CALLIBRATION OF DIRECT BEAM RADIATION MEASURING INSTURUMENT

The direct beam radiation measurement is done with the help of pyrheliometer. Its calibration is done mainly by following two processes.I) Intercomparision II) Calibration with standard instrument.

I) Intercomparision:
In thiis method, one pyrheliometer is taken as reference among the numbers of field instruments. The pyrheliometer which is taken as reference is calibrated against standard one and the calibration factor is determined for all field instruments.

II)Calibration with standard instrument:
An instrument whose calculated performance has been determined with a high degree of precision is called an absolute instrument. Abbot water-flow is taken as absoulute pyrheliometer. Number of pyrheliometers are calibrated against the absolute one are standard pyrheliometer. Generally field instruments are used for daily measurement of radiation. Field instruments are callibrated against standard pyrheliometer. Calibration is done by operating the standard and field instrument simultaneously in a clearday then calibration factor is ratio of measurement from field to that of measurement from the standard pyrheliometer.

CALLIBRATION OF PYRANOMETER

There are no absolute pyranometers like pyrheliometers. All pyranometer are calibrated instruments.There are number of methods of calibrating pyranometers using the sun or laboratory sources. Two of them are as follows.

I)Calibration against a reference standard pyrheliometer:
A perfectly clear day should be choosen to conduct this calibration. The test pyranometer should be fitted with a shading disk to temporarily eliminate the direct component. The disk should be large enough to shade the glass dome at all zenith angles. The test should begin by keeping the pyrheliometer pointed directly at the sun.At the same time, the shading disk should alternately shade and expose the pyranometer for about 10 minutes. The pyranometer calibration factor K is obtained as
 
         K= The ratio of (pyranometer reading average over exposed period- average over the shaded period) and In Cos thitaz
 where In is the direct normal irradiance calculated from the pyrheliometer reading and thitaz is Zenith angle.

II) Calibration  against a reference pyranometer:
In this method, the test and reference pyranometer are both placed in a horizontal position and operated simultaneously in a clear day. The calibration factor is a ratio of the measurement from the test  pyranometer to the measurement from the reference pyranometer.

HOW PYRANOMETER MESURES SOLAR ENERGY ARRIVING EARTH

A simple sketch for half shaded pyranometer is shown in figure. When the shaded part is removed, all the radiation ( direct + diffuse), that is, global fall on it. Inner part of the band is coated with black to remove the multiple reflection. The sensor kept inside the instrument convert radiant energy into heat energy and instrument is able to measure the radiant flux. When shade is placed, the direct radiation coming from sun will be blocked and only diffuse part of the radiation will fall on the instrument. Thus diffuse part of radiation can be measured by it.

CLASSIFICATION OF EARTH'S ATMOSPHERE IN ACCORDANCE WITH HTHE VERTICAL TEMPERATURE PROFILE

Vertical temperature profile is important to understand the actual concentration of atmospheric constituents and atmospheric pressure at pa particular height from the sea level in the earths's atmosphere. The vertical temperature profile for the standard atmosphere is depicted in the figure.

On the basis of vertical temperature profile,, the atomsphere has four layers. These are troposphere, stratosphere, mesosphere and thermosphere. The top of each sphere are respectively called tropopause, stratopause, mesopause and thermopause as shown in the figure above.

Troposphere:

It is the bottom layer where temperature decreases with increase in altitude with average value of lapse rate as 6.6 K/km. The temperature decrease continues to an average height of approximately 12 Km. However, the thickness of the troposphere is not same everywhere. It reaches to a height of 16 km in the tropic region, but in polar region it is about 9 Km. Most of the weather activities, for example cloud, rain, frost, fog, strom etc occur in troposphere. This layer contains nitrogen, oxygen, carbondioxide, dust and water vapour.

Stratosphere:

It is the layer of the atmosphere which lies above the tropopause. On this layer the temperature at first remains nearly constant to a height of about 20 Km. It begins a sharp increase that continues until the stratospause is reached at a height of about 48 km. Ozone occurs chiefly in this layer. In addition thhin layer of aerosol is also observed  to persist for long period of time within certain altitude rane of stratosphere. It is also called Ozonosphere.

