Why does nuclear fusion reaction yield more energy than Nuclear fission reaction?

Fission only produces more energy than it consumes in larger nuclei (eg: Uranium & Plutonium) which have around 240 nucleons.

Fusion only produces more energy than it consumes in small nuclei (in stars, Hydrogen & its isotopes fusing into Helium).

The energy released when four Hydrogen nuclei fuse in to a Helium nucleus is around 27MeV or about 7 MeV per nucleon.

For fission of U or P, energies released are around 200 MeV or so. The energy per event is greater in fission, but the energy per nucleon (fusion = about 7MeV/nucleon; fission = about 1 MeV/nucleon) is much greater in fusion.

Now lets look at fission. An example of fission is when a U-235 atom is split by a neutron into a Ba-144 and Krypton-89 atoms and 3 neutrons. The binding energy per nucleon for Uranium is about 7.6 MeV and for Barium around 8.3 MeV giving an increase in binding energy during fission of about 0.7MeV per nucleon or a total of 164.5MeV in total.

In a fusion reaction firstly two Hydrogens form a Deuteron, a positron and an electron neutrino. Then the Deuterium fuses with  another Hydrogen to form  He-3 and a photon. Finally two He-3's fuse forming a Helium nucleus and two hydrogen nuclei.

Considering the mass of four  protons/hydrogen nuclei and the mass of helium produced we get a mass difference of 24.69 MeV.

Conclusion: We can conclude that fusion reactions give out more energy per reaction. This is 0.7 MeV for fission and 6.2 MeV for fusion.  

I think you have cleared your doubt. Have a nice time ............bye.

Why heavy Nuclei U-238 do not spontaneously undergo fission?

To understand this, one has to realize that the surface area is considerably increased when one large liquid drop breaks into two small liquid drops. In case of U-238, about 6MeV is required to overcome the surface forces and get separated into small fragments before energy is released. This situation is described in terms of potential barrier through which fission fragments have to pass.

The quantum mechanical probability for such a barrier penetration is considered. Because of large masses involved, this probability is extremely small and for U-238 we get a half life of approx. 10^16 years, for decay of spontaneous fission.

Actually for U-238, gamma rays of energy approx 6 MeV are able to induce fission reaction. This reaction is called photo fission for which threshold energy is 6MeV approx. So for all practical purposes, U238 is stable against decay by spontaneous fission.          

What makes a steel stainless?

The addition of Chromium and Nickel to the iron creates a significant percentage of Chromium and Nickel items at the surface.These atoms form tenacious oxides that mask the surface and prevent oxidation of the iron. The process known as passivation for stainless steel, is a common means of improving this protective oxide layer through the use of oxidizing acids.The corrosion resistance of stainless steel depends on the Chromium. 

Why does manufacturers coat inner walls of BF3 counters with activated charcoal?

The charcoal coating is used for tubes filled with BF3 gas and also for He3 filled counters operated in high neutron fluxes. The activated charcoal serves to absorb electronegative gases that build up during nuclear irradiation.

For instance, in a BF3 filled detector three fluorine atoms are released with each neutron capture.The Fluorine atoms will combine with electrons released in subsequent neutron captures. Initially, this process reduces electric pulse amplitude and eventually output pulses are eliminated altogether.

Difference between Ionizing radiation & Non-ionizing radiation

Well as we all know that the term "ionization" refers to dissociating electrons from its atom. In chemistry terminology we do refer as first ionization potential referring to removal of electron from outermost orbit(nth orbit) , second ionization potential refers to removal of electrons from second orbit form (n-1) and so on. Ionization radiation is the flux of energetic particles emanating from various nuclear and atomic transitions capable of ionizing atoms in a medium. Of course the extent of ionization depends on the materialistic medium on which these particles impinges.Ionization gets influenced by physical parameters like charge, mass and energy of incident particles. This is one of the reason that uncharged particles cannot ionize.

Some of the ionizing radiation for your notice are alpha, beta, and many other charged elementary particles.

Non-ionizing radiation on other hand is just converse of the above. The particles which are incapable of doing ionization in a medium are referred to as non-ionization radiation. It is worth to note an important point that energy needed to cause ionization pair in most of the gas medium and human tissues is of order of few electron volt.

Hence most of the non-ionizing radiation comprises of uncharged particle. For instance, Neutron even with high energies cannot ionize the medium.            

Most people confuse about gamma by considering it as ionization particle but the fact is that it doesn't ionize directly. In fact it interacts with medium in 3 types namely photo-electric effect, Compton effect and pair production in which energetic electron gets liberated which actually causes the ionization.    

After differentiating the above differences let us see the advantages of these radiations.

Advantages of ionization radiation:-

i) Treatment of cancer in patients. The utilisation of radiation therapy is increasing world wide and about 30 % of cancers are getting cured with the treatment. Innovation in the technology is rising up like proton therapy to reduce the dose and damage to the normal cells.    

ii) Food sterilisation

iii) Accelerators for  producing radioactive isotopes

iv) research applications like study of elementary particles

Advantages of non-ionization radiation:-

Just observe your sorroundings. The mobile which you are using, microwave oven, laser pointers, fluorescent bulbs etc all of these based on non ionizing radiation.

