INTRODUCTION- In geophysics: Measurements of physical properties either the natural or artificial fields.

-Investigation of the changes in the physical properties (field) measurements.

-Observation of the distortions of these fields which are termed anomalies can lead to the detection of mineralizations and their natures.

-Discovery of mineralizations (targets) which posses physical properties different than these of their surrounding host rocks.The difference between the target and the host rock when large (big) enough, it can create (cause) anomaly.

-The nature and identity of mineralizations can be known from the geophysical investigation.

- The measurement of the physical properties in the field can help in various studies of :i- Geological environment,ii- Mineral exploration ( detection of mineralizations ,ii- Lithological (rock unit) mapping, andiv-Mapping of geological structure (tectonics).

THEORY OF NUCLEAR RADIATION -The Nuclear radiation always surrounds the naturally radioactive (r.a.) materials (elements).

-They are most useful in detection of (r.a.) minerals of the three well-known naturally occurrence r.a. elements; radio-potassium 40K, uranium and thorium.

-There is no other class of minerals whose exploration depends on geophysics to the same extent as radioactive minerals (containing r.a. elements).

-Therefore, a nuclear radiation detector is always required and is a necessity to detect the radioactivity which is the process by which the nuclei of an element (parent or mother) spontaneously decay.

-The resultant is a new type of element and nuclear radiation (s). The newly –formed elements (daughters),are r. a. and yield till an end product newly –formed elements r. a. elements (daughters), till an end product is reached, i.e., a stable element (lead:pb)

-The whole assembly of r.a. elements constitute the decay “or disintegration” series of the first (parent or mother) element in the series (sequence).

-In the field of nuclear exploration geophysics the primary concern is the natural radioactivity (α and β particles as well as γ- rays).There are a number of natural radioisotopes, each of which possesses its decay series.-In this field of study, the primary concern is related to the mentioned three radio-isotopes:40K , 238U and 232Th. The first r. a. element is important for its widespread occurrence in rocks and its significances as a geologic (marker). The latter two r. a. element are important for their strategic, economic and commercial significance.

-The most important and significant natural radiations emitted during r. a. decay-out of the three principal radiations- are the γ- rays, due to their considerable penetration (several hundred meters in the air and several tens of centimeters in the rocks).

-Consequently, these radiation (the γ- rays) can point out (indicate or show) r.a. sources from remote distances through air.

-The β particles- due to their limited penetration and hence their limited range reaching few tens of centimeters in air –are used in assay purposes and face scanning in uranium mines.

-The α particles- for their very limited penetration and therefore their very limited range attaining few centimeters in air –are used in detection of radon “Rn’ gas, which is a gaseous element in the uranium and thorium decay series. It is also used –sometimes- in assay purposes.

-The nuclear radiation possesses two useful parameters:

The (flux) and the (energy)The flux of γ- rays indicates the “amount” of the r.a. materials (elements) present, and can be easily measured by a suitable wide- band instrument

called “Radiometer”.-The units of flux are either the counts per secod

“cps or c/s” or pulses per second “pps or p/s”, or a division of Roentgen per hour(R/h) which is the micro- Roentgen per hour (µR/h). The Roentgen is the quantity of γ- radiation which produces

2.083x109 pairs of ions/c.c of air at N.T.P.. is (eV) .

- The energy of γ- rays is a very useful parameter for determining the element which emits the radiation (its identity).

-The unit of energy extensively employed in nuclear and atomic physics. It is the energy acquired by an electron in falling freely through a potential difference of one volt. It is equal to 1.602x10-19 joule (j).

-The suitable instruments are called (discriminators or radiospectrometers) .

9

Uranium 238Uranium 238Decay SeriesDecay SeriesRadium-226Radium-226

Thorium-230Thorium-230

Uranium-234Uranium-234

Protactinium-234Protactinium-234

Thorium-234Thorium-234

Uranium-238Uranium-238

Lead-206Lead-206(Stable)(Stable)

Polonium-210Polonium-210

Lead-210Lead-210

Polonium-214Polonium-214

Bismuth-214Bismuth-214

Lead-214Lead-214

Polonium-218Polonium-218

Radon-222Radon-222 Bismuth-210Bismuth-210

Radon Decay Products (RDPs)Radon Decay Products (RDPs)

They are electronically complex, narrow-band radiometers. The spectrometers are capable of discriminating the radiations originating from the three r. a. element: K,U and Th.

