Modern Physics For Scientists And Engineers - 4 Edition - Chapter 13 - Problem 48
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# The Los Angeles County Police want to use neutronactivation analysis to look for a tiny

Modern Physics for Scientists and Engineers | 4th Edition

Problem 48

The Los Angeles County Police want to use neutronactivation analysis to look for a tiny residue of bariumin gunpowder. The suspected residue is placed in anuclear reactor, where it is activated by the neutronfl ux. Natural barium contains 71.7% 138Ba. The emitter 139Ba is produced in the 138Ba(n, )139Ba reaction.The half-life of 139Ba is 83.1 min. 139Ba beta decaysto 139La, 72% going to the ground state and 27%going to the fi rst excited state at 0.166 MeV. Scientiststhink they need a count rate for the 166-keV ray(decay to the ground state) of at least 1000 Bq 30 minafter the residue is removed from the reactor in orderto make a positive identifi cation of barium. (a) Howmany 139Ba nuclei must be present at the end of theactivation? (Remember the decay and fraction goingto the fi rst excited state.) (b) How many grams of 139Ba must be produced? If the original amount ofbarium was 0.01 g, what fraction of the 138Ba wasactivated?

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Some on test 2 notes GW1:block mass m1=10 at rest on rough horiz surface.Connected to hanging mass m2=15 by massless string across massless frictionless pulley.Speed of blocks after released and fallen d=1.5m if coeff btwn block 1 and surface is uk=0.5 Same question w angular speed of pulley Ex2) hoop w radius 0.5 and m=2.Rolls w/out slipping 1.5m down plane inclined at 30 deg above horiz.Speed of hoop at bottom Angular speed at bottom • KEtrans- if obj center of mass moving • KErot- if obj it rotating about its center of mass • Work done by constant force/torque. Usually cos180/0(+1,-1) • PEg-When height of center of mass moves (Peg=mgh) h in cm • Static eq- when all F=0(in x & y..Trans eq) andT=0 (torques) probs:

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