dosimetry - objects and methods of treatments, factor influencing mutation dose rate
TRANSCRIPT
AnAssignment
On
Dosimentry –object & Method of treatments
Factor influencing mutation dose rateAcute & chronic irradiation
Thermal neutron Effects
SUBMITTED BY :Patel Nilesh J.
M.sc Agri.3rd Sem.
Dept. Of Genetics and Plant Breeding
C.P COLLEGE OF AGRICULTURE
SUBMITTED TO :
Dr. M.P.Patel
Professor & Head
Dept. Of Genetics and
Plant Breeding
C.P COLLEGE OF
AGRICULTURE
Dosimetry in its original sense is the measurement
of the absorbed dose delivered by ionizing radiation, the term is better known as scientific radiation.
The term is better known as scientific sub-speciality in the fields of health physics and medical physics, where it is the calculation and assessment of the radiation dose received by the human body.
Dosimetry
1925:- First International congress for Radiology in london . Foundation on ICRU “ International commission on Radiation units and measurement”
1928:- Second International congress for Radiology in stock holm. Defination of the unit Roentgen to identify the intensity of radiation by the number of ion pairs formed in air.
1950:- Defination of the dosimetric quantity absorbed dose as absorbed energy per mass.
The rad is the special unit of absorbed dose. 1 rad = 0.01 J/kg
Historical Development of Dosimetry
1975:- Defination of the new SI unit of dose
the Gray (Gy) for the quantity absorbed dose
1Gy = 1 J/kg = 100 rad
Dosimeters is a device that measures directly or
indirectly Exposure Kerma Absorbed dose Equivalent dose
The dosimeter along with its reader is referred to as a dosimetry systems
Object of Dosimeters
Immediate Read
Pocket Ionization Chambers, Solid state detectors, handheld GM/Ionization detectors with dose accumulation function
Delayed read / Personnel monitors Film Badges, TLD (Thermo Luminescent
Dosimeters), OSL (Optically Stimulated Light-emitting Dosimeters)
Types of Dosimeters
Most common type of radiation dosimeter used.
Works by darkening of x-ray film in proportion to radiation absorbed.
Cheap Not durable Short monitoring period per
badge (6 months or less)
Film Badges
“Captures” radiation dose information in a crystal matrix
Releases light when heated, light intensity proportional to radiation dose absorbed
Durable Can be expensive (reusable chips) Information destroyed when
processed
TLD (thermoluminescent dosimeter)
OSL(Optically stimulated luminescence) “Captures” information in an
Aluminum Oxide matrix Releases information by
laser stimulation Can be reread after
processing Durable Landauer Only
Electro-statically charged “leaf” discharges as it is exposed to ionizing radiation
Not considered a “legal” record Low accuracy (+/- 20%) Physical impacts can affect
radiation dose readings
Pocket Ionization
Provides instantaneous
information regarding dose accumulation
Simple to use Not a “legal” record Dose range device dependent
Solid State
Ability to perform both dose
rate and dose accumulation Good for spot checks Direction dependent Not considered a “legal”
personnel dose record
Handheld Radiation Detectors
Accuracy:- Specifies the proximity of the mean
value of a measurement to the true value Precision :- Specifies the degree of
reproducibility of a measurement.
Note:- High Precision is equivalent to small standard deviation.
Properties of Dosimeters
Dosimetry is used extensively for radiation
protection and applied to occupational radiation workers, where a radiation dose is expected but regulatory levels must not be exceeded.
The mutation rate is the frequency with which
a gene changes from the wild type to a mutant. It is commonly expressed as the number of mutations per biological unit,
which may mean per cell division per gamete, or per round of replication.
Factor influencing mutation dose rate
The mutation rate depends on the frequency
of primary changes in DNA. These primary changes may arise from spontaneous molecular changes in the DNA, or be induced by chemical or physical agents in the environment.
Factor 1: Frequency of Primary Changes in DNA
The second influential factor is the probability that,
when a change in DNA takes place, it will be repaired. Most cells posses a number of mechanisms to repair
changes in DNA, so most alterations are repaired before they are replicated.
If these repair systems are effective, mutation rates will be low. If they are faulty, mutation rates will be increased. There are even mutation that increase the overall mutation rate for other genes. Such mutations usually occur in genes that encode components of the repair mechanisms or repair enzymes.
Factor 2: Probability of Repair
Factor 3: Probability of Recognition The third factor is one that influences the ability
to calculate mutation rates. It is the probability that a mutation is recognized and recorded. When DNA is sequenced, all mutations are potentially detectable.
In practice, however, sequencing is still quite expensive, so most mutations are detected by their phenotypic effects. Some mutations may appear more likely to take place simply because they are easier to detect.
In many studies, frequencies of two-break
chromosomal aberrations were found to be proportional to square of the dose of radiation. Chromosome breakage and resulting structural aberrations are generally regarded as associated with gene mutations. Effects of irradiation are cumulative. Therefore prolonged irradiation at low intensities is known as chronic irradiation. It take long time.
Produce the same amount of mutations as that produced by the same dose of irradiation delivered in a short period at a high intensity is known as acute irradiation.
Acute and chronic irradiation
0-25 : No observable effect. 25-5 : Minor temporary blood changes. 50-100 : Possible nausea and vomiting and
reduced WBC. 150-300 : Increased severity of above and
diarrhea, malaise, loss of appetite. 300-500 : Increased severity of above and
hemorrhaging, depilation. Death may occur > 500 : Symptoms appear immediately, then
death has to occur.
ACUTE DOSE(RAD) EFFECT
These are densely ionizing and highly penetrating particles.
Since they are electrically neutral particles, their action is not slowed down by charged (negative or positive) particles of tissues.
They are generated from radioactive decay of heavier elements in atomic reactors or cyclotrons. Because of high velocity , these particles also called fast neutrons.
Their velocity can be reduced by the use of graphite or heavy water to produce thermal neutrons.
Thermal Neutrons
Thermal neutron result in both chromosomal
breakage and gene mutation. Since they are heavy particles, they move in
straight line. Effectively used for induction of mutations
especially in asexually reproducing crop species.
Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the same way that charged
particles such as protons and electrons do (exciting an electron), because neutrons have no charge. However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the gamma ray (photon) subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are more penetrating than alpha radiation or beta radiation.
In some cases they are more penetrating
than gamma radiation, which is depended in materials of high atomic number. In materials of low atomic number such as hydrogen, a low energy gamma ray may be more penetrating than a high energy neutron