•-The detected γ- radiation in the case of K arises from 40K itself which possess a characteristic γ- rays energy of 1.46 MeV.

•In the cases of U and Th , the γ- radiation do not arise from 238 U or 232Th themselves, but from 214Bi &208Tl which possess characteristic of γ- ray energies of 1.76&2.62 MeV respectively. They are chosen for their relative abundance within their decay series and their energetic and characteristic γ- rays

•This method of detecting daughter elements other than parent elements in the two decay series can lead to erroneous results in the case of serious.

disequilibrium.

-The three show the ability of α – and β – particles as well as γ- rays emitted by the three naturally –occurring radioelement K,U and Th. and their decay series (daughter products) to penetrate different substances, which proved to be different types of ionizing radiation

PHYSICAL PRINCIPLES OF RADIOACTIVITY METHODS

13

-Mass and Radioactivity:Structure of the atom:

-The atom is constituted from positively-charged nucleus and negatively-charged electrons (e- ) moving in the electric field of nucleus.

-The atomic nucleus consists of positively charged protons (p+ ) and neutrons (n) without electric change (neutral or non charged).

-The protons (p+ ) and the neutrons (n) are called “nucleons” and represent two states

with different charges.

-The atomic number (z) of the atom is the number of protons (p+ ) in the nucleus. It equals the number of

electrons (e) for a natural non ionized atom . -The atomic mass (M) of the atom is the sum of the

masses of the nucleons ( p+ and n) and electrons (e- ). -The mass of the protons (p+ ):mp=1.67x10-24 gm. -The mass of the electrons (e):me-=9.11x10-28gm.

-The linear dimensions of the nucleus~=10-15 to 10-14m. -The linear dimensions of the cloud of electron~= 10-10 m-The difference between the atoms (atomic nuclei) of the

chemical elements is a result of the difference in the number of protons (p+ ) in their nuclei.

-The difference in the number of neutrons (n) in the atomic nuclei of equal number of protons (p+ ) , does not change the chemical properties of the atoms and are called “isotopes” of the element.

-If the isotope denoted “x”, the atomic mass “M’ , and the atomic number “Z”,

then it is written in the form Mxz, e.g,. 238 U92 , 232Th90 and 40K19, ……etc.

NUCLEAR TRANSFORMATIONS

The radioactivity is the spontaneous emission of nuclear radiation

(i.e. α ,β and γ-) by a substance.

-The radioactive transformations don’t depend on external

condition, but are the properties of the atomic nuclei..

-The α particles have positive charge and are identical to helium

nucleus;4

H2.

-The β particles are stream of fast electrons

with negative charge.

- -The γ rays are hard electromagnetic (e.m.) radiation

with the greatest penetrating power. -All r. a. radiation rays

– have chemical action,

- Cause darkening of the photographic plates and

films,

- ionize gases and sometimes liquids and solids, and

– Cause luminescence of number of solids and liqudes.

These properties represent the bases of the detection of

these radiations.

LOW OF RADIATION DECAY Transformations of atomic nuclei (accompanied) by the emission of alpha and beta particles are called alpha and beta decay. The term "gamma decay" is never used. Natural radioactivity is observed, as a rule, in the heavy nuclei of elements after lead in the periodic table. There also exist light naturally radioactive nuclei: the potassium isotope 40K, the carbon isotope 14C, the rubidium isotope 87Rb, and other. Radioactive transformations are a property of atomic nuclei and do not depend on whether a substance is in the form of a chemically pure element or a chemical compound. The disintegrating nucleus is called the parent; and the nucleus of the product disintegration the daughter one.Alpha decay: MXZ → M-4YZ-2 + 4He2

