怎样设置设置粒子源在几何体中的随机位置

Question

// ge xiang tong xing
G4double Position[3];
G4double R = 0.5 * micrometer;

G4double th = acos(1.0-2*G4UniformRand());
G4double ph =G4UniformRand()*2.0*CLHEP::pi;

Position[0] = sin(th)*cos(ph);
Position[1] = sin(th)*sin(ph);
Position[2] = cos(th);

G4ThreeVector positionPri = G4ThreeVector(Position[0],Position[1],Position[2]);
//define particle position
fpParticleGun->SetParticlePosition(R*positionPri);

请问有谁能解释一下 G4double th = acos(1.0-2*G4UniformRand());
G4double ph =G4UniformRand()2.0CLHEP::pi；这两句的意思吗？看不明白