News
Highest mass capture measurement with radioactive beam
The DRAGON collaboration has recently performed an experiment measuring the strength of the 38K(p, γ)39Ca radiative proton capture reaction. This reaction is significant in oxygen-neon novae and can potentially have significant consequences for the abundances of 38Ar, 39K, and 40Ca. This region of the nuclear chart represents the probable end-point for nucleosynthesis in novae.
In the experiment, the investigators focused on measuring the strengths of three key resonances at center-of-mass energies of 386, 515, and 689 keV. At the highest energy, the experimenters observed a clear signal from 39Ca recoils. At the lower two energies, the team saw no clear sign of recoils, suggesting that the strengths of these resonances are too low to be observed in the approximately two weeks of beam time allotted to the experiment. However, more detailed analysis is required to check for indications of recoils at the lower energies.
The experiment represents a significant technical achievement, setting a new world record as the highest mass radiative capture experiment ever performed with a radioactive beam. Acceleration of the radioactive 38K ions to DRAGON required use of TRIUMF's new charge state booster, the first time this has been utilized for an accelerated beam experiment in the ISAC-I hall. The beam delivery was an astounding success, with approximately 5 × 107 particles per second of 38K delivered to the experiment.
Figure: Plot of time of flight through DRAGON vs. time of flight between the two micro-channel plates located in DRAGON's focal plane. The clustering of events circled in red is indicative of a clear signal from 39Ca recoils.
Recoil Separators Review Article Published
A review article entitled "Recoil separators for radiative capture using radioactive ion beams" authored by DRAGON collaborators has been published in the European Physical Journal A.
The abstract can be seen here:
"Radiative capture reactions involving the fusion of hydrogen or helium are ubiquitous in the stellar history of the universe, and are some of the most important reactions in the processes that govern nucleosynthesis and energy generation in both static and explosive scenarios. However, radiative capture reactions pose some of the most difficult experimental challenges due to extremely small cross sections.
With the advent of recoil separators and techniques in inverse kinematics, it is now possible to measure radiative capture reactions on very short-lived radioactive nuclei, and in the presence of high experimental backgrounds. In this paper we review the experimental needs for making measurements of astrophysical importance on radiative capture reactions. We also review some of the important historical advances in the field of recoil separators as well as describe current techniques and performance milestones, including descriptions of some of the separators most recently working at radioactive ion beam facilities, such as DRAGON at TRIUMF and the DRS at the Holifield Radioactive Ion Beam Facility.We will also summarize some of the scientific highlight measurements at the RIB facilities."
Understanding the Inner Workings of Novae
A measurement of an important nuclear reaction that occurs in novae has been made by the DRAGON group at TRIUMF. Novae are cataclysmic events that occur as a result of material from one star falling onto another. This generates vast amounts of energy causing a violent explosion and the ejection of matter into space. In the aftermath of these outbursts one isotope of interest is 18F. There is hope that its radioactive decay can be observed by satellites which could give us a unique insight to the inner workings of novae.
In order to exploit any future observations, however, the expected amount of 18F formed in nova explosions needs to be properly characterized, requiring experimental knowledge of key nuclear reactions such as 18F + p → 19Ne. The problem with observing these reactions in the laboratory is that as 18F is radioactive, making a target out of it isn't practicable, as it would decay away in a matter of hours. The solution to this is to produce radioactive beams of nuclei, which can be sent to the experiment in a matter of milliseconds after they are produced.
Whilst observing the reaction of interest, 18F + p → 19Ne, DRAGON was able to successfully resolve just two 19Ne particles out of one thousand billion 18F beam particles. This is the first such successful observation of the reaction in a laboratory.
By utilizing a high-energy cyclotron, TRIUMF is one of the few places on earth that can produce the necessary intensity of radioactive ion beams for experiments such as this. The laboratory is also home to the DRAGON Facility, which stands for the “Detection of Recoils And Gammas Of Nuclear reactions”. It consists of a gas target, containing either hydrogen or helium, and a high-resolution mass separator, which can filter out nuclei of interest from other contaminating particles. Beam is delivered to the target where the nuclear reaction of interest takes place, then exits into the separator. Such a facility is required when using radioactive and stable beams as any products produced are hidden amongst the vastly more abundant beam particles (for more information see http://astro.triumf.ca/dragon/system).
The experiment was performed by the local DRAGON Group, Canadian and international collaborators, including those from McMaster University, the University of York, University of Edinburgh, Colorado School of Mines, Michigan State University, Pacific Northwest Laboratories and Oak Ridge National Laboratory. The data were analyzed by University of York/TRIUMF graduate student Charlie Akers (pictured). The experimental results are the first stage in a measurement campaign aiming to make all the necessary experimental measurements of reactions that produce and destroy 18F in the novae environment. The important result was recently published in Physical Review Letters (http://prl.aps.org/abstract/PRL/v110/i26/e262502). The high suppression of contaminants, together with the radioactive beam intensities available, make DRAGON at TRIUMF one of only a handful of facilities on earth that can conduct this kind of research.
Paper on the 20Ne(p, γ)21Na resonance strength published in Phys. Rev. C
A DRAGON measurement of the 20Ne(p, γ)21Na resonance strength was recently published in Phys. Rev. C, Brief Reports. This resonance serves as an important calibration point for direct capture measurements at lower energies. In addition to the new DRAGON measurement, the paper also points out that the "accepted" value of the resonance strength has been misinterpreted for over fourty years due to inconsistencies in the frame of reference.
A link to the online journal publication can be found here, and a freely available preprint is online here.
