Reaction Dynamics Group, University of Hawaii at Manoa

Experimental Setup

The carbon source

The development of the beam in time

1. The Carbon Source

Supersonic dicarbon beams, for example, are generated in the primary source via la- ser ablation of graphite. The 30Hz, 5-40 mJ, 266 nm output of a Spectra Physics GCR-270-30 Nd:YAG laser is focused on a rotating carbon rod. Ablated dicarbon molecules are seeded into argon, neon or helium gas released by a Proch-Trickl pulsed valve. The Proch-Trickl pulsed valve itself is operated with a frequency of 60Hz and pulse lenghts of 80µs. A skimmer of 1mm diameter separates the source chamber from the main scattering chamber. A four slot chopper wheel (not shown in the adjacent pictures) rotating at 120Hz or 240Hz is mounted in the main chamber directly after the skimmer. This chopper wheel selects a segment of the seeded ablation beam. By adjusting the delay time between the chopper wheel triger pulse (given by an IR diode) and the pulsed valve it is possible to select the velocity of the dicarbon beam between 800m/s and about 4200m/s. The atoms (or molecules) in the seeded beam do not have all the same velocity, but instead a narrow distribution of velocities around a center velocity. Due to this velocity-spread, the beam deforms on its way from the pulsed valve to the detector (or interaction region), which is shown in the adjacent animation.

Typical vacuum system of a crossed beams machine
pressure in detector
detector background

2. The Vacuum System

The vacuum system consistes of four diffe- rent pumping systems: the pumps for the evacuation of the two source chambers and the main chamber, the pumping system for the laser port, the gas feedline pumping systems and the pumps for the detector. The main chamber pumping system consists of three 2000l Osaka turbo pumps backed by a Scroll pump; the source chambers have each a 400l and a 2000l turbo pump backed by a Roots and a DryVac pump. Interlock vacuum gauges and pneumatic valves are included in the pumping lines between the backing and turbo pumps. To reach the extreme low pressures in the detector, it is differentially pumped by three Osaka turbo pumps which are all backed by a fourth turbo pump and an oil free scroll pump. To decrease the background level further, we included a helium refrigerator (cold head) in the ionizer region of the detector. In operation, we reach typical pressures of about 8*10^-13 Torr. The adjacent figure shows the background masses with the cold head on and off.

Schematic setup of the detector

The detector at work
3. The Detector

Reactively scattered species are moni- tored using a differentially pumped detec- tor, rotatable in the plane of the beams with respect to the interaction region. The laboratory angle of the detector can be va- ried (avi or mov ) with respect to the carbon beam. Differentially pumped regions I and II reduce the gas load from the maincham- ber, whereas region III contains the Brink- type electron impact ionizer and the helium cold head, surrounded by a liquid nitrogen cold shield. Following the ionizer there are (in region II) the ion extractor, the quadru- pole mass filter (QMS) and the Daly-type scintillation particle detector. This scintil- lation detector itself consists of the so called Daly-knob and a photomultiplier tube (PMT) covered by a scintillating material. The species reaching region III of the de- tector through the aperture are ionized by electron bombardment and extracted into the quadrupole mass spectrometer. Those ions which survive the travel through the QMS are accetelated (25kV) toward the Daly-knob and cause electrons to be re- leased at their impact. These electrons in turn are accelerated toward the PMT and are detected.

The carbon source The development of the beam in time	1. The Carbon Source Supersonic dicarbon beams, for example, are generated in the primary source via la- ser ablation of graphite. The 30Hz, 5-40 mJ, 266 nm output of a Spectra Physics GCR-270-30 Nd:YAG laser is focused on a rotating carbon rod. Ablated dicarbon molecules are seeded into argon, neon or helium gas released by a Proch-Trickl pulsed valve. The Proch-Trickl pulsed valve itself is operated with a frequency of 60Hz and pulse lenghts of 80µs. A skimmer of 1mm diameter separates the source chamber from the main scattering chamber. A four slot chopper wheel (not shown in the adjacent pictures) rotating at 120Hz or 240Hz is mounted in the main chamber directly after the skimmer. This chopper wheel selects a segment of the seeded ablation beam. By adjusting the delay time between the chopper wheel triger pulse (given by an IR diode) and the pulsed valve it is possible to select the velocity of the dicarbon beam between 800m/s and about 4200m/s. The atoms (or molecules) in the seeded beam do not have all the same velocity, but instead a narrow distribution of velocities around a center velocity. Due to this velocity-spread, the beam deforms on its way from the pulsed valve to the detector (or interaction region), which is shown in the adjacent animation.
Typical vacuum system of a crossed beams machine detector background	2. The Vacuum System The vacuum system consistes of four diffe- rent pumping systems: the pumps for the evacuation of the two source chambers and the main chamber, the pumping system for the laser port, the gas feedline pumping systems and the pumps for the detector. The main chamber pumping system consists of three 2000l Osaka turbo pumps backed by a Scroll pump; the source chambers have each a 400l and a 2000l turbo pump backed by a Roots and a DryVac pump. Interlock vacuum gauges and pneumatic valves are included in the pumping lines between the backing and turbo pumps. To reach the extreme low pressures in the detector, it is differentially pumped by three Osaka turbo pumps which are all backed by a fourth turbo pump and an oil free scroll pump. To decrease the background level further, we included a helium refrigerator (cold head) in the ionizer region of the detector. In operation, we reach typical pressures of about 8*10^-13 Torr. The adjacent figure shows the background masses with the cold head on and off.
Schematic setup of the detector The detector at work	3. The Detector Reactively scattered species are moni- tored using a differentially pumped detec- tor, rotatable in the plane of the beams with respect to the interaction region. The laboratory angle of the detector can be va- ried (avi or mov ) with respect to the carbon beam. Differentially pumped regions I and II reduce the gas load from the maincham- ber, whereas region III contains the Brink- type electron impact ionizer and the helium cold head, surrounded by a liquid nitrogen cold shield. Following the ionizer there are (in region II) the ion extractor, the quadru- pole mass filter (QMS) and the Daly-type scintillation particle detector. This scintil- lation detector itself consists of the so called Daly-knob and a photomultiplier tube (PMT) covered by a scintillating material. The species reaching region III of the de- tector through the aperture are ionized by electron bombardment and extracted into the quadrupole mass spectrometer. Those ions which survive the travel through the QMS are accetelated (25kV) toward the Daly-knob and cause electrons to be re- leased at their impact. These electrons in turn are accelerated toward the PMT and are detected.