Science

The breakout of the hot CNO cycle and the onset of the rapid-proton process are of significant importance to our understanding of the nucleosynthesis of proton-rich nuclei in our universe. In particular ¹⁵O(α,γ)¹⁹Ne and ²¹Na(p,γ)²²Mg are both thought to be key reactions for these processes under explosive astrophysical conditions. In this work, an experiment has been carried out at Louvain-la-Neuve, Belgium, in order to test the feasibility of a measurement of the lifetime of the 4.033 MeV state in ¹⁹Ne, which is considered extremely important for the ¹⁵O(α,γ)¹⁹Ne reaction. Also, an elastic-scattering experiment was performed using a newly-developed ²¹Na beam at the ISAC post-accelerated radioactive beam facility in Vancouver, Canada. The experiment represents the first scientific result achieved with this facility. A centre-of-mass energy range of ≈ 0.4-1.5 MeV was investigated using a thick-target scan technique utilising polyethylene ([CH2]n) foils. Data were collected using a silicon charged-particle detector array, enabling the identification of elastic and inelastic resonances in the ²¹Na+p system. Monte-Carlo simulations were used to estimate the experimental resolution effects present in the experiment. These results were then incor- porated into an analysis of the data using a single-channel l = 0 R-matrix code. An analysis of the data enabled the identification of four states in ²²Mg, one of which was previously unobserved. Resonance energies and widths were estimated for each of these states. A comparison of the results with states in the T=1 analogue system was made. The effect a detailed knowledge of these resonances would have on the ²¹Na(p,γ)²²Mg rate under extreme high temperature conditions was also investigated.

ruiz_phd.pdf

Author: Erikson, L.

Journal:

A gamma ray array to detect the characteristic gammas emitted from astrophysically significant, radiative proton and aplpha capture reactions, was built as part of the Detector of Recoils And Gammas Of Nuclear reactions (DRAGON) spectrometer at ISAC/TRIUMF. The DRAGON array consists of a collection of 30 hexagonal BGO detectors measuring 7.62 cm long by 5.58 cm across the face. Experiments at DRAGON are affected by background due to "leaky beam" which reaches the end detector along with the reaction products of interest. In many cases the cross sections of these reactions are so small that it is impossible to distinguish the reaction recoils from leaky beam by using only the electromagnetic separator (EMS) of DRAGON. Further suppression of leaky beam is achieved by demanding a time of coincidence between reaction recoils and the associated gamma emitted from the reaction. To determine the rate of gamma/recoil ion coincidence events it is necessary to have an accutate estimate of the gamma array efficiency. Since it is impossible to measure this rate for all experimental conditions it is necessary to have a simulation which can estimate the efficiency of the array for a given set of experimental parameters (e.g gamma energy). A simulation was built with the particle-tracking program GEANT v3.21. The efficiency of the array was measured using calibrated sources of various gamma energies and compared to simulated results. For the cases where the activity of the source was not well know the sources were calibrated using a standard NaI detector of known efficiency. The agreement between simulation and measured differences is more than adequate for proposed DRAGON experiments. The analysis and results of the comparison between measured and simulated efficiency will be discussed in this thesis.

dariothesis.pdf

Author: Gigliotti, D. G.

Journal:

The DRAGON facility at TRIUMF-ISAC was designed to measure the rates of astrophysically important nuclear reactions involving radioactive reactants. To this end, the mass spectrometer was designed to separate the result of a radiative proton or alpha capture reaction, between beam and target nuclei, from the beam itself. Yields are typically on the order 10⁻⁹ to 10⁻¹⁵, thus, the feasibility of a particular reaction is driven by the suppression of the relatively intense beam, to that of the capture product. In the case of Nova explosions, important resonances occur at low beam energies (0.15 to 1.0 MeV/u) where the DRAGON suppression may be reduced.

