TRIUMF Safety Analysis Report for Experiment 1023, Fusion of 11Li with 70Zn

August 24, 2007

TRIUMF Technical and Safety Report for Experiment 964 The 8Li(α,n)11B reaction at astrophysically relevant energies

Report concerns the test run for the TACTIC (TRIUMF Annular Chamber for Tracking and Identification of Charged particles) chamber using a stable beam of 11B ions. August 28 to September 3, 2007

Safety Coordinator

Patrick Walden TRIUMF Phone: 604-222-7340 Email: [email protected]

Introduction This experiment will measure the total cross section of the 8Li(α,n)11B reaction as well as the partial cross sections for the population of excited states in 11B. This reaction rate is important as it plays a role in seeding the r-process nucleosynthesis in core collapse supernovae, and predicts the light element abundances from inhomogeneous big bang (IBB) models. The later is back in vogue as a result of fallout from the analysis of the WMAP results. This reaction is one of the few ways of bridging the mass 8 gap and its cross section must be accurately known regardless of current theoretical thought. Previous measurements have produced inconsistent results and have been unable to study the lowest resonances of interest. This experiment consists of a specially designed cylindrically symmetric ion chamber (TACTIC) surrounded by an array of γ-ray detectors. The ion chamber tracks charged scattered particles and reaction products in its cylindrical drift region that surrounds the beam cylinder tube. 11B from the 8Li(α,n)11B reaction will be tracked and identified in the chamber and coincident γ-rays will be tagged in the surrounding detectors.

For this run, the newly built TACTIC chamber will be tested for the first time with actual beam. The ion beam will be stable 11B(2+) accelerated to energies which would be typical for 11B emerging from the 8Li(α,n)11B reaction. The reaction we will be looking at in this case is the abundant elastic scattering from the chamber gas (90% He and 10% CO2 by volume) notably from 4He nuclei. There is the possibility, if the 11B tests go well, of running with a 13C beam, which is also a stable, to see actual (α,n) reaction products from 13C(α,n)16O.

Technical discussion and beam tuning The experimental setup is shown in the accompanying figure 1. The location of TACTIC is at the TUDA experimental target location. The TACTIC facility can be removed and exchanged with the TUDA facility. There are locating pins in the floor to put TACTIC

of 7 back aligned. A sketch of the internal TACTIC components for the purposes of explaining how it works (fig. 1) plus a cut away scale figure of TACTIC from solid works (fig.2) can be seen in a preliminary document being prepared for this run by Götz Ruprecht. Pertinent numerical values describing the important dimensions of the chamber are also to be found in this document. This document is attached to this report.

The TACTIC chamber is adjacent both directly upstream and directly downstream to two regular ISAC beam line diagnostic boxes. Both boxes have mounted to it a TUDA style linear drive attached to a target ladder. The upstream box has a viewing port. Both target ladders will be outfitted with a series of collimators, and the upstream ladder will have a scintillator by which the beam can be viewed. The downstream box will mount the faraday cup previously used as the beam dump for HEBT3. The diagnostic boxes mount the vacuum system components, and each element of the setup, the 2 diagnostic boxes and the TACTIC chamber, are connected vacuum-wise via flexible stainless steel hoses. The chamber gas system is connected to the vacuum system.

Figure 1. The TACTIC experimental setup

For the initial tuning the chamber will be replaced with a tube 50mm in diameter. The diameter of the tubes connecting TACTIC to the diagnostic boxes shown is 16mm. Two collimators of equal diameter will be positioned at both the upstream and downstream

of 7 ladders. The beam will be tuned to maximize the transmission through to the faraday cup. The size of the collimators will be reduced until the desired tune is achieved. The diameter of the waist can be confirmed by the limiting collimator size pertinent to the beam energy and desired waist diameter. For a 2mm waist and beam energies of 5.5 and 13 MeV these spot sizes are 7.1 and 4.9 mm respectively using a normalized beam emittance of 0.2 π mm·mrad. The spot size can also be viewed on the upstream scintillator. The distance between the target ladders, as printed in figure 1, is 44 in. or 1118 mm. The beam energies requested will be 13 and 5.5 MeV 11B as stated above. The beam can be tuned with whatever intensities are required to establish a tune. A sketch of TACTIC with all the pertinent dimensions required for beam tuning is shown in fig. 2. This sketch is also attached separately

Figure 2. dimensions for beam tuning purposes

Once the waist is established the TACTIC chamber will be installed. It is mounted on a rugged 3-point stand, commonly used on the TRIUMF and the ISAC beamlines, such that it can be removed and replaced without having to check the alignment. As stated in the Ruprecht document, the tubes or flutes holding the chamber entrance and exit windows are 12mm in diameter. The active length of the TACTIC chamber is 240mm positioned centrally between the diagnostic boxes. However the flutes (see figure 2 ) restrict the length exposed to the beam. This length can be asymmetric with respect to the TACTIC centre. In the present run the entrance and exit windows will come at 455 and 510mm from the upstream collimator. The exit will occur slightly upstream of the middle of the chamber which is 559mm from the collimator. However the waist will move upstream between the windows due to the multiple scattering in the windows and chamber gas. The diameter and thicknesses of the windows have not yet been set. At the present moment, a 5mm square 500nm thick Si3N4 window is being considered for the entrance window and a 5mm diameter 1000nm thick mylar window is being considered for the exit.

