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ALSNews           Vol. 72           March 5, 1997

Table of Contents


    1. OPERATIONS UPDATE
    2. FRAGMENTATION STUDIES REVEAL UNUSUAL DECAY CHANNEL
    3. UEC and USERS' TOWN MEETING MARCH 7
    4. DIRECTOR'S REPORT MARCH 6 -- ALL ARE WELCOME
    5. LONG-TERM SCHEDULE SET THROUGH DECEMBER 1997

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 86.6% overall and 87.7% for user shifts. These figures were affected by repairs to the power supply for the quadrupole defocusing magnets in the booster ring and to the water pump for the storage ring's rf system early in the week of February 17th. The longer fill time required when operating in "camshaft" mode also affected the available beamtime.

Operations Summary for March 5 - March 24

Mar 05, 00:00-08:00           User scrubbing & special operations
                              (1.5 & 1.9-GeV multibunch operations)
Mar 05, 08:00- Mar 10, 07:15  1.5-GeV/400-mA/288-bunch user operations
                                with "camshaft" fill pattern
Mar 10, 07:30-24:00           Maintenance & startup
Mar 11, 00:00-24:00           Accelerator physics
Mar 12, 00:00-08:00           User scrubbing & special operations
Mar 12, 08:00- Mar 17, 07:15  1.5-GeV/400-mA/288-bunch user operations
Mar 17, 07:30- Mar 18, 24:00  Maintenance & startup
Mar 19, 00:00-24:00           Accelerator physics
Mar 20, 00:00-08:00           User scrubbing & special operations
Mar 20, 08:00- Mar 24, 07:15  1.9-GeV/340-mA/288-bunch user operations
                                with "camshaft" fill pattern

** NEW STANDARD MULTIBUNCH FILL PATTERN -- 288 BUNCHES **
The standard multibunch fill pattern for the storage ring is now 288 bunches for both 1.5-GeV and 1.9-GeV operations. This change brings new functionality to the ALS, meeting the special timing needs of users on Beamlines 9.0.2 and 10.3.2 without affecting other users. With 288 bunches, the spacing between bunches is large enough to insert a 20-mA "camshaft" spike with a gap before and after the spike when requested.

2. FRAGMENTATION STUDIES REVEAL UNUSUAL DECAY CHANNEL
(Contact: David Hansen, hansen@nevada.edu)

Researchers at Beamline 9.3.1 have illuminated how molecules interact with hard x rays by studying the x-ray-induced fragmentation of simple molecules. They chose to study hydrogen chloride (HCl) gas because, as a two-atom system, HCl provides a simple model. By analyzing HCl, they are laying the groundwork for greater scientific understanding of how hard x rays interact with more complex molecules, such as DNA. The researchers found that, when the energy of the exciting photons matched a resonance energy (the energy needed to excite an electron from a lower energy level to a higher one), production of neutral hydrogen was a major decay pathway. The finding that a substantial amount of un-ionized hydrogen survived was a bit of a surprise, since the x rays had enough energy to remove up to seven electrons from the chlorine atoms. The researchers likened it to smashing a watch with a sledgehammer only to discover that the glass cover remained whole.

The actual experiment, of course, involved no sledgehammers. Instead, pulses of synchrotron light were used to excite inner-shell electrons in the HCl gas molecules. The molecules could then decay (relax) by several different channels, leading to different decay products. The researchers used ion time-of-flight spectroscopy to identify the products. With time-of-flight detection, crisp start and stop signals (specific events with a known relationship to the movement of an ion) can be used to calculate how long the ion took to move from the point where it was formed to the detector. From this time of flight, one can calculate the ion's mass-to-charge ratio and from the ratio identify the type of ion.

When the x-ray energy was on a resonance below the chlorine 1s threshold, the researchers observed several effects. The strongest was a sharp decrease in the amount of hydrogen ion (H+) produced. The yield dropped from 50% of the total ions produced off resonance to 35% on resonance. This result can be explained by the greater production of neutral hydrogen once a few alternate possibilities are ruled out. First, the decrease in H+ yield could not have been the result of hydrogen ions being "lost" in the analyzer, because it occurred only on resonance. Second, formation of a molecular ion (HCl+) could account for a decrease in detected H+, but the measured yield of molecular ion was always less than 1%, not enough to account for the difference in H+ yield. The remaining alternative is the production of neutral, excited HCl. Even this is unlikely, however, because this product could only be formed by radiative decay (where an excited molecule loses energy by giving up photons), and the typical rate of radiative decay for HCl is not enough to account for the difference in H+ yield. Another notable result when operating on resonance was an increase in the detected amount of chlorine ion with a +3 charge (Cl+3). The researchers therefore surmise that formation of Cl+3 is correlated with the formation of neutral hydrogen.

