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Sunday, May 20, 2012

What are PROMPT and DELAYED Neutrons in Nuclear Physics?

In the process of fission, the capture of a Neutron leads to formation of excited compound nucleus and thus breaks into two nuclear fragments having excess neutrons and energy of about 8 MeV which is sufficient to expel neutrons. Such neutrons which gets emitted in time scale of order 10^-14 sec are called prompt neutrons. They have energies of order of few MeV generally 1-2MeV.

     On the other hand, some fission fragments decay in various modes to become stable. In this process, fragments which undergo beta decay, some times forms product nucleus left in excited state with an energy more than average binding energy which then emits neutron to reach stable state. Aas this neutron emission follows beta decay of preceding nucleus and so neutron activity of that element will have some apparent half life due to beta activity of parent nuclide, usually of order of mSec to few Sec. This type of emission is called Delayed Neutron Emission and neutrons emitted are called delayed neutrons.     

You can see the decay scheme for two well known fission fragments Br-87 & I-137 exhibiting delayed neutron activity.

DELAYED NEUTRON EMISSION

 

(Click on images to Zoom)




Saturday, May 19, 2012

ELECTROMAGNETIC SPECTRUM

Gamma Rays:

Wavelength Range:  from 0.0001 to 1Angstroms.

Production: Nuclear origin. Emitted on disintegration of nuclei of atoms.

Properties: Highly penetrating and uncharged. Exhibits fluoroscence, Phosphoroscence, ionization and chemical reaction on photographic plates.

Application: Gives information about structure of nuclei.

X-Rays:

Wavelength Range:  from 1 to 100 Angstroms.

Production: by striking high speed electrons on heavy target.


Properties: All properties of gamma rays holds good for these rays also, but less penetrating.


Application: helpful in medical diagnosis, study of crystal structure. 

Ultraviolet Rays:

Wavelength Range:  from 100 to 4000 Angstroms.

Production: by sun, arc, spark and ionized gases.

Properties:  All properties of X-rays but less penetrating. They produce photoelectric effect.

Application: used in medical applications. Detection of finger prints, forged documents. 


Visible Region:

Wavelength Range:  from 4000 to 7800 Angstroms.

Production: radiated from ionized gases and incandescent bodies.

Properties:  ehibit reflection, refraction, interference, diffraction, polarization, photoelectric effect.

Application: used in LASER technology.

Infrared radiation:

Wavelength Range:  from 7800 to 0.001 meter.

Production: by hot bodies.

Properties:  heating effect on thermopiles and bolometer. Exhibit refelction, refraction, and photographic emulsion.

Application: used in industry, astronomy & medicine etc.

Hertzian waves:

Wavelength Range:  from 0.001 to 1 meter.

Production: by spark discharge, by electronic devices such as Klystron & magnetron.

Properties:  reflection, refraction & diffraction. Produces spark in gaps of receiving circuits.

Application: used in radar and Masers and also to reveal finer details of atomic and molecular structure.

Radio waves:

Wavelength Range:  from 1 to 10000 meter.

Production: oscillating circuits and electronic devices.

Properties:  reflection, refraction & diffraction.


Application: used in television and radio broadcast system.