A certain radioactive element has a half-life of 20 years. If we have a block with 10 g of the element in it, after how many years will there be just 2.5 g of the element in the block?

• 80 years

• 40 years

• 100 years

• 60 years

The activity of a radioactive sample decreases to
(1/3)^rd of its original value in 3 days. Then, in 9 days its activity will become

• (1/27) of the original value

• (1/9) of the original value

• (1/18) of the original value

• (1/3) of the original value

If r₁ and r₂  are the radii of the atomic nuclei of mass numbers 64 and 125 respectively, then the ratio (r_1/r₂) is

• 64/125

• $\sqrt{\frac{64}{125}}$

• 5/4

• 4/5

A radioactive element forms its own isotope after 3 consecutive disintegrations. The particles emitted are

• 3 β-particles

• 2 β-particles and 1α-particle

• 2 -βparticles and 1 γ-particle

• 2 α-particles and 1 β-particle

A radioactive substance contains 10000 nuclei and its half-life period is 20 days. The number of nuclei present at the end of 10 days is

• 7070

• 9000

• 8000

• 7500

A radioisotope has a half-life of 5 yr. The fraction of the atoms of this material that would decay in 15 yr will be

• 1

• 2/3

• 7/8

• 5/8

A particle of mass m at rest decays into two particles of masses m₁ and m₂ having non zero velocities. The ratio of de-Broglie wavelengths of the particles $\frac{{\mathrm{\lambda }}_1}{{\mathrm{\lambda }}_2}$ is

• m₁/m₂

• m₂/m₁

• 1.0

• $\sqrt{\frac{{\mathrm{m}}_2}{{\mathrm{m}}_1}}$

Which of the following nuclei is fissionable but not possible?

• ${}_{92}\mathrm{U}^{235}$

• ${}_{92}\mathrm{U}^{233}$

• ${}_{92}\mathrm{U}^{238}$

• ${}_{94}\mathrm{Pu}^{239}$

279.

In nuclear reaction

${}_{72}{}^{180}\mathrm{X} \stackrel{-\mathrm{\alpha }}{\to } \mathrm{Y} \stackrel{-\mathrm{\beta }}{\to } \mathrm{Z} \stackrel{-\mathrm{\alpha }}{\to } \mathrm{A} \stackrel{-\mathrm{\gamma }}{\to } \mathrm{P}$

the atomic mass and number of P are, respectively:

• 170,69

• 172,69

• 172,70

• 170,70

A radioactive substance has activity 64 times higher than the required normal level. If T_1/2 = 2h, then the time, after which it should be possible to work with it, is:

• 16 h

• 6 h

• 10 h

• 12 h