News

TRIUMF has long been addressing big questions about the origins of matter in our universe by studying the interactions among elementary particles or essential nuclei.  The DRAGON experiment at TRIUMF is an apparatus designed to measure the rates of nuclear reactions that are important in astrophysics and the formation of the chemical elements. The big question we are asking is, "Where do the elements around us come from?" and "What happens inside a supernova and what does it produce?"  One new experiments at TRIUMF, S1227, recently looked at a process that creates lithium and neutrinos within ancient stars.

Researchers working at DRAGON have successfully measured the rate of the 3He + 4He -> 7Be + γ radiative capture reaction, an important reaction in various areas of nuclear astrophysics.  The 7Li observed in ancient stars was created via the radioactive decay of 7Be nuclei formed in the 3He + 4He -> 7Be + γ reaction just minutes after the big bang.  For this reason, measurements of the reaction rate are an important step in resolving the discrepancy between the big bang nucleosynthesis prediction of 7Li abundances and astronomical observations.  In addition, 7Be decay is one process by which stars produce neutrinos.  This process even occurs within our own Sun, so the 3He + 4He ->7Be + γ measurement will lead to a better understanding of the solar neutrinos reaching us on Earth.

TRIUMF experiment S1227 was performed from September 8 to September 14, 2011.  The measurement was done using the DRAGON recoil separator, during which a 4He beam bombarded a 3He gas target.  The 7Be recoils were then separated from the incoming beam particles using DRAGON's world-record beam suppression capabilities and subsequently detected at the focal plane using a silicon detector sensitive to the energy and position of the incident particle.  The 3He + 4He -> 7Be + γ reaction rate was measured at 3 different relative energies (Erel = 1.5 MeV, 2.2 MeV and 2.8 MeV) and a total of approximately 100,000 7Be recoils were collected, a number never reached in previous DRAGON runs.  The completion of the experiment at these energies will add another reaction rate measurement in an energy range where previously only two discrepant measurements existed.

Another first for DRAGON was the use of pure 3He gas in the target chamber, a gas whose use in US homeland security applications has made it both difficult and expensive to obtain.  A complex 3He recycling system was created prior to the September run and successfully operated during the 3He + 4He -> 7Be + γ reaction rate measurement.  It allowed us to retain ~5/6 of our $15 000 3He inventory, leaving a large portion of 3He gas available for future measurements at DRAGON.  In completing this 3He + 4He -> 7Be + γ reaction rate measurement, DRAGON and its collaborators continue to stay on the forefront of measurements important in nuclear astrophysics.

-- by Sarah Reeve, SFU MSc Student

DRAGON achieved two records earlier this year with the highest mass proton capture reaction measured at the facility immediately followed by the highest mass beam delivered to the facility. These measurements were part of tests designed to demonstrate the feasibility of a measurement of the ⁷⁶Se(a,g)⁸⁰Kr reaction, which would be the highest mass capture reaction ever measured at DRAGON and a very technically challenging experiment. 

A technical description of the tests follows, taken from a report issued to TRIUMF's Experimental Evaluation Committee in summer 2011: 

⁵⁸Ni run

In early April of 2011, DRAGON took 12 shifts of beamtime in order to study the feasibility of measurements at high mass and high charge state. 8 shifts of ⁵⁸Ni beam at 1.42 MeV/u were taken in order to (a) determine the magnitude of the beam suppression of DRAGON for a proton capture reaction that has roughly the same Dp/p (~1%) for neighboring charge states as the proposed ⁷⁶Se(a,g)⁸⁰Kr reaction (as it is expected that the dominant source of background will be neighboring charge states scattered into the energy acceptance of the separator), (b) detect ⁵⁹Cu recoils at the end of the separator using local time-of-flight and DE-E ionization chamber methodology as would be used in the final experiment, and (c) re-familiarize the group with the use of post-target silicon-nitride charge state booster foils required to achieve charge-states high enough to overcome the rigidity limitations of the separator. 

The ⁵⁸Ni beam was delivered at q=10+ with an intensity of 700 epA (4.4E8 pps). Transmission through the DRAGON gas target was 100%, and an operating pressure of 6 Torr H₂ was used. A 50 nm thick SiN charge state booster foil was used on the downstream side of the gas target enabling q=20+ beam to be bent round the first dipole magnet to the energy dispersed focus. The separator was then scaled for ⁵⁹Cu recoils and the leaky beam rate at different separator slit settings was investigated.