An MCP (Micro Channel Plate) detection system has been commissioned to be used in a local time-of-flight approach for particle identifcation at the focal plane of the DRAGON recoil mass separator. It is the goal of this additional detection system to enhance the current suppression systems without signifcant loss in effciency. Three properties of the MCP system have been investigated: the timing resolution, the effciency and the position resolution. Two sources, ⁶⁸Ge and ¹⁴⁸Gd, were used off-line to test the detection system performance. The timing studies were performed with the use of a fast PMT (Photo Multiplier Tube) as a second detection system. A DSSSD (Double Sided Silicon Strip Detector) was used for the effciency tests and masks were used during the position resolution studies. These off-line tests were followed by on-line studies of the well known resonance (Ecm = 258.6 keV) in the ²¹Ne(p,γ)²²Na reaction. A simulation using the RELAX3D software along with a custom made tracking code, both written at TRIUMF, has also been studied, and its results pertaining to the three aforementioned important properties will be discussed.

lamey_thesis.pdf

Author: Lamey, M. J.

Journal:

The ¹³N(p,γ)¹⁴O reaction is important as it determines the breakout from the CNO cycle to the HCNO cycle. Studying the ¹³C(ρ,γ)¹⁴N reaction was important for the DRAGON facility at TRIUMF for their future analysis of the ¹³N(p,γ)¹⁴O reaction, not only because pure radioactive ion beams of 13N are impossible to create without contamination from ¹³C due to the very small mass difference between these two elements, but also it was a good test for the DRAGON due to the fact that the ¹³C(p,γ)¹⁴N reaction has been measured before.

Early analysis of the ¹³C(p,γ)¹⁴N reaction data collected by DRAGON, showed that not all the ¹⁴N recoils made it through the DRAGON separator to the end detector (an ionization chamber), because they were being clipped due to the large cone angle for this reaction. A GEANT simulation of DRAGON was used to simulate the ¹³C(p,γ)¹⁴N reaction so that it could be compared to see what fraction of the recoils were being lost within the DRAGON due to this clipping, and also to see where the clipping occurred.

The creation of an ionization chamber in the GEANT simulation for the first time, meant that simulations of the ¹³C(p,γ)¹⁴N reaction could test the DRAGON’s acceptance also, by simulating different mistunes of the DRAGON’s reference tune, in x and y position, x and y angle, and percentage of energy. These mistunes showed that the maximum acceptance for DRAGON is achieved when the beam is not mistuned in x and y position, but mistuned to -0.5% of the energy, and -1.5 mrad and -0.5 mrad in the x and y angular position respectively. They also showed that there is a large acceptance loss, with the maximum acceptance being roughly 78-79%.

aaron_thesis.pdf

Author: Bebington, A. M.

Journal:

The DRAGON electromagnetic recoil mass separator located at TRIUMF-ISAC in Vancouver, Canada was built to study resonant radiative proton and α-particle capture reactions relevant to nuclear astrophysics. In DRAGON experiments, a stable or radioactive ion beam in the energy range 0.15 MeV/amu to 1.5 MeV/amu from ISAC impinges on a differentially pumped, windowless gas target of hydrogen or helium. Recoiling reaction products are separated electromagnetically from beam ions and detected in the final focal plane, typically with a double sided silicon strip detector. A bench test facility for testing and improvement of the double sided silicon strip detectors and related systems has been assembled. The facility has been used to measure the efficiency and energy resolution of the detectors using an α-particle source, and to assess the effects of radiation damage on the detectors. The pulse height defect and the effect when particles pass through the gap between strips have been measured. Data from α-particle tests have been compared with data from stable beam experiments in the mass range A = 12 to A = 25, and used to predict detector performance in future DRAGON experiments. A 1 MeV/amu ¹⁶O beam has been used to measure timing resolution. DRAGON has used this detector system successfully in measurements of the resonance strengths of several astrophysically relevant resonances in the ²¹Na(p,γ)²²Mg reaction with a radioactive beam of ²¹Na. A hybrid thermoelectric/liquid cooling system has been designed, built and tested with the purpose of cooling the double sided silicon strip detectors and improving their performance. The future possibility of using silicon detectors for particle identification by pulse-shape discrimination has been researched.

wredethesis.pdf

Author: Wrede, C. L. H.

