The deexcitation or quenching of the metastable 2S state of atomic hydrogen in collision with atoms and molecules has been studied using a beam-attenuation method in conjunction with a time-of-flight technique at velocities between 0.4)& 10' and 4 X 10 cm/sec (0.08 and 8 eV). In this regime, transfer of the metastable to the 2P state of hydrogen, followed by radiative decay to the ground state, is the dominant destruction mechanism. Absolute cross sections are reported for the quenching of H(2S) atoms in collision with the noble gases (helium-xenon), with molecules that have permanent electric-quadrupole moments (hydrogen and nitrogen), and with molecules that have permanent electric-dipole moments (ammonia, methanol, and acetone). For molecules with dipole moments, the cross sections are on the order of 10 ' cm and vary approximately as v ', For the noble gases and the quadrupole-moment molecules, the cross sections are on the order of 10 " cm' and vary approximately as v " where 0.3 & n & 0.7. Measurements of the relative cross section for the production of ultraviolet radiation in collision with nitrogen and argon are reported, and the cross sections for the quenching of H(2S) and D(2S) in argon are compared. Data for the noble gases indicate that large-angle elastic scattering is probably not responsible for the discrepancy between theory and experiment. The data for molecular hydrogen suggest that short-range forces are important in collisions with molecules possessing a quadrupole rnornent. I. INIODUCTIONThe long-lived 2S metastable state of atomic hydrogen is easily deexcited or quenched by collisions with other atoms and molecules. These collisions are of particular interest since the cross section for this quenching process can be calculated from first principles and compared to an absolute measurement of the inelastic scattering. It can be shown using the adiabatic criterion that at low kinetic energies (&50 eV) the physics of the collision is dominated by the presence of the nearby 2P states of hydrogen which are nearly degenerate with the 2S metastable state. A perturbing electric field during the collision can cause an admixture of the 2S and 2P states by the Stark effect. After the collision, therefore, there is a finite probability that the hydrogen atom has made a transition to the 2P state, in which event the atom decays to the 1S ground state emitting a Lyman-n photon. The metastable has thus been quenched (or inelastically scattered) in the reaction H(2S) + X-H(2P) + X -H(1S) + h v+ X.On the other hand, if the atom remains in the 2S state after the collision, the metastable will have received a net impulse from the gradient of the electric field during the collision and thus be deflected or elastically scattered. Elastic and inelastic scattering are therefore complementary processes; the relationship between the two depends on the relative velocity in the collision and the nature of the perturbing electric field.In the low-energy regime where the 2P states dominate the inelastic scattering, col...

External beam proton-induced X-ray emission (PIXE) analysis has been used to verify the presence of lead in the finger bone of a murder victim. The deceased, who had been buried several years, was known to have suffered a bullet wound to his right hand several yers before death. X-ray radiographs of the right second proximal phalanx revealed the possible presence of metal fragments below the surface of the bone. To verify the presence of lead in a nondestructive manner, the bone was scanned with a 1.5-MeV proton beam. PIXE analysis showed that lead was present only in the vicinity of the fragments previously detected in the radiographs. A study of gunshot residue in bone shows that the distribution of lead around the bullet hole is independent of the firing distance for distances greater than 0.6 m.

We describe a solid-state mutual inductance bridge suitable for low-temperature thermometry and susceptibility measurements. The design features an electronically simulated variable mutual inductance. Particular care is taken to reduce spurious capacitive effects. The bridge is highly stable and linear. It has a resolution of 10(-4) over the range 15 microH to 150 mH, and a noise limited sensitivity of 55 I(p) (-1) nH with a l-s time constant, where I(p) is the rms primary current in mA. Application to thermometry is discussed.