following processes, in which there is an overall decrease in mass.

1. Alpha decay: a-particle is emitted and the daughter nucleus has an atomic number, Z ,
two units less, and a mass number, A, four units less than the parent's. Thus



2. β- decay: An electron is emitted and the daughter has a Z-value one unit greater than the
parent's, with no change in A . Thus

3. β+ decay: A positron is emitted and the daughter has a Z-value one unit less than the

parent's, with no change in A. Thus


4. Electron capture: By capturing an orbital electron within its own atom, a nucleus can
reduce its Z-value by one unit without a change in A . Thus

Radiation Risk

Because the HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html" \l "c1"energies of the particles emitted during HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html" \l "c1"radioactive processes are extremely high, nearly all such particles fall in the class of HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c2"ionizing radiation.




Old and new radiation units
Activity of sourceAbsorbed doseBiologically effective doseIntensityOld standard unitHYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c3"CurieHYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c5"RadHYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c6"Rem HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c4"Roentgen SI unitHYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c3"Becquerel HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c5"GrayHYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c6"Sievert ...
Ionizing Radiation
The practical threshold for HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c1"radiation risk is that of ionization of tissue. Since the ionization energy of a hydrogen atom is 13.6 eV(1J = 6.241509 1018 eV), the level around 10 eV is an approximate threshold. Since the energies associated with HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html" \l "c1"nuclear radiation are many orders of magnitude above this threshold, in the MeV range, then all nuclear radiation is ionizing radiation. Likewise, HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html" \l "c6"x-rays are ionizing radiation, as is the upper end of the HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html" \l "c5"ultraviolet range.
In addition, the upper end of the electromagnetic spectrum is ionizing radiation.

All nuclear radiation must be considered to be ionizing radiation!
Activity of Radioactive Source
The curie (Ci) is the old standard unit for measuring the activity of a given radioactive sample. It is equivalent to the activity of 1 gram of radium. It is formally defined by:

1 curie = amount of material that will produce 3.7 x 1010 nuclear decays per second.
1 Becquerel = amount of material which will produce 1 nuclear decay per second.
1 curie = 3.7 x 1010 Becquerel.
The Becquerel is the more recent SI unit for radioactive source activity. Intensity of Radiation
The roentgen (R) is a measure of radiation intensity of x-rays or gamma rays. It is formally defined as the radiation intensity required to produce an ionization charge of 0.000258 coulombs per kilogram of air. It is one of the standard units for radiation dosimeter, but is not applicable to alpha, beta, or other particle emission and does not accurately predict the tissue effects of gamma rays of extremely high energies. The roentgen has mainly been used for calibration of x ray machines.
Absorbed Dose of Radiation

The rad is a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. The more recent SI unit is the gray, which is defined as 1joule of deposited energy per kilogram of tissue. To assess the risk of radiation, the absorbed dose is multiplied by the relative biological effectiveness of the radiation to get the biological dose equivalent in HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c6"rems or HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c6"sieverts. Biologically Effective Dose
The biologically effective dose in rems is the radiation dose in HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html" \l "c5"rads multiplied by a "quality factor" which is an assessment of the effectiveness of that particular type and energy of radiation. For HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html" \l "c2"alpha particles the relative biological effectiveness (RBE) may be as high as 20, so that one rad (of HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html" \l "c2"alpha ) is equivalent to 20 rems. However, for HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html" \l "c6"x-rays and HYPERLINK "http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact2.html" \l "c1"gamma rays, the RBE (relative biological effectiveness ) is taken as one so that the rad and rem are equivalent for those radiation sources. The sievert is equal to 100 rems. 

Radiation Units of Measure

UnitAbbreviationDefinitionCommentRoentgenHYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "roentgen"RThe amount of energy absorbed in airFor x-rays and gamma rays onlyRadiation absorbed doseHYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "rad"radThe energy absorbed per gram of material 1 rad = 100 ergs/gramImportant because it represents the amount of energy that is absorbed by the material of interest-e.g., person, organ, tissue, cellsRoentgen equivalent man HYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "rem"RemThe product of the amount of energy absorbed (rad) times the efficiency of radiation in producing damage rem = rad x (Wr) Accounts for the different degrees of damage produced by equal doses of different radiations, for example:
Radiation
Radiation Weighting Factor (Wr)
x rays=1gamma raysbeta particles
neutrons range 2-20
alpha particle 20 Gray*HYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "gray"Gy1 Gy = 100 rad 1 Gy = 1 joule/kilogramSievert*HYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "sievert"Sv1 Sv = 1 Gy x Wr 1 Sv = 100 remCurie HYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "curie"CiThe number of radioactive decays (disintegrations)/ unit of time1 Ci = 2.2 x 1012 disintegrations/minute 1 Ci = 3.7 x 1010 disintegrations/secondBecquerel*HYPERLINK "http://www.remm.nlm.gov/dictionary.htm" \l "becquerel"BqThe number of radioactive decays (disintegrations)/ unit of time1 Bq = 60 disintegrations/minute 1 Bq = 1 disintegration/second 


Mechanisms of Damage
Injury to living tissue results from the transfer of energy to atoms and molecules in the
cellular structure. Ionizing radiation causes atoms and molecules to become ionized or excited. These excitations and ionizations can:
Produce free radicals.
Break chemical bonds.
Produce new chemical bonds and cross-linkage between macromolecules.
Damage molecules that regulate vital cell processes (e.g. DNA, RNA, proteins )

The cell can repair certain levels of cell damage. At low doses, such as that received every day from background radiation, cellular damage is rapidly repaired.
At higher levels, cell death results. At extremely high doses, cells cannot be replaced quickly enough, and tissues fail to function.