Initially, with no local time-of-flight system in place, and regular DRAGON (large acceptance) slit settings, it was found that the rate of leaky beam particles reaching the focal plane detector was 3 kHz for this selected recoil charge state and beam intensity. With a modest reduction in mass slit settings to 10mm, far above what would be close to cropping recoil transmission efficiency, this was reduced to 60% of that rate. With the MCP local-TOF system in place the rate was reduced to 300 Hz, implying that the leaky beam is dispersed in the horizontal plane and more intense on the low energy side of that plane, thus being mostly blocked by the 1” aperture of the initial MCP foil (note that recoils are focused to a small cone angle at this point so are not in danger of being cropped by this aperture), equivalent to using the final slits for suppression purposes.

Further tests with charge slits and mass slits were performed to optimize the reduction in leaky beam particles without a subsequent reduction in recoil transmission. In this way optimum settings of 20 mm and 8 mm were found for the horizontal charge slits and mass slits respectively. It was found that at 6 mm the mass slits began to cut into recoil transmission at the level of about 33%, close to what is predicted by simulation. The maximum recoil cone angle of the ⁵⁸Ni(p,g)⁵⁹Cu reaction is around 1.6 mrad, well below the 20 mrad acceptance of DRAGON.

Analysis of focal plane energy spectra showed qualitatively that indeed there were several different leaky beam peaks, expected to be the peaks from neighboring charge states to the selected one, and that these were dominating the rate.

Eventually, leaky beam rates were found to be on the order of 5 Hz. This means that if the beam intensity were scaled up by around 500 (as required for the proposal), the rates for this reaction would still be within the envelope that the DRAGON DAQ system can handle.

Detection of Recoils

Fig. 1 shows the separator time-of-flight between a detected particle at the focal plane in coincidence with a gamma ray at the target position, with no other cuts applied, for a selection of the ⁵⁸Ni data. It clearly shows a sharp peak corresponding to ⁵⁹Cu recoils (the smaller, broader peak is caused by events which are detected by the Ionization Chamber but not the MCP, thus having worse timing resolution and an offset, reflecting the efficiency of the MCP). The time-uncorrelated leaky beam events can be seen as a flat background, showing the true nature of the excellent beam suppression in coincidence mode of DRAGON.

Figure 1: time-of-flight between detected particle at DRAGON focal plane and coincident gamma ray at target position. X-axis range is 10 microseconds.

Rough numbers have been extracted for the overall beam suppression performance of DRAGON during this run. They are:

Average beam suppression in ‘singles’ mode ~ 1.2E8

Average beam suppression in ‘coincidence’ mode ~ 3.0E10.

The conclusions from this run are:

(a) the leaky beam rate per incident beam particle for this reaction, with similar Dp/p to the proposed ⁷⁶Se reaction is sufficiently small that a scale up to the ⁷⁶Se experiment at higher intensity is not deemed problematic;

(b) the coincidence mode beam suppression is extremely good, better than anticipated, at this high mass (it should be noted that at this point, ⁵⁸Ni was the highest mass beam ever delivered to DRAGON).

⁸⁴Kr charge-state distributions and suppression tests

In addition to the ⁵⁸Ni beam, a 1.2 enA ⁸⁴Kr 15+ (5E8 pps) beam of 1.25 MeV/u was delivered to DRAGON and sent through the gas target with the SiN booster foil inserted and helium gas used. Charge state fractions for q=23,24,25,26 were measured. The results are shown in table 1. The measurement was repeated with a beam energy of 1.46 MeV/u. The measured charge state fractions are lower than previously calculated. However, the conditioning tests indicate that higher than anticipated fields are feasible, thus allowing us to use a lower charge state than expected (23+ or 24+). As the charge state fraction is higher for lower charge states, there is an effective increase in yield.

Table 1: Measured Kr charge state fractions at 1.25 MeV/u after SiN booster foil.

The Electric Dipole maximum voltage achieved during this run was 210 kV (although it had previously reached 220 kV in a conditioning test). This enabled us to then scale the separator for ⁸⁸Sr recoils that would be present if we were trying to measure the ⁸⁴Kr(a,g)⁸⁸Sr reaction. We used a beam energy to 1.25 MeV/u for this test and could select q=25 in the separator settings. The purpose of this test was to determine what the beam suppression was at the settings for these even higher mass recoils. The result was a measured suppression of ~1.1E9 in singles mode, and 1.4E10 in coincidence mode. Again, these numbers show sufficient ‘raw’ (meaning no further cuts applied) suppression in order to perform a measurement of the ⁷⁶Se reaction at similar energies. The leaky rate here was much lower than for the nickel runs (~0.5 Hz) and there was little indication of background from lower charge states (presumably due to these charge states being outside the envelope of DRAGON). These conditions are similar to those of the ⁸⁴Kr(a,g)⁸⁸Sr experiment (i.e. close to the limits of DRAGON) and the leaky rate here is thus more representative of expected running conditions. Scaling up the beam intensity to 2.5E11 pps would thus suggest a singles leaky rate of around 250 Hz, well within the DAQ capabilities.

