Number of nuclei of a radioactive substance at time t = O are 200

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281.

In a mean life of radioactive sample :

  • about 13 of substance disintegrates

  • about  23 of substance disintegrates

  • about 90% of the substance disintegrates

  • almost all the substance disintegrates


282.

If the half-life of any sample of a radioactive substance is 4 days, then the fraction of the sample will remain undecayed after 2 days, will be

  • 2

  • 12

  • 2 - 12

  • 12


283.

For the stability of any nucleus:

  • binding energy per nucleon will be more

  • binding energy per nucleon will be less

  • no. of electrons will be more

  • none of these


284.

Nuclear fusion is common to the pairs :

  • thermonuclear reactor, uranium based nuclear reactor

  • energy production in sun, uranium-based nuclear reactor

  • energy production in sun, hydrogen bomb

  • the disintegration of heavy nuclei, hydrogen bomb


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285.

A radioactive substance containing N atoms emits n α - particles per second. Half-life of the substance is :

  • Nn sec

  • nN sec

  • 0.693nN sec

  • 0.693Nn sec


286.

In a radioactive decay,

           92U238 α  X  α  ZYA

The values of Z and A

  • 92 , 236

  • 88 , 230

  • 90 , 130

  • 88 , 234


287.

In a sample of radioactive substance, what percentage decays in one mean life time?

  • 69.3 %

  • 63 %

  • 50 %

  • 36 %


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288.

Number of nuclei of a radioactive substance at time t = O are 2000 and 1800 at time t = 2s. Number of nuclei left after t = 6s is

  • 1442

  • 1554

  • 1652

  • 1458


D.

1458

From           N = N0eλt 

             1800 = 2000 eλ × 2

                    910 = e-2λe-λ = 9101/2

Number of nuclei left after 6s,

                   N = N0eλt'

                      = 2000 eλ × 6

                          = 2000 (eλ )6

                      = 2000 9101/26

                      = 2000 × 9103

                      = 2000 × 7291000

                      = 1458


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289.

92U238 on absorbing a neutron goes over to 92U239. This nucleus emits an electron to go over to neptunium which on further emitting an electron goes over to plutonium. The resulting plutonium can be expressed as

  • 94Pu239

  • 90Pu219

  • 93Pu240

  • 92Pu240


290.

The radius of nucleus of silver (atomic number = 47) is 3.4 x 10-14 m. The electric potential on the surface of nucleus is (e = 1.6 x 10-19 C)

  • 1.99 x 106 V

  • 2.9 x 106 V

  • 4.99 × 106 V

  • 0.99 x 10V


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