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The Origin of Oxygen in the Universe - A new approach to an Old Question

Carbon and oxygen are not only important elements for the existence of life on Earth, but they also play an important role in the evolution of stars towards the end of their life cycle. The formation of 12C through the so-called triple-α reaction is quite well understood. The next step, the formation of 16O through the a capture reaction 12C(α, γ)16O on the other hand, still has an experimental uncertainty of ∼ 30%. Direct measurements of the 12C(α, γ)16O reaction by detecting either the outgoing γ radiation in a high acceptance Ge-detector array or the residual 16O nuclei in a mass spectrometer do not allow for order-of-magnitude improvements. In this contribution, the possibility of using superheated bubble detectors for a measurement of the time-inverse 16O(γ, α)12C reaction is being discussed. The first results of a 'proof-of-principle' experiment of the 19F(γ, α)15N reaction are also being presented.


Introduction
The isotopes 16 O and 12 C are, after 1 H and 4 He, the third and fourth most abundant nuclei in the visible universe. Most of the carbon and oxygen which we observe today is produced by helium burning in red giant stars. These two elements are not only crucial for all living organisms, but their relative abundances, which are determined by the competition between the triple-α and the 12 C(α, γ) 16 O reactions, is also an important parameter for the evolution of a massive star at the end of its lifetime during the carbon-, neon-, and oxygen-burning phases [1,2,3]. The cross section for the triple-α process is experimentally quite well determined [4]. Our knowledge of the 12 C(α, γ) 16 O reaction under typical helium burning conditions (T 9 ∼ 0.2 or E c.m. ∼ 300 keV), however, is still limited by its small cross section and by the crucial role played by two sub-threshold states in 16 O [5].
The history of experiments studying the 12 C(α, γ) 16 O reaction goes back more than four decades [6,7]. The magnitude of this cross section, however, is still a hotly debated issue, both experimentally and theoretically, and many recent publications can be found in the literature [8,9,10,11,12,13,14,15,16].
Contrary to the triple α reaction which is dominated by the contribution from a resonant state (the so-called Hoyle state) the 12 C(α, γ) 16 O reaction proceeds mainly through two radiative capture modes to the 16 O ground state. One is E1 capture with contributions from the 1 − state at E x =9.585 MeV (E r =2.418 MeV) and the sub-threshold 1 − state at E x =7.117 MeV (E r =-45 keV). The other is E2 capture which is dominated by the contributions from direct capture and the sub-threshold 2 + state at E x =6.917 MeV (E r =-245 keV). At energies corresponding to the Gamow window for red giant stars (E c.m. ∼300 keV), the cross sections are on the order of 10 −17 b. For that reason, all direct measurements so far were done at higher energies above E c.m. = 890 keV [4]. These data are then extrapolated into the energy region of astrophysical interest using R-matrix theory. Since the higher-energy data are not very sensitive to the contributions from sub-threshold resonances, the published S-factors in the past 30 years range from 1 to 288 keVb for S E1 (300) and 7 to 120 keVb for S E2 (300) [17].
Two different techniques have been used in the past for direct measurements of the 12 C(α, γ) 16 O reaction: • A highly enriched 12 C target is bombarded with high intensity α beams and the outgoing γ's are detected in large-acceptance Ge-detector arrays. The measurements performed at the lowest c.m.energies (E cm =890 keV) have used this technique. The main drawback of this technique is the low efficiency that can be achieved for the detection of 8 MeV γ rays.
• A higher detection efficiency can be obtained by studying the reaction in inverse kinematics, i.e. bombarding a 4 He gas target with a 12 C beam and detecting the final 16 O nucleus in a mass spectrometer. The difficulty with this technique comes from a small 16 O beam contamination, which currently restricts the lowest energies to about E cm ∼ 1.8 MeV.
Good agreement of the measured cross sections is observed in the energy region studied by both techniques. In addition to these direct techniques, several indirect methods have been investigated in order to improve the reliability of the extrapolations into the astrophysically important energy region. These techniques include data from elastic α scattering on 12 C [18,19], α-transfer reactions to 16 O [20], and β-delayed α decay of 16 N [22, 23, 21, 11, 24].
In order to discuss the limits and possible improvements for future direct measurements of the 12 C(α, γ) 16 O reaction, we introduce the concept of a 'modified' luminosity L = I · N · , where I is the beam intensity (particles/sec), N the areal target density ( target nuclei/cm 2 ) and the detection efficiency. When multiplied by the cross section σ, the product L · σ gives the count rate expected in an actual experiment. Sometimes only an order-of-magnitude estimate of the luminosity can be extracted from the information provided in the various publications. The 'modified' luminosities for some of the experiments performed during the last 20 years are shown in Fig. 1. They are typically of the order of 10 31 sec −1 cm −2 which, when multiplied by a cross section of 1 pb, translate into an expected count rate of 1 count/day. Improvements by factors of 2-5 are possible by choosing higher beam currents or better Ge detector arrays (perhaps located deep underground). The choice of higher efficiency BaF 2 detectors has also been investigated. However, based on the small cross sections and the relatively small luminosities improvements by orders of magnitude do not seem possible with these techniques in the foreseeable future.