Journal:

The charge state of an ion passing through matter fluctuates as a result of electron capture and loss in the collisions with the target atoms. Despite the existence of a large number of theoretical and experimental studies on this complicated atomic collision system, an accurate prediction of the charge state distribution is still not available. To meet DRAGON’s future experimental needs, the non-equilibrium and equilibrium charge state distributions resulting from the collisions of ¹⁶O, ²³Na, ²⁴Mg ions passing through a windowless hydrogen and helium gas target with the beam energy in the range of 0.138−0.875 MeV/u, 0.200−0.478 MeV/u, 0.200−0.800 MeV/u, respectively, have been measured using the differentially pumped gas target facility at Naples University, Italy, and DRAGON/ISAC, TRIUMF, Canada. It is determined that the equilibrium distribution is established at low target thickness. The equilibrium distribution depends on the projectile species, its energy and the nature of the target, while independent of the incident charge state. The uncertain- ties of the normalized charge state fractions are estimated to be less than 5% except for very low fractions. The equilibrium distribution is shown to be close to the Gaussian distribution. Semi-empirical formulas have been derived for the average equilibrium charge state and distribution width. Assuming that the probability of multiple electron capture and loss in a single collision are negligible, single electron capture and loss cross sections have been estimated using the least-squares method in cases where sufficient experimental charge state fraction data are available. The dependence of cross sections on projectile energy and charge state has been studied.

wenjiethesis.pdf

Author: Liu, W.

Journal:

A proof-of-principle approach for the production of intense (~10⁸/s) radioactive ion beams, which differs from the standard ISOL (Isotope Separation On-Line) technique, has been demonstrated successfully using ¹¹C at the TRIUMF laboratory. This approach uses 13 MeV protons produced by a medical cyclotron and should be useful for a range of radioisotopes of interest to the nuclear astrophysics research programme.

alternativeproduction.pdf

Author: Trinczek, M. et al.

Journal: Canadian Journal of Physics

The available data on nuclear fusion cross sections important to energy generation in the Sun and other hydrogen-burning stars and to solar neutrino production are summarized and critically evaluated. Recommended values and uncertainties are provided for key cross sections, and a recommended spectrum is given for 8B solar neutrinos. Opportunities for further increasing the precision of key rates are also discussed, including new facilities, new experimental techniques, and improvements in theory. This review, which summarizes the conclusions of a workshop held at the Institute for Nuclear Theory, Seattle, in January 2009, is intended as a 10-year update and supplement to 1998, Rev. Mod. Phys. 70, 1265.

adelberger11.pdf

Author: Adelberger, E. G.; García, A.; Robertson, R. G. Hamish; Snover, K. A.; Balantekin, A. B.; Heeger, K.; Ramsey-Musolf, M. J.; Bemmerer, D.; Junghans, A.; Bertulani, C. A.; Chen, J.-W.; Costantini, H.; Prati, P.; Couder, M.; Uberseder, E.; Wiescher, M.; Cyburt, R.; Davids, B.; Freedman, S. J.; Gai, M.; Gazit, D.; Gialanella, L.; Imbriani, G.; Greife, U.; Hass, M.; Haxton, W. C.; Itahashi, T.; Kubodera, K.; Langanke, K.; Leitner, D.; Leitner, M.; Vetter, P.; Winslow, L.; Marcucci, L. E.; Motobayashi, T.; Mukhamedzhanov, A.; Tribble, R. E.; Nollett, Kenneth M.; Nunes, F. M.; Park, T.-S.; Parker, P. D.; Schiavilla, R.; Simpson, E. C.; Spitaleri, C.; Strieder, F.; Trautvetter, H.-P.; Suemmerer, K.; Typel, S.

Journal: Review of Modern Physics, vol. 83, Issue 1, pp. 195-246

The new DRAGON facility at TRIUMF is designed to measure alpha and proton capture reactions with radioactive ion beams in inverse kinematics. For nucleo-synthesis in astrophysical scenarios, the relevant energies lie in the 0.15-1 MeV/u range, where very low cross sections are expected. Therefore the separation of the recoil products from the beam particles will be a difficult task. This pare focuses on the end detectors, which will be used to distinguish recoils from beam particles at the end of the DRAGON separator.

sabine.pdf

Author: Engel, S. et al.

Journal: Nuclear Physics A