Conclusions

Both tests demonstrate that the DRAGON separator has sufficient suppression to perform the ⁷⁶Se(a,g)⁸⁰Kr experiment as proposed and so stage 2 approval is requested at this time. Development of a ⁷⁶Se beam of the necessary ( 2.5E11 pps) intensity is required and it is requested that resources are allocated to allow this be done during schedule 120. The first measurements of the ⁷⁶Se(a,g)⁸⁰Kr experiments could then be undertaken in the spring of 2012 before RIB is available.

(Vancouver, BC) – Boldly going where the universe has not gone before, scientists at the CERN laboratory near Geneva, Switzerland have succeeded in capturing anti-matter atoms. In a paper published today in Nature, physicists of the ALPHA Collaboration, including key Canadian contributors, describe how they succeeded in containing for the first time atoms of antihydrogen, the antimatter partner of ordinary hydrogen. This breakthrough will allow future detailed measurements of antihydrogen, giving scientists a powerful new tool to help solve the age-old question: “Why is there something, rather than nothing, in the universe?”

Antimatter, or the lack of it, remains one of the biggest mysteries of science. At the Big Bang, matter and antimatter should have been produced in equal amounts, but since they annihilate upon contact, shortly thereafter nothing should have remained but pure energy (light). However, to date all observations suggest that all the antimatter has vanished. To try to understand what happened to “the lost half of the universe”, scientists are eager to determine whether there is a difference in the properties of matter versus antimatter that might offer an explanation. The approach taken by the ALPHA collaboration will be to compare a well-known system in physics, the hydrogen atom, consisting of one proton and one electron, with its antimatter counterpart, antihydrogen, consisting of an antiproton and an antielectron.

Antihydrogen atoms were first made at CERN eight years ago, but couldn’t be stored, since the anti-atoms touched the ordinary-matter walls of the experiments within millionths of a second after forming and were instantly annihilated. The ALPHA collaboration succeeded by developing a sophisticated “magnetic bottle” using a state-of-the-art superconducting magnet to suspend the antiatoms away from the walls. The experiment showed definitive proof of antihydrogen atom capture for about a tenth of a second. Very few were captured (nowhere near enough to power a starship engine!), but their longevity was more than enough to allow study. This result is the crucial step before commencing detailed studies of antihydrogen. These antihydrogen atoms very well may be the first contained antiatoms in the history of the universe.

A well-known aphorism proclaims that to understand the hydrogen atom is to understand all physics. Makoto Fujiwara, spokesperson for the ALPHA-Canada group, points out, “That is only half right - we still have to understand antihydrogen.” CERN Director General Rolf Heuer said, “These are significant steps in antimatter research and an important part of the very broad research programme at CERN.” CERN is the only laboratory in the world with a dedicated low-energy antiproton facility to enable this type of research.

ALPHA-Canada scientists have been playing leading roles in the antihydrogen detection and data analysis aspects of the experiment, and also the development towards forthcoming antiatomic structure studies. Richard Hydomako, a Ph.D. student of Prof. Rob Thompson at the University of Calgary and a scholar visiting Prof. Scott Menary at York University, played a crucial role in the data analysis of the reported result. He said “It’s been a rare privilege and learning experience taking part in this groundbreaking international endeavor.” Important infrastructure support came from TRIUMF in Vancouver, BC, which enabled Canadian scientists to participate in an international project at the level beyond what is normally possible by a single university group. TRIUMF Director Nigel Lockyer was enthusiastic, “This is an historic achievement and a real testament to the imagination, ingenuity, and inspiration of the scientists and students from TRIUMF, Canada, and around the world.”

The ALPHA Collaboration is already exploiting the fruits of their labour. Fujiwara notes that “As we speak, we are trying to measure, for the first time, what colour antimatter atoms shine,” referring to initial attempts to apply microwave spectroscopy on the trapped antihydrogen, an effort led by Prof. Michael Hayden of Simon Fraser University, and Prof. Walter Hardy of the University of British Columbia. This effort is the next step in determining the detailed atomic structure of antihydrogen, which could give new clues on why there is so much something, rather than a lot of nothing, in the universe.

Financial support for ALPHA-Canada and its members comes from NSERC (National Science and Engineering Research Council), NRC and TRIUMF, AIF (Alberta Ingenuity Fund), the Killam Trust, and FQRNT (Le Fonds québécois de la recherche sur la nature et les technologies).

For More Information

ALPHA Collaboration website:             http://alpha.web.cern.ch/alpha

CERN antimatter information:    http://angelsanddemons.cern.ch/