And finally the most important thing to mention is that ionization most of the radiation detectors work on principle of ionization as ionization radiation do have more impact on human tissue.

What ever it may be, either the ionization or non-ionization radiation is beneficial to mankind if used in a innovative technological ways.  

What is Radiation?

Well, some times  I see many people struggle to answer basic questions even after going through toughest parts of the concept. This is the reason that I try to focus on such basic questions.

Radiation, is a flux of energetic particles emanating from matter due to Nuclear or Atomic transitions.People always discuss about three important radiations Alpha, Beta, Gamma but the term radiation comprises vast range of energetic particles.

You might have seen different terms used for the word as "background radiation", "extra terrestrial radiation", "Solar radiation" etc.

Background radiation refers to the flux of particles reaching surface of earth due to certain events like explosions in stars etc which is referred to as cosmic radiation or extra terrestrial radiation.

of course solar radiation as name indicates has its origin from events happening in sun. UV, Infrared rays from sun fall in this category.

You can see below some types of Radiation

 EB refers to electron beam.

The picture shows only some types of radiation but actual list is vast.


 

How a GM tube operates?

The counter is named as GM counter based on its developers ‘Geiger’ and ‘Muller’ in 1928. They are oldest type of gas filled radiation detectors.GM counters were operated in the Geiger discharge region of gas filled ion chambers.

Construction: - The counter is usually a leak tight assembly of two electrodes electrically isolated. The counter is filled to sub atmospheric pressures of few mm of mercury. A high voltage is applied to the anode electrode. 

Principle: - This counter also works on the principle of ionization caused by incoming energetic particle in the gas medium filled between anode and cathode. The electron liberated in the primary ionization event would get accelerated towards anode because of its high potential. The electron may gain sufficient energy to cause ionization of other gas molecule. This leads to a chain of ionizing events which is usually referred to as Townsend avalanche. During this process, there may be interactions in which excitation of atoms may occur due to sufficient energy of impinging electrons. Such atoms while de-exciting may emit photons which normally fall in UV or visible region. These photons which are emitted may again lead to photo electrons due to ionization of gas atoms or due to photoelectric interaction with walls of counter. Each photo electron would again cause Townsend effect. Such a series of Townsend avalanches would lead to discharge in the tube called Geiger-discharge. In such a state there is formation of dense envelope of electron-ion pairs distributed on either side of anode.

The voltage applied to anode shall be such that it is enough to trigger the avalanche mechanism and collect total charge (electrons) pertaining to single event leading to Geiger discharge.

Concept of quenching: - Practically the process would not be as simple as above. During the Geiger discharge, there is dense envelope of electrons and ions. The electrons would drift towards anode and positive ions would drift towards cathode. The positive ions which drift towards cathode having ionization potential (E) greater than the work function (W) of cathode material leads to exchange of electron from cathode and becomes neutral. The excess energy may be dissipated in two forms, one by emission of photon or an electron form cathode if excess energy is greater than the work function of the cathode material. This would again initiate another Geiger discharge. The result of this is that the tube would always be in continuous Geiger discharge and hence will not able to measure any radiation.

To overcome this problem, concept of quenching is introduced. There are two types of quenching

i)                   Organic quenching

ii)                 Halogen quenching

Organic quenching: -

This involves addition of small quantity of organic gas having complex molecule structure. This prevents the continuous Geiger discharge mechanism by charge transfer collision principle.   The positive ions on their path collide with organic molecules to get neutralized. This makes only ions of organic gas reach cathode and gets neutralized. If there is any excess energy released leads to dissociation of organic molecules. Thus multiple Geiger discharges could be avoided.

A typical filling of organic quenched GM tubes is 90% Argon and 10% of ethyl alcohol. When organic gas gets depleted to a sufficient extent there is occurrence of multiple discharges frequently and thus the plateau length gets decreased, with slope increased.

Thus the organic quenched GM tubes are characterized by short life time and thus not suitable for operation in very high fields which leads to large count rate.  To overcome this, technique of Halogen quenching is introduced.

Halogen quenching: - This involves the addition of small quantity of Halogen gas such as Chlorine or Bromine. A typical filling is about 0.1% of chlorine to Neon. The quenching action is same as that in Organic quenching process. The diatomic halogen gas molecules too gets dissociated in quenching but gets recombined to replenish the gas molecules and thus counter life gets extended.

Characteristics of GM tubes:-

The important parameters which decide the quality of functioning of Gm tubes are

i)                   Dead time

ii)                 Recovery time

iii)               Plateau length &Plateau slope

i)           Dead time: - As discussed above, the positive ions take considerable time to reach cathode tube compared to electrons. The reason is that the mobility of electrons is about 1000 times greater than that of electrons.