Beta decay MXZ → MYZ+1 + oe-1

K capture MXZ + oe-1 →

MYZ-1Where X is the symbol of the chemical element corresponding to the parent nucleus,Y is the symbol of the chemical element corresponding to the daughter nucleus, 4He2 is the nucleus of a helium isotope, & 0e-1 is the designation of an electron

Daughter nuclei, as a rule, are radioactive themselves. A radioactive series (family) is defined as a sequence of radioactive transformations from a certain parent nucleus. There are three naturally radioactive families, which- according to the parent nucleus-are called: the uranium family 238U, the thorium family 232Th, and the potassium family 40K. A chain of α -and β- decays takes place in each of the radioactive families. In the first two naturally radioactive series, the radioactive transformations terminate on stable nuclei of lead isotope

The law of radioactive decay indicates that the radioactive transformations of atomic nuclei are statistical processes. It is impossible to predict which nucleus of a radioactive isotope will decay at a given instant. The decay of any of nuclei is an event having an equal probability.

The fundamental law of radioactive decay is: Nt=No e

-λt

Where No is the initial number of radioactive nuclei at the initial moment of time t =

o Nt is the number of radioactive nuclei at any moment of time, and

λ is the decay constant that determine the rate of radioactive decay and = -ΔN/N /ΔtThe minus sign indicates that the number of nuclei diminishes as a result of

radioactive decay. The quantity τ = 1/λ is called the mean live time of a radioactive isotope .The half

life T is defined as the time during which half of the original nuclei decay

T= 0.693 / λ = 0.693/ λ or τ = 1/ λ = 1/0.693 T = 1.443 TIt is difficult in practice to determine N, but since It is well known that Nα I, the intensity I of the radiation is used instead of N.

I0 , N0

I0 /2 , N0 /2

Natural Sources of Radioactivity The disintegration of natural radioactive elements is accompanied by the emission of alpha particles, beta particles and electromagnetic radiation. The electromagnetic radiation is generally referred to as gamma radiation when emitted by the nucleus, and X-radiation when origination from the electrons orbiting the nucleus.

All common rock types and the soils derived from them contain a significant amount of natural radioactive elements (radioelements) that emit gamma radiation of which the three major sources are the three naturally occurring radioelements:a) Potassium-40, 0.012% of the total potassium and emits gamma rays of an energy of 1.46 Mev;b) Decay products in the uranium-238 and uranium-235 decay series;c) Decay products in the thorium-232 decay series.

Table showing the radioelement concentration in crustal rocks (Clark et al, 1966).

The natural radioactive elements could be divided into:1)Natural radioactive element (n.r.e) forming disintegration series: 238U, 235U and 232Th.

2) Natural radioactive elements, which do not have any disintegration series: 40K.

1a) And 1b) UraniumNatural uranium consists principally of two isotopes, 238U and 235U, of which the first one is the most abundant (99.73%) and is the only one of concern under field survey conditions. The radioactive decay of 238U is complex and passes through 14 steps, before it reaches the final stable end product. Uranium with its decay products is for more radioactive than potassium. The principal gamma emission is associated with 214pb and 214Bi and not directly with 238U. The cumulative half-lives of decay products up to their formation (214pb & 214Bi) are approximately 330000 years. It should be noted that one of the steps in the decay chain preceding the formation of 214pb and 214Bi is the gas 222Rn, and also that eight of the disintegrations are accompanied by the formation of alpha particle which are helium nuclei.

IsotopeHalf-life Principal radiation

Principal gamma ray energy(Mev)

238U92 4.51*109 y Α234 Th

90 24.1 d Β234Pa91 2.18 m Β Protactinium234U92 2.48*105 y Α230Th

90 8.0*104 y Α226Ra88 1.622*103 y Α222Rn86 3.82 d Α218Po84 3.05 m Α Polonium214pb82 26.8 m β, γ 0.29 , 0.35214Bi83 19.8 m β , γ 0.61 , 1.12 , 1.76214Po84 1.6*10-3

s Α210pb82 21.3 y Β210Bi83 5.01 d Β210Po84 138.4 d Α206pb82 stable

Table showing the radiometric family of 238U, and the sequence of its conversion into

other elements, as well as the principal characteristic of the series.