Studies of time-inverse Reaction 16 O(γ, α) 12 C Reaction
For experiments studying astrophysically important proton-capture (p,γ) reactions it has been pointed out [25] that measurements of the time-inverse (γ,p) reactions can yield higher count rates when compared to standard (p,γ) experiments. This is caused by two effects: (a) the possibility to use thicker targets and (b) by a more advantageous phase space factor. This has made Coulomb dissociation studies a competitor to direct (p,γ) experiments. Similar advantages also exist for measurements of (α, γ) and (γ, α) reactions as was pointed out in Ref. [26,27]. Using the principle of detailed balance, the cross sections for (α, γ) and (γ, α) reactions are (for the system 12 C(α, γ) 16 O) related by the equation where µ and E cm are the reduced mass and the c.m. energy in the 12 C + α system, and E γ is the corresponding γ energy in the 16 O + γ system (typically 8 MeV for E cm ∼1 MeV). For the lowest energies studied so far the enhancement factor σ(γ,α) σ(α,γ) is about 50.
Another advantage for studying the time inverse 16 O(γ, α) reaction originates from the large range of an 8 MeV γ which has a mean absorption length of about 40 cm in water. Thus, in (γ, α) experiments, target thicknesses of tens of g/cm 2 can be used, which results in yields that are higher by about 6 orders of magnitude when compared to direct 12 C(α, γ) 16 O experiments where target thicknesses of typically 10 µg/cm 2 are used.
The implementation for measuring the time-inverse 16 O(γ, α) 12 C reaction requires a tunable γ beam with the highest possible intensities and a detector system that can identify the occurrence of a breakup of 16 O into 12 C and α while being insensitive to γ's at the level of at least 10 −11 . In Ref. [26] an optical TPC has been discussed. While it can identify the occurrence of a 16 O(γ, α) 12 C reaction it operates with gases at pressures of ∼100 mbar and thus does not make full use of the large range of 8 MeV γ's. In the following we will discuss the advantageous of using a bubble chamber filled with superheated liquids.

The use of a bubble chamber for nuclear astrophysics experiments
Bubble chambers were one of the principle detectors for tracking energetic particles in high energy physics experiments. Invented more than 50 years ago, their operational principles are well understood [28]. These type of detectors have recently seen a renaissance in Dark Matter searches and three groups are utilizing superheated liquids in a search of nuclear recoils induced by possible WIMP interactions [29,30,31]. The liquids used in these experiments are fluorocarbon compounds which are superheated close to room temperatures. However, water, (the Contrary to bubble chambers used in high energy physics which are superheated only for a short period of time, Dark Matter searches require detectors that are superheated for longer periods (e.g. days). This can be achieved by choosing a glass vessel as a container for the superheated liquid. When located deep-underground, background rates of ∼counts/day have been achieved. In order to avoid the high-pressure/temperature conditions required for a water-filled bubble chamber we have therefore, for a first 'proof-of-principle' experiment, studied the timeinverse of the reaction 15 N(α, γ) 19 F by bombarding fluoro-carbon liquids with γ beams from the HIγS facility at Duke University. Fig. 2 shows a schematic of the experimental setup. A γ beam from the HIγS facility collimated to a diameter of 1 cm hits a cylindrical glass vessel (diameter ∼3 cm) filled with C 4 F 10 . This fluoro-carbon compound is superheated at T∼30 C and p∼3 atm. If an incident γ causes a 19 F(γ, α) 15 N reaction the outgoing α-15 N pair is stopped in the liquid and acts as a nucleation center producing a bubble. The glass vessel is continously monitored by two CCD cameras taking pictures every 10 ms. If a bubble is detected, the pressure in the vessel is immediately increased which brings the liquid out of the superheated state, thus leading to a quenching of the bubble. After a time delay of typically 1 s, the pressure is again reduced bringing the detector back to the superheated state for the detection of another event. Fig. 3 shows a sequence of photographs taken 10 ms apart showing the growth and the quenching of a bubble produced via the 19 F(γ, α) 15 N reaction in a superheated C 4 F 10 liquid. The energy of the γ beam was 5.3 MeV.
The normalization of the incident γ flux is achieved by using a high efficiency Ge detector located after the bubble chamber. With this setup, an excitation function of the 15 N(α, γ) 19 F reaction has been measured in the energy range E γ =5-6 MeV. The uncertainty of the incident  γ-beam is determined by the energy spread of the electron beam in the storage ring and by the amount of collimation and was about 1% for this experiment. A preliminary excitation function for the 15 N(α, γ) 19 F reactions is shown in Fig. 4. The cross section scale covers almost four orders of magnitude ranging from 1 nb to 10 µb. The cross sections in Fig. 4 have not been unfolded with the experimental resolution. In order to measure the yield at the maximum of the excitation function, the intensity of the incident γ beam had to be reduced to 5×10 3 γ's/sec. For a 20 cm long vessel filled with water and assuming a beam intensity of 5x10 7 γ's/sec, the calculated count rate for the 12 C(α, γ) 16  The next step involves a similar test using a water-filled bubble chamber. A setup that allows measurements at 250 • C and p=75 atm is presently being assembled at Argonne. Contrary to the 19 F(γ, α) reaction which used the mono-isotopic 19 F, reactions on the target contaminants 17,18 O can contribute to the 16 O(γ, α) reaction. For this reason water, which is highly enriched in 16 O, has been purchased. Similar to the study of the 19 F(γ, α) reaction we will start the measurement of the 16 O(γ, α) reaction in the vicinity of the known 1 − resonance at E γ = 9.585 MeV, where several cross sections measurements using other techniques have been performed in the past and will extend the (γ, α) measurements to the lowest energies possible. Reactions induced by cosmic ray neutrons can be eliminated by using an acoustical readout system as done e.g. in Ref. [32].