Nuclear Physics : Some questions ??

Question

In my Nuclear Power Plants lecture some students put a few questions. I asked them to allow me some time to answer. Can some one give me right answers ? Questions are as follows :-

If time rate of flow of elctrons through a given cross section of a conductor is known as Electron current, then what shall we call " The Time Rate of flow of Protons ? Can we make Protons to flow through a metalic conductor ? Justify your answer.

Here we know that it is the time rate of flow of Free elctrons through the conductors that is termed as "Electron Current". It may be pleaded that protons cannot be obtained as " Free Protons " to flow through; but is it really so that we can't obtain Free Protons that can flow ?

Answer 1

Proton pumping in thylakoid membranes and backflow of protons through the active ATP synthase CF0-CF1 (where CF0 is the proton channel and CF1 is the catalytic portion) were investigated by flash spectrophotometry. A steady pH difference across the membrane was generated by continuous measuring light, supplemented by voltage transients that were generated by flashing light. In the presence of Pi and ADP, the electric potential transients elicited transients of proton flow via CF0-CF1, typically 1.3 H+ per CF1 and flash group. Proton flow was blocked by CF0-CF1 inhibitors: N,N′-dicyclohexylcarbodiimide, acting on the channel component CF0, and tentoxin, acting on the catalytic component CF1. The half-rise time was 40 ms in 1H2O and 78 ms in 2H2O. ATP synthesis under conditions of flashing light and transient proton flow was characterized by a Km(Pi) of only 14 μ M, contrasting with a Km of several hundred micromolar for continuous ATP synthesis at high rate. This might reflect a resistance to Pi diffusion. The degree of proton delocalization in the chemiosmotic coupling between redox reactions and ATP synthesis is under debate. In thylakoids, it has been proposed that intramembrane proton buffering domains act as ducts for protons between pumps and ATP synthases. In this work, transient proton flow by way of CF0-CF1 was completely tracked from the lumen, across the membrane, and into the suspending medium. Proton uptake from the lumen and charge flow across the membrane occurred synchronously and in stoichiometric proportion. The uptake of protons from the lumen by CF0-CF1, half completed in 40 ms, was preceded by release of protons from water oxidation into the lumen, half completed in 1 ms. Hence, pumps and ATP synthases were coupled through the lumen without involvement of intramembrane domains.  Hope I helped.

Answer 2

In nuclear power plants the flow of neutrons (not protons) is moderated to make them flow a thermal velocity (2200 m/s or from Maxwell's distribution v0² = 2kT/m) or at velocities commensurate with  temperature of 293 K (room temperature). The neutrons have an area probability of fission known as the cross-section 'σ', or to be absorbed, or of being scattered by a U235 or Pu239 nucleus and this is expressed in units of the barn (10^-28 m²). The drift of the neutrons is normally called the diffusion flux, and the flux distribution for thermal neutrons is given by: -

n(v)vdv = 4nv³exp(-v²/v0²)dv/(v0³√π)

Where v is the neutron's velocity with v0 as the thermal velocity, with n(v) as the neutron density.

Thus, the reaction rate of fission is given by: -

dN/dt = n(v)vσ(fission)(v)N

Where ‘N’ is the number of nuclei in the target sample.

The rate of neutron diffusion, in a given direction ((say) the x-direction), across or into a region of different neutron density - is given by Fick's Law: -

J = -Ddn/dx

Where 'J' is the current density and D is the diffusion coefficient.

From the kinetic theory of gases, and here the neutrons are a gas of sorts, the mean free path 'λ' of a neutron between scatterings is given by: -

Λ(transport)=1/Nσ(transport)

Where N is the number of nuclei per cubic metre.

Hence, the diffusion coefficient, for a given neutron velocity v, can be shown to be expressed as: -

D = (1/3).λ(transport).v

I hope this very, brief survey of neutron (reactor) physics is of some use.

Turning, now, to protons beams! The L(arge) H(adron) C(ollider) at CERN (LHC for short) will soon be colliding bunched beams of protons - head on. The flow rate of the proton beams is called the flux and the number of particles per beam bunch is referred to as the luminosity, which is a measure of the reaction rate or cross-section.

Answer 3

The basic statement is  "the flow of CHARGE through a given cross-section is known as an electric current".  This applies to any charges, electrons, protons, ions or any charged particle of matter.  Protons cannot flow through metallic conductors, but they can flow through a vacuum and some accelerators use proton beams.

The reason protons cannot flow through conductors is that their energy gap is too large.  The protons are too tightly bound to the nucleus to break free under the influence of thermal or electrical forces.