Isotope Half-life Principal radiation235U92 713*108 y α, γ231Th90 25.54 m Β231Pa91 3.43*104 y Α227Ac89 22 y β Actinium227Th90 18.6 d Α

β α 223Fr87 21 m β Francium223Ra88 11.2 d Α219Rn86 3.92 s Α215Po84 183*10-3 s Α211pb82 35.1 m Β211Bi83 2.16 m Β ,γ211Po84 α 4.79 m α β 207Ti81 0.52 s Thallium, Β ,γ207pb82 Stable

Table showing the radiometric family of 235U, and the sequence of its conversion into other elements, as well as the principal characteristic of the series.

Isotope Half-life

Principal radiation

Principal gamma ray energy Mev

(Mev)232Th90 1.39*1010 y Α228Ra88 5.75 y Β228Ac89 6.13 h Β, γ 0.91, 0.96228Th90 1.91 y Α224Ra88 3.64 d Α220Rn86 55.3 s Α216Po84 0.15 s Α212pb82 10.64 h Β 0.24212Bi83 60.6 m Β 0.73212Po84 α 3*10-7 s Α α

208Tl81

3.1 m Β 0.51, 0.58, 2.62

208pb82 β Stable

Table showing the radiometric family of 232th, and the sequence of its conversion into other elements, as well as the principal characteristic of the series.

2) Potassium There are three natural isotopes of potassium. Only one (40K) of the several natural isotopes of potassium is radioactive. It has a relative isotopic abundance of only 0.0118%. It decays by B- (89% of all disintegrations) and K-capture (11%). Because of the simple decay scheme, it is characterized by single gamma-ray energy (1.46Mev). The decay is said, therefore to be monoenergetic. The energy of beta radiation is up to 1.33 Mev (maximum).

Decay scheme Half-lifeRadiatio

n

Gamma ray

energy

(Mev)

40K191.25*109 y β, γ, 1.4608

40Ca20 +β, γ Stable B-

(89%

40Ar18 + β, γ Stable K-

capture (11%).

Table shows the characteristics of the 40K-decay scheme.

Background Radiation: Several other sources of gamma radiation can also be detected in the field. Cosmic radiation, a flux of charged particles and neutrons of high energies, because of its interaction with the atmosphere and the detectors produces a measurable signal. - The intensity of this cosmic contribution increases with altitude. -Radioactive gases from the decay series of uranium-238 and thorium-232 escape into the air and their decay products are significant emitters of gamma radiation. Radon-222, because of its relatively long half-live of 3.8 days, can be transported over considerable distances in the air. ------- Products of nuclear weapon tests, deposited on the surface of the ground are found up to depth of 15 cm. The energy of the gamma radiation from this nuclear fallout is generally less than 1 Mev.

-Traces concentrations of radioactive elements in the instrument and noise associated with the electronics are further contributions of the measured radiation. -The normal crustal material, most of these ”background” sources of radiation represent between 5 and 15% of the total gamma radiation.-The mother element disintegrates in 2 directions: α- & β- disintegration. The elements of the beginning of the series have big or long half-lives, while the elements at the end of the series have short half-lives. Uranium itself does not emit γ-radiation, but its daughter products (end of the series); i.e. of Ra produces γ-radiation.

.

238U 5% of γ-radiation produced by the series.226Ra 206pb 95% of γ-radiation

produced by the series

Background Radiation:The use of the atom "background" in geoscience should be

restricted to radiation, which does not originate from the lithosphere.

For geoscientific purposes, radioactive background is defined as the some effect of cosmic rays, atmospheric radioactivity, and the radioactivity of the measuring system. (B = C R + I R + R n)

There are many objections to the use of "background " as a bases for reporting. What is included in the background? How large an area? and what rock types?

Is the background itself abnormal relative to other rocks of the same type elsewhere, or neighboring areas?. Such method of reporting is inadequate for any serious scientific work.

Instrument Calibration: It is called, sometimes, the standardization of the instrument. There are three types of calibration.1) By means of standard "Radium point source":

We know that 1 mg "Ra" gives 840 μR/h at 1m.