For these studies, the researchers took advantage of the ALS's "timing" mode of operation, in which only two bunches of electrons circle the storage ring, creating pulses of synchrotron light about 328 nanoseconds apart. Measurements in this mode used the arrival of an ion at the detector as a start signal and the following synchrotron pulse as a stop signal. The short bunches (100 ps or less) produced by the ALS allowed a very high overall resolution (200 ps) and put the researchers in the unusual (but pleasing) situation of having their resolution limited more by their detector than by bunch length. Further studies at the National Synchrotron Light Source, which produced results consistent with the formation of neutral hydrogen on resonance, did not use the timing mode of operation. Instead, the arrivals of two ions produced in the same fragmentation event served as start and stop signals.

This work was conducted by D.W. Lindle (principal investigator) and D.L. Hansen (University of Nevada Las Vegas), S.B. Whitfield (University of Wisconsin Eau Claire), M. Simon (UniversitŽ Paris-Sud), P. Neill and J. Cotter (University of Nevada Reno), Y. Uehara (Mitsubishi Electric Corp.), R.C.C. Perera and G.R. Fisher (Berkeley Lab), J.C. Levin and I.A. Sellin (University of Tennessee Knoxville), and T. Leung (University of Waterloo, Canada) at the x-ray atomic and molecular spectroscopy (XAMS) endstation on Beamline 9.3.1.
Funding: National Science Foundation, Nevada DOE Experimental Program to Stimulate Competitive Research, Research Corporation, and the Petroleum Research Fund.

3. UEC and USERS' TOWN MEETING MARCH 7
(contact: terminello1@llnl.gov)

The next meeting of the ALS Users' Executive Committee (UEC) is scheduled for March 7. Afterward, the UEC and members of ALS management will meet with users in a Users' Town Meeting from 3:00 to 4:30 p.m. in the Building 4 conference room. Discussion topics for the Town Meeting include shutdown planning, operations, the independent investigator process, and accelerator physics and beam stability. Users are invited to come with comments or requests. Please send any issues you may wish to discuss at the meeting to ALS Users' Executive Committee Chair Lou Terminello (terminello1@llnl.gov) or to Fred Schlachter (fred_schlachter@lbl.gov) in advance of the meeting.

4. DIRECTOR'S REPORT MARCH 6 -- ALL ARE WELCOME

Tomorrow, March 6, at 10:30 a.m. in the Building 50 auditorium, ALS Director Brian Kincaid will present his Director's Report to ALS staff and other interested members of the ALS community. This lively, frank presentation and business meeting will cover several areas, including -- The fiscal 1998 budget and its impact on the ALS: outlook following the State of the Union Address -- Recent accomplishments of ALS groups -- Plans for new projects -- The large number of important reviews of the ALS to occur this year

Participants will also be welcome to ask questions or bring up other topics for discussion with ALS management. Coffee, tea, and soft drinks will be provided.

5. LONG-TERM SCHEDULE SET THROUGH DECEMBER 1997
(contact: fred_schlachter@lbl.gov)

The long-term schedule for May-December 1997 has been confirmed and will be distributed to ALS users this week. The schedule is unchanged from the draft schedule that has been circulated for comment, except that it extends one month further because of a decision to postpone the next major shutdown until January 2, 1997.

A summary of the new schedule, giving the type of operations scheduled for each week, follows. The table is best viewed in a monospaced font. The first part of each heading is the storage ring energy in GeV. The second part specifies the bunch pattern: MB for multibunch (288 electron bunches) or 2B for two-bunch mode. For example, 1.9/2B refers to 1.9-GeV operation with two electron bunches. Accelerator physics time, maintenance/startup, and holidays occupy the time not accounted for in the "Dates" column, as well as some shifts on the first and last days of most periods listed.

Dates (MM/DD)  1.5/MB    1.9/MB    1.9/2B    Shutdown

05/01-06/09                                  Shutdown/startup
06/10-06/16    1.5/MB
06/18-06/23    1.5/MB
06/25-06/30              1.9/MB
07/02-07/03              1.9/MB
07/05-07/07              1.9/MB
07/09-07/14              1.9/MB
07/16-07/21              1.9/MB
07/24-07/28    1.5/MB
07/30-08/04    1.5/MB
08/06-08/11              1.9/MB
08/13-08/18                        1.9/2B
08/20-08/25                        1.9/2B
08/27-08/31              1.9/MB
09/04-09/08              1.9/MB
09/10-09/15              1.9/MB
09/17-09/22              1.9/MB
09/25-09/29    1.5/MB
10/01-10/06    1.5/MB
10/08-10/13              1.9/MB
10/15-10/20              1.9/MB
10/22-10/27              1.9/MB
10/29-11/03    1.5/MB
11/05-11/10    1.5/MB
11/12-11/17              1.9/MB
11/20-11/26              1.9/MB
12/03-12/08              1.9/MB
12/10-12/15              1.9/MB
12/17-12/23              1.9/MB