Mesosphere:

It is the layer of atmosphere above the stratopause in which temperature decreases with altitude wntil at the mesopause to about 80 km above the surface. Temperature of this layer decreases upto about -100*C. It has very less ozone. Mesosphere has very strong winds blow eastward in winter and westward in summer. The upper boundary of this layer is Mesopause.

Thermosphere:

Extending upward from the mesopause and having no well defined upper limit in the atmosphere is the thermosphere.In the extremely rarified air of this outermost layer, temperature again increases as a result of the absorption of very short wave of solar radiation by the atoms of oxygen and nitrogen. Dute to which temperature increases sharply and reaches more than 1000*C.

Ozone is mostly found in stratosphere. A maximum in O3 is achieved near 30 Km. The radiative energy budget is controled by shortwave heating  due to Ozone absorption in the 9.6Km. Ozone thus determines the thermal structure of the middle atmosphere.

PYRHELIOMETER AND PYRANOMETER

PYRHELIOMETER
A special type of pyrheliometer is used to measure direct solar flux at normal incidence. it is a telescope type of instrument with narrow opening called aperture. This instrument faces the sun and follows its motion. The interest in establishing the value of the solar constant has been the main force behind development of this instrument. The instruments called water-flow, water-stir and silver disk pyrheliometers are based in the calorimetric principle.

A) Abbot water-flow and water-stir pyrheliometers:

• Inlet and outlet temperatures are measured at D1 and D2
• Accurate measurement of rate of water flow and temperature difference in D1 and D2 gives the measure of solar flux.
• The principle of calorimetry is used.
• Solar irradiance is calculated in true heat units from physical parameters of the instrument. For this reason they are called absolute pyrheliometers.

B) Abbot Silver-Disk pyrheliometers:

Sensor is silver disk coated with black paint

Temperature rises after the exposure is callibrated with the absolute one.

    Field Instruments:

The field instruments ( direct solar flux measuring devices) are calibrated on the basis of absolute pyrheliometer as reference. The field instruments are:

• Eppley pyrheliometer
• Kipp and Zonon pyrheliometer

Eppley Pyrheliometer

It is popularly called (NIP) Normal incidence pyrheliometer.

It works in the principle of thermopiles.

It has quartz window.

Kipp and Zonon pyrheliometer

It is also called actinometer

It uses Moll thermopiles

It measures long wave and short wave radiations, so long wave cut off filters are used.

Pyrheliometric Scales

• Angstrom scale is used as standard scale (A*S 1905)
• Smithsonian scale is used as revised scale(SS 1913)
• In 1956, Davos IRC ( International Radiation Commision ne Scale (IPS 1956)
• IPS 1956 = 1.015 (AS 1905)
•                = 0.98 (SS1913)
• WRR ( World Radiometric reference)
•               WRR= 1.022 (IPS 1956)

PYRANOMETERS

Global solar irradiance is measured by radiometer with hemispherical fields of view called pyranometers.Sensing elements of most common pyranometers are based on thermoelectric, thermo-mechanical or photovoltaic principles. Unlike the canonical absorbers, some of the pyranometers are flat sur flat surfaces. In routine meterological measurements, pyranometer are always placed in horizontal position.

Cosine Effect:

The sensing element of pyranometer is invariably coated with some form of highly absorbent black paint. Although it is easier to achieve high absorptance in the spectal sense, it is difficult to achieve the same in the directional sense ie. the absorpatance varies with the angle of incidence. Absorptance remain almost constant until the incident angle exceeds about 70*. Beyond this point absorptance increases considerably  and then drops rapidly when angle of incidence approaches 90* . This angular dependence of absorptance  is called the cosine effect.

RADIATION SENSORS

Detectors of various instruments can be classified as calorimetric, thermomechanical, thermoelectric and photoelectric.
(I) Calorimetric Sensors:
In the calorimetric instrument, the radiant energy is incident on a high conductivity metal coated with a non selective black point of high absorptance. The radiant energy is converted into heat that can be measured by a variety of means.

• The heat can be carried away by a flowing fluid whose change of enthalpy is measured. The change of enthalpy is an indication of the incident radiant flux.
• The heat gives rise to a change in the enthalpy of the absorbing metal(sensor). Again this increase in enthalpy (or increase in temperature) can be measured easily.
• In the modern cavity type instruments the temerature difference across a transducer is maiintained constant by additional elecrical heating required between shielded and exposed phases. The irradiance is then proportional to the difference in classify heating in two phases.