X is the exposure rate radiation in μR/h, m is the mass of the "Ra" source in mg, R is the distance in meters, and •840 is the value in μR/h emitted by 1 mg “Ra” at one meter.The first problem: It is the dependence on the type of the detector (whether a GM counter or a scintillation one).

The efficiency of both counters for gamma radiation from a radium point source is different, when for example detecting potassium; the GM counter registers 16 μR/h, while the scintillation detector records only 9 μR/h. The real problem now is: the lower the discrimination level, the higher the count rate we obtain. This creates real problems when using different devices with different lower energy discrimination levels in the mapping of an area.

As a consequence from the previous discussion, it could be stated that the calibration using point radium sources is not good one. The different efficiencies and the different lower discrimination level of the different instruments make it difficult to survey an area by numerous instrument.

2) Water Model This is the second type of calibration of instruments. The water model is composed of a tin or a barrel filled with water. The lengths of the sides of this container are 40 cm approximately. The radium point source “Ra” is placed inside the bottle at a certain depth (or level) equal s nearly 18 cm from the top. In this case, the gamma rays scatter in all direction similar to what happens in the nature (rocks or layers). The detector “D” of the instrument to be calibrated is put at a distance of about 10 cm from the top of the barrel

The total- count rate in cps of the gamma radiation emitted by the “”Ra” point source is givenby the following empirical formula:

NF,T -= C QeU/ M * NS,T

Where NF,T is the total count rate measured in cps in the field, above the rock or layer with equivalent

uranium concentration QU in percent,

QeU is the equivalent uranium concentration of rock or the layer in the field in eU%

NS,T is the total count rate measured in cps, above the standard source (“Ra” point source in

the water model), M is the mass of “Ra” in the point radium source in gm, and C is a constant =0.66*10-3 gm Ra /1% eU. Therefore, the more simple formula could be determine the equivalent uranium concentration:QeU = M. NF,T/ C . NS,T

If we want to measure QU in ppm eU and m in mg, the formula could be:

QeU =1* 104 / 0.66* 10-3 *103 . M * NF,T/ NS,T

=1.515 .104 . M * NF,T/ NS,T

The value of (M. NS,T) is always constant for a specified source and instrument, while the values

(NF,T and QeU) depends on the rock or layer in the field and are proportional to each other.

Consequently, the calibration curve showing the dependency Consequently, the calibration curve showing the dependency of the total count rate in c/s, on the equivalent uranium concentration in ppm, could be drawn. Therefore, using the total count or gross count survey devices, the measured total gamma activity (K, U, and TH) in cps could be expressed (or transformed) into ppm eU using the calibration curve. This kind of calibration is only possible for portable (ground) instruments having small detector.

3) Rock Standards: A series of three (or more) three dimensional (cylindrical) calibration sources (or models) should be constructed and having a height of 50 cm and radius depending upon the type of instrument (100 cms are enough for detectors placed over the sources directly). They must approximate to the composition of the natural geologic sources and have three different concentrations of U. The radioelement (U) concentrations or radiometric equivalent concentrations of these calibration facilities should be satisfactorily established by chemical or radiometric analysis. The range of radiometric concentration of U is from 50 to 500 ppm eU.

In order to obtain a constant gamma-ray flux, precautions must be taken in the construction of the concrete calibration pads to minimize the migration of radon through the pore spaces. The pads can be made of concrete mixtures to which a uranium mineral with low emanation characteristics has been added.

c- Environmental Studies The exposure rate (E) can be calculated from the apparent concentrations of K (%), eU (ppm), and eTh (ppm) using the expression (IAEA, 1991):Exposure rate (μR/h) = 1.505 K (%) + 0.653 eU (ppm) + 0.287 eTh (ppmThe radiation exposure rate can be converted to equivalent radiation dose rate through the use of a simple conversion factor (IAEA, 1979), as follows.•Dose rate (mSv/y) = 0.0833 * Exposure rate (µR/h) (1 Ur = 1 ppm U = 0.6 μR/h = 4.3 * 10-14 A.kg-1). 1 % K = 1.6 to 2.2 Ur1 ppm eTh = 0.44 to 0.45 Ur