Due to the low drift velocity of positive ions, there is formation of cloud of positive ions which tend to electric field opposite to that of actual field. This reduces the electric field intensity due to anode potential and thus affects gas multiplication factor. This in turn affects the pulse heights.       

             In high count rates, it is more worse that there is formation of dense  positive cloud which makes the electric field intensity in the vicinity of anode wire reduce by great margin thus multiplication goes down by big margin. During this phase of detector, any new ionizing event caused by incoming particle cannot be recorded. Thus the time interval during which any event caused by newly incoming particle would not get counted and called as dead time of the country.

ii)            Recovery time: - After certain time, all the positive ions tend to reach cathode wall and thus the electric field begins to restore to actual value. When the electric field goes beyond a critical value there is again formation for pulses. But the process requires some time to give maximum pulse heights. Hence the total time required for GM tube to give maximum pulse height pulses is Recovery time.

iii)           Plateau length & Slope: In order to decide the operating voltage of the GM tube, a graph between anode voltage (X axis) and count rate (Y axis) is plotted. After applying minimum voltage to initiate Geiger discharge, the no. of pulses shall remain same in fixed radiation field exposure. But due to formation of short pulses during recovery time there is variation in count rate. Thus one of the quality parameters deciding the operation of GM tube is that plateau slope shall be less. Usually 2-3% plateau slope is a good choice.  As we go on applying voltage to the anode, the tube starts entering continuous   discharge region. Thus the slope gets worsened. The region or length of voltage region during which the plateau slope remains in desired value is called as plateau length and usually the operating voltage is chosen at the midpoint of plateau length.

You can download complete details of these counters here.  

Difference between Voltage & Current

Usually Voltage is the unit for electric potential, of course in real world we always deal with potential difference and not a single potential. Potential is the capacity to do work in a field. The field may be gravitational where it is called gravitational potential, electric field where it is called electrostatic potential and so on.   

In electrostatics there should always exist a potential difference to move charge hence the work done to move unit charge from one place to another in the electric field is called potential difference. The magnitude of potential difference is measured in "VOLTS".

  

                  The basic definition of "current" is flow of charge in unit time. For flow of charge, there should exist a potential difference. Electrons always move from lower to higher potential and thus a potential difference is required.As every material has some finite resistance to flow of charge, magnitude of current (flow of number of electrons per second) would be decided by magnitude of potential difference. 

Conclusion:  Voltage is measure of potential difference which is responsible for current i.e flow of charge.            

Why do mirrors show left as right and not top as bottom?

It is a misconception that most of the students pose the question as above.

Mirror only shows the reflected image of the object before it. 

This is more confused when we think of example of ourselves

standing before the mirror. It is the bi-symmetry of the human being which is exactly reflected in mirror as our left being right and right being left. Hence we get confused with it. To be more clear the light reflected from our left side will be falling on the mirror's right side relatively.Where as reflected light from top falls on the top side of mirror and from bottom falls on mirror's bottom side. Hence we see that top is not reversed to bottom.

   

 

Fukushima reactor blast in Japan

Japan suffered disaster aftermath of the destruction made by Tsunami due to failure of its Nuclear reactors.

The failure of cooling system is the reason for explosions in the reactors. We know that the enormous heat generated during fission process is compensated by coolant under flow in reactors.

The failure of cooling system called as Loss of coolant accident would lead to severe effects in reactor. Even after shutdown there is decay heat which shall be cooled to protect the core of the reactor.

In failure of this the pressure inside the core may increase by many folds and result in blast. The consequence is the escape of radioactive gases containing traces of Iodine, Cesium-137 etc which pose serious threat to health of people in the vicinity of reactor.

Already it is announced that the natural background in Japan is increased by many folds giving us a serious alarm..

Technology is evidently a best resource for development but at the same time periodic review is must to tackle such disasters.

Hopefully the entire world would wake up to improve the existing system of technology for eliminating such disasters in the future.  
 



 

Internal energy

Two solid copper spheres, having different radii, undergo the same temperature change. A student states that the change in internal energy of the two objects would be the same. Briefly discuss this statement.

Internal energy  is the stored energy and is not apparently shown by any system. The energy is possessed by the system due to its molecular constitution and the position of atoms/molecules. In the above query, though the sizes are different the change in temperature is uniform and the change in energy at atomic/molecular level is same and hence the change in internal energy would be same. This is because the atomic structure is same in both cases.
So effect of temperature is same in both cases. 

Speed of Light faster than sound

Sound is generated due to vibrations in atoms/molecules in a medium and thus always require materialistic medium. The physical parameters like Temperature, Pressure etc., effects the sound velocity.

On the other hand Light doesn't require a materialistic medium. It transmits in space through field.

The concept of field is still a mysterious thing to explain meticulously.

Light is transmitted by 'photons' which are treated as zero mass entity. The speed of sound is negligible when compared to speed of light. This is one of the reasons that while thunderbolts, we see flashing light fast and then hear sound. This is a best example.

In fact light falls in different spectrums and light which gets generated at a point travels through the universe changing its presence int he spectrum.