To check the tune, beam will be run though the chamber while it is evacuated and the HV is off. It is probable that up to 1nA can be run through the chamber. If tuned to a 2mm waist, the beam spot sizes on both the exit and entrance windows will be under 3mm. For

of 7 13 MeV most of the beam will make it to the faraday cup while for 5.5 MeV the beam envelope will blow up to fill the 16mm diameter beam line at approximately 170mm downstream of the TACTIC active length. For this run we are not concerned about losing beam. We are checking out the operation of TACTIC and are not making a cross section measurement.

After the check of the tune, the chamber will be filled with gas. If tuned to a 2mm waist, the beam spot sizes on both the entrance and exit windows for both 5.5 and 13 MeV will increase but remain under 3.5mm. However the beam spot sizes will blow up considerably downstream of the exit window due to multiple scattering in the gas. For 13MeV there could still be a substantial amount of beam current making it to the faraday cup as the beam envelope will not fill the 16mm diameter beam pipe For the 5.5 MeV beam there is no possibility of seeing much beam. The envelope exceeds that of the beam diameter 6cm downstream of the active length of the chamber. However it is a moot point. Due to the expected event rate and amount of data per event, the beam flux will be kept in the 104 pps region until we can ascertain if we can handle the data flow. We will require the pepperpots. In any event the requested beam will most likely not exceed 106 pps. At these fluxes it is not expected to able to measure any current in the faraday cup. A TACTIC event rate will be sent to the ISAC control room.

It is not envisioned that there will be a call on very much of the TRIUMF technical resources except for assistance in alignment of the diagnostic boxes and the chamber. There will be the usual rush jobs in the machine shop as the facility goes together.

There may be an issue with the data rates coming from TACTIC; one conservative estimate is about 40 Mbytes/s. The data transfer from the TUDA shack over the optical fibre connection will be strained. Other solutions may have to be incorporated and will require TRIUMF technical assistance. Solutions for this run could compromise the electrical isolation of the TUDA shack, however this could probably be tolerated over the week of the run. An optical coupling solution eventually would have to be installed.

Vacuum and Gas Handling System The vacuum and gas handling system (VGHS) is shown in figure 3. The vacuum system is basically the same as that for TUDA except that instead of the cryopump being hooked up through RV2, the gas handling system (GHS) is hooked up. Because of the similarities the EPICS interlocks are almost identical.

The GHS is not hooked up to EPICS, but runs by itself. The mass flow controller (FC) opens and restricts the valve according whether the pressure measured in the chamber (PA) falls within the preset limits. The rest of the GHS system is manual.

The pressure differential gauge (PD) is hooked up to EPICS. Whenever the differential pressure set points are exceeded, valve RV2 will be opened in order to equalize the pressure across the chamber entrance and exit windows. The mass flow controller will be hard wired shut whenever RV2 is open. This will prevent chamber gas being dumped into

of 7 the vacuum system while pumping. This is the protection against popping chamber windows. If they do pop, the ISAC-I fast acting valve will close and GV1 will be shut.

Figure 3. The TACTIC vacuum and gas handling system

Safety Concerns There are no real safety issues for this current run. The requested beam is stable and will be at a low flux. The beams, beam energies, and fluxes requested for this run are shown in table I. Except for 11B scattering on 4He at 13 MeV, beam energies will be below the coulomb barrier and thus there will be no concern regarding isotope activation. The coulomb barrier energies in MeV/u for 11B on various targets are given in table II.

Table I ion 11B 11B 13C Energy 13 5 13 MeV E/A 1.182 0.455 1.000 MeV/u intensity tuning >1 >1 >1 nA no gas ≤1 ≤1 ≤1 nA gas 104 104 104 pps min gas 105 105 105 pps max pressure 400 200 400 mbar

of 7 Beam Request and Chamber operating pressures

The 4He case needs only to be considered when TACTIC has been filled with gas. However in this case activation such as neutron production will not be measurable because the target is quite thin (722 μg/cm2) and the beam fluxes will be kept low so that the DAQ can handle the data rates. For 13C on 4He, the coulomb barrier comes at 1.0781 MeV/u which is above our beam request.

The experimental data runs will call for beams of 8Li, 9Be, 7Be, and 12C. These runs will also use different gas mixtures such as H2 and isobutane. The safety assessment for each of these beams and gas mixtures will be made as the runs are scheduled.

Table II target Ec/A 1H 1.8195 4He 0.9680 12C 1.2537 14N 1.3270 16O 1.4003 28Si 1.8240 56Fe 2.5298 Coulomb barriers for 11B (MeV/u)

The chamber gas, a mixture of 90% He and 10% CO2, is not flammable, not toxic and is mostly lighter than air so that there are no explosion, poisoning or asphyxiation concerns. The HV power supplies are current limited and the HV is routed through the appropriate cables and connectors. There is no possibility of receiving an electric shock much less a debilitating shock. With this said there are no other outstanding safety issues pertinent to this test run.

Conclusion The safety concerns for this TACTIC test run are quite minimal. Technical problems do not overly involve TRIUMF except for possibly the data rate issue. The TACTIC facility is new and can be expected to have its share of initial start up problems as the run proceeds, but these should be resolvable. A 3D rendering of the TACTIC facility is given in figure 4.

Attachments 1. TacticRunReport.pdf (preliminary): useful information for experimenters. 2. Tactic_setup.pdf : layout of the experimental setup with dimensions 3. kirchner_report.pdf: preliminary design note. 4. debrecen_2006.pdf: Debrecen conference submission Eur. Phys. J. A 27, s01, 315–320 (2006) 5. NIM_A_573_306_2007.pdf: journal article NIM A 573 306–309 (2007)

of 7 Figure 4. 3D projection of the TACTIC facility with its stand