# Law of Radioactivity

## Trending Questions

**Q.**The half-life of a radioactive substance is 30 minutes. The time (in minutes) taken between 40% decay and 85% decay of the same radioactive substance is

- 60
- 15
- 30
- 45

**Q.**Radioactive material A has decay constant 8λ and material B has decay constant λ. Initially they have same number of nuclei. After what time, the ratio of number of nuclei of material B to that A will be 1e?

- 19λ
- 1λ
- 17λ
- 18λ

**Q.**the half life of a radioactive nucleus is 50 days. the time interval (t2-t1) between the time t2 when 2/3 of it has decayed and the time t1 when 1/3 of it had decayed as

**Q.**

X and Y are two radioactive substances having ${\mathrm{N}}_{1}\mathrm{and}{\mathrm{N}}_{2}$nuclei. The half-life of X is the half-life of Y.After three half lives of Y , The ratio of ${\mathrm{N}}_{1}\mathrm{and}{\mathrm{N}}_{2}$will be equal to?

$\frac{8}{1}$

$\frac{1}{8}$

$\frac{3}{1}$

$\frac{1}{3}$

**Q.**5.) The half life of radioactive radon is 3.8 days. The time at the end of which (1/20)th of radon sample will remain undecayed (given log10e= .4343) is

**Q.**The binding energy per nucleon for C12 is 7.68 MeV and that for C13 is 7.5 MeV. The energy required to remove a neutron from C13 is

- 5.34 MeV
- 5.5 MeV
- 9.5 MeV
- 9.34 MeV

**Q.**In a radioactive sample 4019K nuclei either decay into stable 4020Ca nuclei with decay constant 4.5×10−10 per year or into stable 4018Ar nuclei with decay constant 0.5×10−10 per year. Given that in this sample all the stable 4020Ca and 4018Ar nuclei are produced by the 4019K nuclei only. In time t×109 years, if the ratio of the sum of stable 4020Ca and 4018Ar nuclei to the radioactive 4019K nuclei is 99, the value of t will be? [Given: In 10=2.303]

- 1.15
- 9.2
- 2.3
- 4.6

**Q.**

Two radioactive substances A and B have decay constants $5\lambda $ and $\lambda $ respectively. At $t=0$ they have the same number of nuclei. Find the time interval at which the ratio of the number of nuclei of A to those of B will be $\frac{1}{{e}^{2}}$.

**Q.**200 MeV of energy may be obtained per atom, in the nuclear fission reaction of U235. A nuclear reactor is generating a constant power of 1000 kW. The number of atoms undergoing fission per second is,

- 3.125×1016
- 6.25×1016
- 1.25×1016
- 2.5×1016

**Q.**Half-life of Bi210 is 5 days. If we start with 50, 000 atoms of this isotope, the number of atoms left after 10 days is

- 5, 000
- 25, 000
- 12, 500
- 20, 000

**Q.**Ten percent of a radioactive sample has decayed in 1 day. After 2 days, the decayed percentage of nuclei will be

- 81%
- 19%
- 20%
- 50%

**Q.**A freshly prepared radioactive source of half life 2 hours 30 minutes emits radiation which is 64 times the permissible safe level. The minimum time, after which it would be possible to work safely with source, will be

**Q.**Two radioactive materials A and B have decay constants 10 λ and λ, respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of A to that of B will be 1e after a time:

- 19λ
- 110λ
- 111λ
- 1110λ

**Q.**The activity of a radioactive material is 6.4×10–4 curie. Its half life is 5 days. The activity will become 5×10–6 curie after

- 25 days
- 35 days
- 15 days
- 7 days

**Q.**In 4g atom of Ag, calculate 1. Amount of Ag 2. Weight of one atom of Ag.

**Q.**The half-life of 215 At is 100 μs. The time taken for the radioactivity of a sample of 215 At to decay to 116th of its initial value is

- 300μs
- 400μs

- 6.3μs

- 40μs

**Q.**A radioactive nucleus A with a half-life T, decays into a nucleus B. At t=0, there is no nucleus B. At some time t, the ratio of the number of B to that of A is 0.3. Then, t is given by:

- t=Tlog(1.3)
- t=Tlog1.3log2
- t=Tlog(1.3)
- t=Tlog(1.3)

**Q.**

Radioactivity 131I has a half-life of 8.0 days. A sample containing 131I has activity 20μCi (a) What is its activity at t = 4.0 days ? (b) What is its decay constant at t = 4.0 days ?

**Q.**The radioactive sources A and B of half lives of 2 hr and 4 hr respectively, initially contain the same number of radioactive atoms. At the end of 2 hours, their rates of disintegration are in the ratio :

- 4:1
- 2:1
- √2:1
- 1:1

**Q.**

The activity of a radioactive substance falls from 700 s^{-1} to 500 s^{-1} in 30 minutes. Its half-life is close to

$66min$

$62min$

$52min$

$72min$

**Q.**What is the mass of one Curie ofU234

- 3.7×1010gm

- 1.44×10−11gm

- 2.348×1023gm

- 6.25×10−34gm

**Q.**

What is ${}^{n}C_{r}$ in probability?

**Q.**The radioactivity of a sample is X at time t1 and Y and a time t2. If the mean life time of the specimen is τ, the number of atoms that have disintigrated in the time interval t1−t2 is

- Xt1−Yt2
- X−Y
- X−Yτ
- (X−Y)τ

**Q.**

Natural water contains a small amont of tritium (31H). This isotope beta-decays with a half-life of 12.5 years. A mountaineer while climbing towards a difficult peak finds debris of some earlier unsuccessful attempt. Among other things he finds a sealed bottle of whisky. On return he analyses the whisky and finds that it contains only 1.5 per cent of the (31H). radioactivity as compared to a recently purchased bottle marked '8 years old'. Estimate the time of that unsuccessful attempt.

**Q.**Some nuclei of a radioactive material are undergoing radioactive decay. The time gap between the instances when a quarter of the nuclei have decayed and when half of the nuclei have decayed is given as :

(where λ is the decay constant)

- ln(2)λ
- ln(32)λ
- 2 ln(2)λ
- 12ln(2)λ

**Q.**

The wavelength of the radiation emitted by a body depends upon

the nature of its surface

the temperature of its surface

all the above factors

the area of its surface

**Q.**What is the mass of KClO3 that will liberate 11.35litre O2 at STP?

**Q.**

90% of the active nuclei present in a radioactive sample are found to remain undecayed after 1 day. The percentage of undecayed nuclei left after two days will be

- 85 %
- 81 %
- 80 %
- 79 %

**Q.**In an ore containing uranium, the ratio of U−238 to Pb−206 is 3. Assuming that all the lead present in the ore is the final stable product of U−238. If age of the ore is 1.868×10n years. The value of the n is

(Take the half life of U−238 to be 4.5×109 years. (ln43=0.2876)

**Q.**half-life of 1 month has thea331. A radioactive sample withlabel: Activity=2 microcurie on 1-3-2013. What was its(AIPMT)activity before 2 months?(b) 4 microcuriefa) 8 microcurie(d) 0.5 microcurie(c) 1 microcurie