II) Thermomechanical sensors:
In the instruments based on the thermomechanical principle, the radiant flux is measured through bending of a bimetallic strip. The metal strips with different thermal expansion properties are rigidly held together, one end is fastened  and the other is free to move. One stripe is coated with a highly absorbent black paint and the other given a highly reflective coat. The blackened strip is exposed to solar radiation and the other is shielded from it. The two strips are insulated from each other to prevent heat flow from one to another. The unequal temperature and unequal coefficient of thermal expansio cause bending of the plates into a curve. The distortion is transmitted optically or mechanicall to an indicator.

III) Thermoelectric sensors;
A thermoelecctric device consists of two dissimilar metalic wires with their ends connected.An electromotive force(emf) is developed when the two junctions are at different temperatures. The emf developed is proportional to the temperatue difference and depends on material of the two metals.
                                   E proportional to change in temperature and material
One unction is exposed to sunlight and other is shielded. Since emf by one thermocouple is low in practice many thermocouples are joined ( used in such an arrangement is called thermocoil.) The hot junction is coated with black paint and cold with white to shield from solar radiation. The cold junctions are kept as constant temperature.
IV) Photoelectric sensors:
Among the photoelectric devices, photovoltaic instruments are the most numerous in the field of solar radiation measurement. A photovoltaic device is made of a semiconducting material such as silicon. There si a formation of pn junction between p-type and ntype material of a semiconductor as shown in figure.
When radiation at an energy level capable of ionising atoms falls on pn junction an electric current is produced from the continous movement of excess elctrons and holes.
A major disadvantage of silicon cell device is their spectral response which is strong only in the red and near infrared portions of the solar spectrum. However, their advantages are lower cost and faster response times for instantaneous measurements.

Prevention and Control of Radiation Hazard

Although it is difficult to eradicate the total effects of radiation, it can be minimized by employing some prevention schemies. These are helpful to control the radiation hazards. Following are the methods for the control of radiation hazards.

1)For the control of radiation hazards form external sources of radiation, it can be done by:

• Structural shielding design
• Radiation protection survey
• Nuclear Regulatory commision regulation
• Personal monitoring

2) Hazards associated with radioactive nuclides deposited internally are controlled by minimising the absorption, inhalation and injection of radioactive material into the body.

3)Building and fuseliges of aircraft provide little protection from the cosmic rays.
4) Preventing random production
5) Applying ICRP radiation protection principle
6) Applying basic safety standard (BSS) principle
7)Minimising unjustified particles such as

• Addition of radioactive materials to food, beverages or cosmetics
• Use radioactive materials in toys and jewellery

8) Reducing nuclear weapons testing and radioactive waste from nuclear power station
9)Minimising the burning of coal
10)Reducing nuclear power industry in number if possible. If not keeping them at seperate place from the public area.
11)By personal monitoring and performing equipment survey in hospitals, power stations etc.

Radaition Hazards-Sources, Prevention and Control of Radiation Hazards.

Radiation is a part of environment. The background radiation is contribute by tree sources namely terrestial radiation, cosmic radiation and radiation from radioactive elements in our bodies. Since ionising radiations have the capacity to ionise the molecules, so when  ionisiong radiation enters into our body, they ionise the tissue molecules and chemical dissocialn takes place. Due to this there is distortion of DNA chain and body cells take place. The cell production rare increases and malignant tumour is formed. These type of radiations are unnecessary to the body. Hence the damages due to such unwanted radiation to the body parts is termed as radiation Hazards. Radiation Hazards mainly take place due to two types of sources.

1. Internal sources of radiation
2. External sources of radiation

The damaging of body cells due to radiation from external source of radiation is much less than that of internal sources or radiation.

Radiation Hazard from the Internal source of radiation

This type of radiation hazard are due to:

• deposition of radioactive nuclide by injection to the body
• depositions of radioactive nuclide by inhalation in the body
• deposition of radioactive nuclide bye swallowing

Radiation Hazard from External source of radiation

It is due to the careless handling and use of radioactive source and therapy unit of 60 Co etc.

