A freshly prepared radioactive sample of half-life 4 hours emits radiation of intensity which is 64 times the safe level. The minimum hours after which it would be safe to work with it is

• 4

• 6

• 24

• 16

A radioactive decay can form an isotope of the original nucleus with the emission of particles

• one α and four β

• one α and two β

• one α and one β

• four α and one β

A radioactive sample at any instant has its disintegration rate 5000 disintegrations per minute. After 5 min, the rate becomes 1250 disintegration per minute. Then, its decay constant (per minute) is

• 0.8 log_e 2

• 0.4 log_e 2

• 0.2 log_e 2

• 0.1 log_e 2

The distance of closest approach of an α-particle fired towards a nucleus with momentum p, is r. If the momentum of the α-particle is 2p, the corresponding distance of closest approach is

• $\frac{\mathrm{r}}{2}$

• 2r

• $\frac{\mathrm{r}}{4}$

• 4r

If the binding energy per nucleon of deuteron is 1.115 MeV, its mass defect in atomic mass unit is

• 0.0048

• 0.0024

• 0.0012

• 0.0006

The set which represents the isotope, isobar and isotone respectively is

• (₁H², ₁H³), (₇₉Au197, ₈₀Hg¹⁹⁸) and (₂He³, ₁H²)

• ( ₂He³, ₁H¹), (₇₉Au¹⁹⁷, ₈₀Hg¹⁹⁸) and (₁H¹, ₁H³)

• (₂He³, ₁H³), (₁H², ₁H³) and ( ₇₉Au¹⁹⁷, ₈₀Hg¹⁹⁸)

• (₁H², ₁H³), (₂He³, ₁H³) and (₇₉Au¹⁹⁷, ₈₀Hg¹⁹⁸)

Two samples X and Y contain equal amount of radioactive substances. If $\frac{1}{16}$th of the sample X and $\frac{1}{256}$th of the sample Y, remain after 8 h, then the ratio of half periods of X and Y is

• 2 : 1

• 1 : 2

• 1 : 4

• 1 : 16

Radioactive ${}_{27}{}^{60}\mathrm{Co}$ is transformed into stable ${}_{28}{}^{60}\mathrm{Ni}$ by emitting two γ-rays of energies

• 1.33 MeV and 1.17 MeV in succession

• 1.17 MeV and 1.33 MeV in succession

• 1.37 MeV and 1.13 MeV in succession

• 1.13 MeV and 1.37 MeV in succession

The activity of a radioactive element decreases to one-third of the original activity A₀ in a period of 9 yr. After a further lapes of 9 yr, its activity will be

• A₀

• $\frac{2}{3}{\mathrm{A}}_0$

• $\frac{{\mathrm{A}}_0}{9}$

• $\frac{{\mathrm{A}}_0}{6}$

Two nucleons are at a separation of 1 fermi. The net force between them is F₁ if both are neutrons, F₂ if both are protons and F₃ if one is proton and the other is a neutron. Then

• F₁ > F₂ > F₃

• F₁ = F₃ > F₂

• F₂ > F₁ > F₃

• F₁ = F₂ > F₃