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33 Cards in this Set
- Front
- Back
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13. The radiation emitted by a natural radioactive material
can be separated into three parts in an electric field. What kind of particles constitute these types of radiation? |
Alpha radiation consists of helium nuclei with two
positive charges; beta radiation is composed of high speed electrons; gamma radiation is an electromagnetic radiation consisting of high energy photons. |
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14. What is the direction of changes in the atomic index and the mass number of nuclei during an alpha, a beta and a
gamma decay? |
In the case of alpha decay the mass number and the
atomic indices decrease by four and two, respectively; in the case of beta decay the mass number remains constant, the atomic number increases by one (in the case of negative beta decay) or decreases by one (in case of positive beta decay and electron capture); in the case of gamma decay neither the atomic index nor the mass number change. |
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15. Give the equation describing the number of undecayed
nuclei as a function of time. |
N=No e^−λt
No: number of radioactive nuclei at t=0, N: number of undecayed radioactive nuclei at the time of investigation, λ: decay constant, t: time. |
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16. What is the physical meaning of the radioactive decay
constant? |
Radioactive decay constant is equal to the inverse first
power of the mean lifetime of a radioactive nucleus. |
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17. What is the relationship between the radioactiv decay
constant (λ) and the half life (T) ? |
T=ln2/λ
ln 2: the natural logarithm of 2. |
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18. Why is the spectrum of beta decay continuous?
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Besides an electron (or a positron) an antineutrino (or a neutrino) is also emitted, and the energy of the decay is
shared randomly by the two particles. |
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19. What is the definition of specific radioactivity?
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Radioactive isotopes are mixed with non-radioactive
isotopes in radioactive preparations. Specific radioactivity is the radioactivity of the material in unit volume or mass (Bq/g, Bq/mole, Bq/mL). |
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20. What is responsible for the energy loss of an alpha
particle along its path? |
Ionization.
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21. Write the formula describing the attenuation of gamma radiation in an absorbing material.
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I = Io e^−μ x
where Io denotes the incident intensity and I is the transmitted intensity after passing through an absorber of thickness x. μ is the absorption/attenuation coefficient. |
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22. How does the intensity of α-radiation change as a
function of the distance from the radiation source? |
It is constant in the beginning then suddenly decreases
to zero. |
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23. What kind of radioactive radiations can be detected by a
GM-counter? |
α-, β- and γ-particles can be detected.
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24. What is the principle of the archeological age
determination based on measurement of 14C? |
The relative concentration of the radioactive 14C isotope
is constant in the atmosphere and therefore in the living body. After the death of the organism active metabolism ceases and due to the disintegration of the isotope the relative concentration of 14C gradually decreases. |
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25. List the isotopes of hydrogen with their mass number
and the constituents of their nuclei! |
Mass
number Composition Hydrogen 1 1 proton Deuterium 2 1 proton+1 neutron Tritium 3 1 proton+2 neutron |
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26. Give the definition of isotopes!
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Isotopes are the variants of a chemical element with a
given atomic number whose mass numbers are different. |
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27. What is the isotope effect and what are the elements in
the case of which it is important? |
If the relative mass difference of isotopes of a given
element is significant, it results in distinct biological effects. This effect is important in the case of light elements, especially in the case of hydrogen (H,D,T). |
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28. What is the meaning of biological half life?
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Biological half life is the time period during which half of
the initial quantity of the radioactive isotope leaves the living system undecayed due to metabolism, secretion or excretion. |
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29. What is the meaning of effective half life?
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Effective half life gives the time during which the initial
activity of a given type of radioactive nucleus decreases to half of its original value either by physical decay or metabolism. |
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30. Describe the relationship between the effective (Teff), the physical (Tphys) and the biological (Tbiol) half lives!
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1 1 1
eff phys biol T T T = + |
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31. Describe the relationship between the physical (λphys),
the biological (λbiol) and the effective (λeff) decay constants! |
λeff = λphys + λbiol
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32. What is the basic principle of operation of a
photomultiplier tube ? |
Electrons liberated from a light sensitive cathode by
photons are accelerated in an electric field and collide into other electrodes (dynodes) whose potentials are increased in succession along the length of the tube. The energy of this collision is sufficient to free several secondary electrons. In this way the number of electrons increases at each dynode. |
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33. What is the basic principle of operation of ionization
detectors? |
Ion pairs produced by the ionization process are
separeted by the electric field of the detector. The ions are attracted towards the appropriate electrodes and generate electric impulses. |
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34. What is the principle of detection of radioactive radiation
by a scintillation detector? |
In certain organic and inorganic substances the energy
of radioactive particles is converted to luminous energy, i.e. they generate visible light flashes. |
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35. How does a γ-camera work?
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A substance labeled with a radioactive isotope is
injected into the body (radiopharmacon). The emitted γ photon passes through a collimator and collides into a scintillation detector. The distribution of the radiation source can be mapped by the counts of photomultiplier tubes connected to the detector. |
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36. What is the principle of SPECT?
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Images are taken from different directions by a γ-
camera. The three-dimensional distribution of the radiation source is calculated from these images. |
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37. What kind of isotopes can be used in PET?
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Only nuclei with positive β-decay can be used.
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38. What is the principle of determination of the location of a
radioactive isotope in PET? |
A positron is generated in a positive beta decay. It
collides with an electron in a distance shorter than 1 mm from its generation and the two particles are annihillated. Two gamma photons generated in the annihillation reaction leave the place of annihillation in opposite directions. A circular array of detectors senses these gamma photons. If two detectors opposing each other signal at the same time (coincidence), annihillation took place along the line connecting the two detectors. The intersection of many such lines gives the exact location of the radioactive source. |
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39. What is the mass defect of nuclei?
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The mass defect equals the difference between the
mass of a nucleus and the total mass of its constituents (Z pieces of protons and A-Z pieces of neutrons, where Z and A are the atomic index and the mass number of the nucleus, respectively): Δm = (Z mprot + [A-Z] mneutr) - matom where Δm is the mass defect, mprot, mneutr and matom are the masses of a free, unbound proton, a free, unbound neutron and the given atom, respectively. |
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40. What is the relationship between the total binding energy
(ΔE) and the mass defect (Δm) of a given nucleus? |
ΔE=Δm⋅c2, according to Einstein's equivalence principle
(c is the speed of light in vacuum). |
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41. Does the binding energy per nucleon increases
proportionally to the number of nucleons? |
No, binding energy per nucleon has a maximum at
nuclei with mass numbers 55-60 (e.g. Fe). |
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42. What are the properties of nuclear forces (their range,
strength and direction)? |
Nuclear forces have limited range, their effect is
negligible at a distance of more than a single nucleon and they are independent of charge. They are very powerful attractive forces whose magnitude excels that of electrostatic forces. |
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43. On what kind of energy level does a nucleon in a
nucleus reside compared to the energy of a free particle? |
The bound nucleon has negative energy compared to
the free particle. |
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44. What is electron capture and what does it produce?
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Some nuclei are capable of capturing an electron
residing on the K shell decreasing their atomic number by one. The vacancy generated this way on the K shell is filled by an electron from a higher shell. This transiton generates characteristic X-ray. |
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45. What are the energetic requirements of alpha and beta
decay? |
The total energy of the system must decrease during the
radioactive decay, that is the total mass of the decay products (daughter nucleus and the emitted particles) has to be smaller than the mass of the undecayed (mother) nucleus. |