These two categories of hazards are due to the use of radioactive sources by workers or individuals. Also depending upon the roduction of radiation, the hazards are:

• Due to natural radiation
• Due to artifical radiation

Hazard due to natural radiation

1. Cosmic radiation to earth's atmosphere gives some radiation effect. The global yearly dose (average) due to cosmic radiation is 0.39 MSV.
2. Earth crust is made of up of radioactive materials such as uranuim. Rock soils contain uranium ie. radioactive material so they irradiate the whole body more or less uniformly. The global dose per year is 0.46 MSV.
3. Radon is naturally radioactive gas coming from Uranium that is widespread in earth crust. It is emitted from rocks or soils. When radon is inhaled it can lodge in lung and irradiate tissue. The global dose is 1.3 MSV.
4. Food and water contain radioactive material when food and water are taken, these radioactive materials are taken, they enter int he body and irradiate tissue. The global dose is 0.23 MSV.

Hazard due to artifical source of radiation

The radiation hazard take place due to artificial source of radiation such as:

Medical
Radiation is used in medicine to diagnosis disease and to kill cancer cells. During treatment or diagnosis indivisuals are exposed to radiation that may cause hazards.

Institutions
Many medical institutions discharge radioactive materials into environment that causes radiation hazard.

Environmental radiation
Due to testing of atom bombs and other nuclear activities, radioactive materials set free in atmosphere. These cause irradiation in human due to radioactive material deposition on the ground or from inhalation of air borne radioactivity and from injectio of radioactive material in food and water.

Nuclear power industry
The release radioactive material at each stage int he nuclear fuel cycle.

Non-Nuclear industries
They produce radioactive discharge in the processing of ores containing radiation. The discharged products processing of ores containing radiation. The discharged products transfer throough the food chain from the population.

Accidental release of radioactive material
In normal operation the radioactivity can be widely dispersed accidentally which causes radiation hazard.

Radiation in consumer products
In consumer goods such as smoke detectors, luminious watches, dolls etc may emit radiaton to cause hazard.

Maximum Permissible Dose

The persons who are exposed to radiation should not have the dose greater than upper limit and this limit is called maximum permissible dose (MPD) or smiply the dose limit. The MPD is different for occupationally exposed persons and general public. MPD include contributions from radiation source both inside and outside th ebody but excludes contributions from medical exposure and background radiations. The total dose in rems accumulated by an occupationally exposed person should not exceed 5(N-18) where N is the age of the person in years.

An area where a yearly whole body dose of 1.5 rem or more should be considered as a controlled area. Persons working in a controlled area should carry personal monitors such as TLD, pocket ionisation chamber etc. For the measurement of radiation dose personal monitors are not feasible for members of the public exposed occasionally to radiation. The protective measures for these persons is evaluated indirectly by sampling the air, water, soil and other elements of environment.

Dose Limit and ALARA

Dose limit gives support for the protection principle. Dose limit represents the judgement at the level of the individual harming or not harming which is on the border line of acceptability and unaacceptability. Continued exposure at a level just above a dose limit would result in risks which could be resonably ve described as unacceptable. Thus dose limits are those limits above which any person may cause radiaton hazard. Dose limits for the different people are different according to their employment or their occupation.It is categorized into three,they are:

1. Employees aged 18 years or above(Employees)
2. Trainees aged under 18 years(Trainees)
3. Any other person (Public)

The scientific basis for setting dose limit is  the comparative risk. There are signigicant differences between the risk of radiation and  other occupation al hazards. The persons whose occupation requires exposure to the radiation are called occupational exposure such as radiation oncologist, radiologists,nursing staffs etc working in a radiology or radiotherapy department are occupational exposure. On one hand, the staff has to receive the radiation dose and on other hand, the body has some tolerance limit for occupation of the injured cells. Hence many national and international organizations have recommended the maximum permissible limit radiation.

But ALARA principle should be followed in each department which means the risks we keep as low as reasonably achieavable taking into account the social and economic functions.

