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