New 23Mg lifetime experiment completed
By Oliver Kirsebom
Novae are thermonuclear explosions that take place on the surface of white dwarf stars with a close companion star. With a handful detected in the Galaxy every year, these powerful explosions constitute a very common phenomenon and contribute to the enrichment of the interstellar medium. One of the isotopes thought to be produced in novae, 22Na, emits a characteristic gamma ray which, if observed, would provide a crucial test of our current understanding of novae.
In november 2012 we conducted an experiment at TRIUMF to determine the lifetime of certain excited state of the nucleus 23Mg, using the Doppler shift attenuation method at the DSAM facility. This piece of information will allow us to make a more precise estimate of the amount of 22Na produced in novae and thus a more precise estimate of the expected gamma-ray flux - which will put novae models to a stronger test if and when astronomers succeed in detecting the gamma ray from 22Na. The analysis of the experiment is in progress and we expect to have preliminary results soon.
The DSAM facility at ISAC II
New CO nova paper accepted for publication
The first work from the Nova Framework on CO novae has been accepted for publication in the Astrophysical Journal, led by Pavel Denissenkov of UVic, TRIUMF and JINA. The abstract of the paper is as follows:
Novae are cataclysmic variables driven by accretion of H-rich material onto a white-dwarf (WD) star from its low-mass main-sequence binary companion. New time-domain observational capabilities, such as the Palomar Transient Factory and Pan-STARRS, have revealed a diversity of their behaviour that should be theoretically addressed. Nova outbursts depend sensitively on nuclear physics data, and more readily available nova simulations are needed in order to effectively prioritize experimental effort in nuclear astrophysics. In this paper we use the MESA stellar evolution code to construct multicycle nova evolution sequences with CO WD cores. We explore a range of WD masses and accretion rates as well as the effect of different cooling times before the onset of accretion. In addition, we study the dependence on the elemental abundance distribution of accreted material and convective boundary mixing at the core-envelope interface. Models with such convective boundary mixing display an enrichment of the accreted envelope with C and O from the underlying white dwarf that is commensurate with observations. We compare our results with the previous work and investigate a new scenario for novae with the 3He-triggered convection.Report on Nova Framework
A February 2012 short report on the status of the UVic-TRIUMF-JINA nova project has been released. The document can be found here.
A New Look Inside Ancient Stars
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
2011 Nobel Prize in Physics
Yesterday morning, the Nobel Prize in Physics 2011 was awarded, "For the discovery of the accelerating expansion of the Universe through observations of distant supernovae," with one half to Saul Perlmutter (Lawrence Berkeley National Lab) and the other half jointly to Brian P. Schmidt (Australian National University),and Adam G. Riess (Johns Hopkins Univ & Space Telescope Science Institute).
The Nobel Prize website says,
"The research teams [of Perlmutter and Schmidt] raced to map the Universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and new digital imaging sensors (CCD, Nobel Prize in Physics in 2009), opened the possibility in the 1990s to add more pieces to the cosmological puzzle.
"The teams used a particular kind of supernova, called type Ia supernova. It is an explosion of an old compact star that is as heavy as the Sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research teams found over 50 distant supernovae whose light was weaker than expected - this was a sign that the expansion of the Universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion."
TRIUMF and other labs pursuing nuclear astrophysics are playing key roles in understanding the nuclear physics of SN Ia so that eventually the yardstick technology can be fully understood. How does this work? TRIUMF studies the detailed nuclear reactions that occur in Type 1a supernovae; this in turn allows observers to make more precise comparisons between what they expect to see in the night sky and what is actually observed.
Type Ia supernovae have recently been proposed as a major source of so-called "p-nuclei". These are nuclei of around 35 different kinds between mass numbers 74 and 196 that are slightly neutron-deficient, but stable, and are separated from their more neutron-rich stable neighbours by one or two radioactive isotopes. This means they are unlikely to be able to be produced in a standard mechanism of stellar nucleosynthesis. However in SN1a they can be produced in something called the "gamma process", where they are created via the disintegration of slightly more neutron-rich nuclei when bombarded with high energy photons (gamma rays).
In order to understand nucleosynthesis of these nuclei, the nuclear reactions which create and destroy them have to be measured in the laboratory. Theoretical models are so far insufficient to calculate the reaction rates with good precision. In particular, proton and alpha fusion reactions on these nuclei are important. The DRAGON facility at TRIUMF was built to study these proton and alpha fusion reactions with nuclei of lower masses than the p-nuclei, but recently it was determined that the DRAGON facility can perform well at these higher masses to do measurements for the gamma process as well. A program in this area will kick off in 2012 with a measurement of alpha capture on selenium-76. It is hoped the data from these measurements will help elucidate astrophysical simulations of SNIa, making them ever closer to the observations, and allowing us to have deep insight into the working of these objects.
Our highest praise and congratulations go to Professors Perlmutter, Riess, and Schmidt! We learn more about ourselves and the universe in which we live through pioneering work such as theirs.
--by T.I. Meyer, Head of Strategic Planning & Communication
New High Mass Records for DRAGON
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.
|
Selected Charge state |
Current on faraday Cup at energy-dispersed focus (enA) |
Percentage of incident current (normalized to particle-nA) |
Dipole Field (Gauss) |
|
23 |
0.170 |
9.29% |
Above limit of NMR probe head |
|
24 |
0.090 |
4.7% |
5618.76 |
|
25 |
0.035 |
1.75% |
5407.09 |
|
26 |
0.010 |
0.5% |
5206.34 |
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.
