Question

A radioactive material has a mean lives of 1620 year and 660 year for aand b emission respectively. The material decay by simultaneous aand bemission. The time in which 1/4^th

of the material remains intact is-

• A. 4675 year
• B. 720 year
• C. 650 year
• D. 324 year

Answer 1

Answer: (C)

650 year

Answer 2

t=$\frac { \tau _ { 1 } \tau _ { 2 } } { \tau _ { 1 } + \tau _ { 2 } }$=$\frac { 1620 \times 660 } { 2280 }$ = 469 $\frac { \mathrm { N } } { 4 }$ = t = 2T = 2 t ln 2

= 2 × 0.693× 469

= 650 years