Harmful Effects of Ionising Radiation

An individual working with a source of radiation should be familiar with radiation hazards. When ionizing radiation enters the body tissue, it produces ionisation and cause teh chemical dissociation of the tissue molecules. When chemical dissociation takes place in vital components of the body cell, it may be destroyed by the destruction of DNA chain due to unwanted radiation. The cell production rate may be increased and malignant tumor may be formed. Exposure to ionising radiation can produce several effects in an individual depending on:

The type and amount of radiation producing the exposure.
The amount of body that is exposed.
The general helath of the exposed individual.
The quality of medical care available in the event of a relatively high exposure.

The effects of radiation can be divided into following two types:

1.  Stochastic(long term or delayed ) radiation effect
2.  Non Stochastic ( Short term or immediate or acute) radiation effect

STOCHASTIC EFFECT:

These are effects which show a random process ie. they can occur at any dose level. The probabililty of the effect occuring is dose dependent. The higher the dose the greater the probability. Stochastic effects are cancer and genetic effects. Of these most important is cancer.

Delayed effects in addition to cancer induction include teratogenesis ( the induction of birth effects by irradiation of foetus). and mutagenesis(the induction of genetic disorders in future generation by irradiation of germ cells). Data on stochastic effects are limited. The most important set of data from the surviors of the atom bo b explosion in Japan at Hirosima and Nagasaki. Cancer birth defects and genetic mutaions all occur naturally at relatively high rates in human population ad identifying an increase in these rates caused by exposure. Too small amoounts of ionising radiation is subject to considerably uncertain.

NON-STOCHASTIC EFFECTS:

The short term effecrts of radiation are associated with the levels of radiation far above those received by persons working in modern radiation facility. Hair loss, skin burns, sterility etc are the examples of deterministic effects. These effects are a result of loss of functional cells, in tisures or organs ie. cell killing. The body may loose its ability to combat infection. Following an absorbed dose of seeral gray diarrhoea, electrolyte imbalance, dehydration adn other gastointestinal effecrts may appear within a few days as a seequence of cell damage.
 Moreover there are some of the serious health harzard caused by heavy exposure to ionising radiations. These are discussed below:

Genetic mutation
When the gene in the DNA chain of a cell is distorted or changed, the same effect is reproduced in the subsequent divisions of the cell. If the affected gene belongs to a cell taking part in reproduction, the progeny(offspring) may show some mental or physical disabilities. The harmful genetic mutatioin get transmitted to the future generation.

Cancer
When there is uncontrolled multiplication of cells whose genes are damaged, a tumor may be formed in any part of the body.

Leucopenia
It is caused when the number of white blood cells in the blood becomes low. This reduces the resistance of the body prone to infections.

Bone Nerosis
It results when the bone marrow is damaged bye the radiations. The bone marrow is responsible for productng red blood cells int he body.

Sterility
It results when there is severe radiaton which damage the gonads.

Epilation
With this,the hair start to fall off which is caused by heavy exposure to radiation, vomiting, fever,  diarrhoea, headache, radiation sickness may result on early effect.

Electromagnetism in Science and Technology

The study of matter and energy without iunterference to chemical change occuring is said as physics. In general physics can be subdivided in two ways which are called traditional physics and Modern physicsTraditional physics covers mechanics, heat, light, sound, electricity and magnetism etc. While modern physics extends the study to atomic,nuclear and particle physics, relativity and quantam mechanics.

त्यो आयो

साथी!

तिमी त्यो नआवोस भन्थ्यौ
तर त्यो आयो
फेरी एक पटक आयो
साउनको खहरेजस्तो

त्यो आयो

गड्गडाएर आयो

तिम्रो नजिक आयो
ठिङ उभियो

हाँस्यो, एक्लै हाँसेन
उश्ले दुनिह हसयो 

अफ्सोच!

त्यो आयो, निचिक्क हास्यो
दुनियाँ हसयो
तिमी हास्न सकेनौ
तिम्रो मुस्कान हरायो
तिम्रो खुशी बगायो


सुरेश पौडेल
काठमाडौं

त्यो ब्यर्थ मुर्ति किन कुदियो?

