Question

A physicist has developed a device that generates a beam of alpha particles (each of which is a helium nucleus, consisting of two protons, two neutrons, and no electrons). The device ejects a steady stream of 4910 alpha particles per second. You are testing different shielding materials for a test chamber that will house the alpha particle generator. You prepare a sheet of one potential material in an electrically neutral state, mount it on a perfectly insulating stand, and fire the alpha particle device at it for 17.3 seconds. Afterward, you measure that the charge of the sheet is 6.39 × 10-15 coulombs. Assume that none of the alpha particles bounced back from the sheet, meaning that the alpha particles either were captured by the shielding or penetrated completely through the shielding. What percentage of the alpha particles penetrated completely through the shielding material?

Answer 1

As device ejects a 4910 alpha particles/seconds,

and we fire the alpha particle device at sheet of potential material for 17.3 seconds

Hence,

Total no. of alfa particles emmited in 17.3 second =4910 (particles/sec)*17.3 sec = 84943 particles

84943 alpha particles are emmited in 17.3 seconds.

now,

we know each alpha particle has 2 protons and 2 neutrons.

Therefore,

Total charge on alpha particle =(2*charge on proton+2*charge on neutron)

=(2*1.6*10^-19C+2*0) = 3.2*10^-19C

.........( charge on proton = 1.6*10^-19C & charge on neutron is 0.)

Now we calculate total number of alpha particles captured,

alpha particles captured = 6.39*10^-15 C * (1 alpha particle / total charge on alpha particle)

= 6.39*10^-15 C * (1 alpha particle / 3.2*10^-19 C)

alpha particles captured = 2*10⁴ alpha particles

Therefore,

Percentage of the alpha particles penetrated completely through the shilding material is

percentage of alpha particles penetrated through shielding material is = 76.45 %.