NATURE OF RADIOACTIVE RADIATIONS
Rutherford, a British physicist and others studied the nature of radioactive radiations, in 1898. They found that the radiations can be classified into three distinct types.
They named these three types as
gamms (g) rays.
Experiments show that
a-rays consist of positively charged particles,
B-rays consist of negatively charged particles (steam of electrons) and
y-rays (energy) are electrically neutral.
All radioactive atoms emit either a-rays or B-rays, never both,
and this may be accompanied by y-rays.
Alpha rays( or a-particles) are nothing but helium nuclei, i.e.
an a-particle consists of 2 protons and 2 neutrons bound to together.
Beta rays are electrons, identical to those that orbit the nucleus. But they are created within the nucleus. Gamma rays are high energy photons whose energy is higher then that of X-rays.
For example half-life of radium-226 is about 1600 years. Suppose we start with1 mg of radium, In 1600 years it reduces to 0.5 mg, 0.25 mg in 3200 yrs. 0.125 mg in 4800 years and so on.
Rutherford, a British physicist and others studied the nature of radioactive radiations, in 1898. They found that the radiations can be classified into three distinct types.
They named these three types as
alpha (a) rays,
bata (b ) rays and gamms (g) rays.
Experiments show that
a-rays consist of positively charged particles,
B-rays consist of negatively charged particles (steam of electrons) and
y-rays (energy) are electrically neutral.
All radioactive atoms emit either a-rays or B-rays, never both,
and this may be accompanied by y-rays.
Alpha rays( or a-particles) are nothing but helium nuclei, i.e.
an a-particle consists of 2 protons and 2 neutrons bound to together.
Beta rays are electrons, identical to those that orbit the nucleus. But they are created within the nucleus. Gamma rays are high energy photons whose energy is higher then that of X-rays.
TRANSMUTATION
When a radioactive nucleus emits an a-particle, the remaining nucleus will be different from the original because there is loss of two protons and two neutrons. For example, radium 226 (A=226, Z=88) is an a. emitter. It changes (decays) to a nucleus with z=(88-2)=86 and A=(226-4)=222. The new nucleus is radon (Rn). Note :
To specify a given nucleus (or nuclide), we need to give only A and Z, where A represents the mass number, sum of the numbers of protons and neutrons and Z represents the atomic number the proton number, (A-Z) being neutron number.
A nucleus X with mass number A and atomic number, Z is represented by the symbol zXA
Radium 226 is represented as 88Ra225..
The original nucleus is called parent nucleus and the new nucleus is Called daughter nucleus. The changing of one element into another is known as transmutation.
The transmutation of radium 226 into radon 222 can be written as
88Ra226 --- > 86Rn222
Transmutation also occurs when a nucleus decays with the emission of a b particle. For example, Carbon-14 decays with the emission of a B particle; this can be written as
6C14 --- > 7N14
During b decay it is assumed that a neutron in a nucleus changes to a proton and an electron. Newly formed electron is emitted and proton remains in the nucleus. This is how Z increases by 1.
Half –life is found to be a characteristic feature of a radioactive element. Half-life is represented by T/2. If a sample of radioactive element contains N nuclei at a time (taken as Zero), then after a time equal to half –life, the number reduces to N/2; during the next half life that is after two half-lives N/4 and after three half-lives N/8 and so on. This is represented in a graph.
A nucleus X with mass number A and atomic number, Z is represented by the symbol zXA
Radium 226 is represented as 88Ra225..
The original nucleus is called parent nucleus and the new nucleus is Called daughter nucleus. The changing of one element into another is known as transmutation.
The transmutation of radium 226 into radon 222 can be written as
88Ra226 --- > 86Rn222
Transmutation also occurs when a nucleus decays with the emission of a b particle. For example, Carbon-14 decays with the emission of a B particle; this can be written as
6C14 --- > 7N14
During b decay it is assumed that a neutron in a nucleus changes to a proton and an electron. Newly formed electron is emitted and proton remains in the nucleus. This is how Z increases by 1.
HALF-LIFE
Theoretically, the decay process of a radioactive element in never complete and there is always some residual radioactivity. In other words, time taken for all the atoms to disintegrate, is infinite. For this reason, the half-life of a radioactive element is considered. Half-life of a radioactive element is defined as the time taken by a radioactive sample of that element, to get reduced to half its initial amount. Half-life is useful in comparing the activities of different radioactive elements. Half-life varies from micro seconds to billions of years. Half –life is found to be a characteristic feature of a radioactive element. Half-life is represented by T/2. If a sample of radioactive element contains N nuclei at a time (taken as Zero), then after a time equal to half –life, the number reduces to N/2; during the next half life that is after two half-lives N/4 and after three half-lives N/8 and so on. This is represented in a graph.
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| Half-Life of a radioactive element |
Half life of Polonium 214 is only 164 m s (m s = micro second = 10-6S). Half-life of uranium 238 is 4.5 billion years which accounts for its presence in nature. The half life of radium is 1622 yrs and the 200 mg sample of Radium separated by the Curies in 1898 now contains about 190 mg.
more details:
Radioactivity elements radiations isotopes for std 8 to 10
http://simplewayoflearnphysics.blogspot.in/2012/05/radioactivity-elements-radiations.html
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