त्यो ब्यर्थ मुर्ति किन कुदियो?
न तेसलाई आवाज भर्न सकियो
न बार्ता गर्न मिल्यो
खाली आकार दिईओ
सोखिनहरु को मोल मोलाई
को बस्तु बनाइयो

बरु ढुंगा बोक्दा कमिज च्यातियो
काध को खाला फ्याकियो
सहयोगी को हात च्यापियो
गरिखाने मान्छेलाई अपाङ्ग बनायियो
त्यो ब्यर्थ मुर्ति किन कुदियो

न तेसलाई आवाज भर्न सकियो
न बार्ता गर्न मिल्यो
खाली आकार दिईओ
सोखिनहरु को मोल मोलाई
को बस्तु बनाइयो
त्यो ब्यर्थ मुर्ति किन कुदियो?

suresh poudel
kathmandu

SCOPE AND METHODS OF BIOPHYICS

Living organisms are multilevel complex systems.Large and small molecules, cell organells, cells, tissues, organs, organisms, population, the biosphere are all levels that must be dealt with biology and biophysics.

Biophysics has been classified as:

1. Molecular Biophysics
2. Biophysics of cell
3. Biophycis of complex system

Molecular Biophysics

It is concerned with the study of the structure and physiochemical properties of biologically functional molecules; primarily biopolymers, proteins and nucleic acids. Its objective is to disclose the phycisal mechanism responsible for the biological functionality of the molecules say for catalytic activity of protein enzymes. Molecular biophycics can't be separated from molecular biology and  chemistry.
Molecular biophysics on one hand rests on biochemical disciplines and on the other hand , on the phycis of small and large molecules.


The theoritical apparatus of molecular biophysics consists of equilibrium thermodynamics, statistical mechanics and quantum mechanics. For an experimental study of biologically functional molecules use is made up of wide range of phycisal methods.

The first group includes methods employed in macromolecular physics for the determination of molecular masses, sizes and shapes- sedimentation in the ultra centrifuse, scattering of light and X-rays by collision of the substances to be studied etc.

The second group includes method used for investigarion of molecular structure and based on interaction of matter with light in the broad sense of these words from X- rays to radiofrequency radiation.NMR ( Nuclear Magnetic Resonance) and EPR( Electron Paramagnetic resonance) are also important calorimetric methods for macromolecules.

The third group includes calorimetric methods used for the study of the conversion of bioilogical macromolecules.Finally the structure of proteins and nucleic acids are directly studied by means of electron microscopy.

RELATIONSHIP BETWEEN PHYSICS AND BIOLOGY

We know biology covers all living things in the world and Physics examines the basic concpt such as energy, force space, time and all that dervices from mass, charges, matter and their motion. Broadly it is the general analysics of nature.While Bilogy is the science dealing with living organisms which are immeasurably more complicated than non living matters.

Biophysics is the application of physical laws to explan the properties of living organisms. It is the physics of life phenomena studied at all levels from molecules and cells to biosphere as a whole.

According to A.V. Hill-"Biophysics is the study of biiological function, organisation and structure of physical and physiochemical ideas and method."

According to vital force theory-Biological phenomena are basically incompressible on the basis of phsics and chemistry. Since there exists a certain mysterious vital forece or biological field which are not amenable to a physical interpretation.

The first law of thermodynamics tells conservation of energy was based on the observation of living organisms. According to uncertainity princliple the physio-chemical properties of living organism and life phenomena cant be studied. Simultaneously physiochemical and biological investibation are complementary to each other.

Schrodinger gave the concept of thermodynamical foundation of life. He stated that the living organisms"feed on negative entropies ie. the living organisms and the biosphere as a whole are not isolatedc but open system which exchange both matter and energy with the outisde world. He posed the question of stability of the substance of genes which is made up of light atoms; C, H, N,O.

Still the modern physics is not approching its limits of applicability for the treatment of biological phenomena but it points to the unlimited possibilities.

The living systems are basically open and therefore non equilibrium. A living organism is a thermodynamically open system which undergoes chemical reactions.Biochemical reactions are catalytic at all stages and protein-enzymes serve as catalyst. The change in entropy of such system is expressed by the sum  of the entropy produced inside the system  dis and the entropy supplied externally or given off the surrounding des
                                                                   ds =d is - d es

The quantity dis  is always positive according to the second law of thermodynamics. If the organism is placed in an insulating shell, the entropy flow des    will be equal to zero and the entropy can only incerease . The entropy of the system is not at the maximum. For the stationary state, we have 

                                                                                 ds = 0             ie. des = - dis <0   


In other words, the entropy produced is given up completely to the surrouonding.