Tissue Sensitivity

In general, the radiation sensitivity of a tissue is:
proportional to the rate of proliferation of its cells
inversely proportional to the degree of cell differentiation
For example, the following tissues and organs are listed from most radiosensitive to least radiosensitive:

Most Sensitive: Blood-forming organsReproductive organsSkinBone and teethMuscleLeast sensitive: Nervous systemThis also means that a developing embryo is most sensitive to radiation during the early stages of differentiation, and an embryo/fetus is more sensitive to radiation exposure in the first trimester than in later trimesters.

Prompt and Delayed Effects

Radiation effects can be categorized by when they appear.
Prompt effects: effects, including radiation sickness and radiation burns, seen immediately after large doses of radiation delivered over short periods of time.
Delayed effects: effects such as cataract formation and cancer induction that may appear months or years after a radiation exposure


Prompt Effects
High doses delivered to the whole body of healthy adults within short periods of time can produce effects such as blood component changes, fatigue, diarrhea, nausea and death. These effects will develop within hours, days or weeks, depending on the size of the dose. The larger the dose, the sooner a given effect will occur. 

Effect

The background Dose
0.36 remBlood count changes50 remVomiting (threshold)100 remMortality (threshold)150 remLD50/60* (with minimal supportive care)320 360 remLD50/60 (with supportive medical treatment)480 540 rem100% mortality (with best available treatment)800 rem*(LD 50 is the abbreviation for HYPERLINK "http://en.wikipedia.org/wiki/Lethal_dose" \o "Lethal dose"lethal dose 50%).
Partial Body Exposure
These acute effects apply only when the whole body is relatively uniformly irradiated. The effects can be significantly different when only portions of the body or an individual organ system are irradiated, such as might occur during the use of radiation for medical treatment. For example, a dose of 500 rem delivered uniformly to the whole body may cause death while a dose of 500 rem delivered to the skin will only cause hair loss and skin reddening.

Delayed Effects of Radiation Exposure

Cataracts
Cataracts are induced when a dose exceeding approximately 200-300 rem is delivered to the lens of the eye. Radiation-induced cataracts may take many months to years to appear.

Cancer

Studies of people exposed to high doses of radiation have shown that there is a risk of cancer induction associated with high doses.
The specific types of cancers associated with radiation exposure include leukemia, multiple myeloma, breast cancer, lung cancer, and skin cancer.
Radiation-induced cancers may take 10 - 15 years or more to appear.
There may be a risk of cancer at low doses as well.

cancer risks at low doses are uncertain

It has been difficult to estimate cancer induction risks, because most of the radiation exposures that humans receive are very close to background levels. At low dose levels of milli rems to tens of rems, the risk of radiation-induced cancers is so low, that if the risk exists, it is not readily distinguishable from normal levels of cancer occurrence. In addition, leukemia or solid tumors induced by radiation are indistinguishable from those that result from other causes.


Cancer Risk Estimates
Using the linear model of stating this risk:
A dose of 10 mrem creates a risk of death from cancer of approximately 1 in 1,000,000.

Genetic Effects

There is no direct evidence of radiation-induced genetic effects in humans, even at high doses. Various analyses indicate that the rate of genetic disorders produced in humans is expected to be extremely low.

Prenatal Radiation Exposure

Rapidly proliferating and undifferentiating tissues are most sensitive to radiation damage. Consequently, radiation exposure can produce developmental problems, particularly in the developing brain, when an embryo/fetus is exposed prenatally.
The developmental conditions most commonly associated with prenatal radiation exposure include low birth weight, microcephaly, mental retardation, and other neurological problems. These effects are related to the developmental stage at which the exposure occurs. The threshold dose for developmental effects is approximately 10 rems.

problems

1- How many protons, neutrons, and electrons are there in each of the following atoms: (a) 3He, (b) 12C, (c) 206Pb?
2- What is the total binding energy of 12C and what is the average binding energy per nucleon?the actual
mass is 12
3- Complete the following nuclear equations

a- 147N + 42He = 178O +?

b- 94Be + 42He =126C + ?


c- 94Be + 11P = 42He +?

d- 3015P = 3014S + ?

e-31 H =32 He + ?

f- 4320Ca + 42He = 4621Sc + ?
4- 18F is found to undergo 90 percent radioactive decay in 366 min. What is its computed half-life from this
observation?
5- Estimate the age of a carcass from Mesopotamia was analyzed and found to have a 14C activity of 8.1 counts
(disintegrations) per minute, per gram of carbon. The half-life of 14C is 5730 years. And the recent activity is 15.3 counts per
minute, per gram of carbon..