While JENSA was designed explicitly for performing reaction studies in inverse kinematics with rare isotope beams, early science measurements focused on demonstrations of the system performance and comparison to existing reaction data. One such measurement was a study of ¹⁵N elastic scattering on a ⁴He jet target. This measurement was undertaken to constrain R-matrix analysis of the ¹⁵N + α entrance channel, relevant to the astrophysically-important ¹⁵N( α ,p) and ¹⁵N ( α , γ ) reactions. The JENSA data, taken in inverse kinematics with a pure beam of ¹⁵N produced “batch-mode” from the Holifield Radioactive Ion Beam Facility (HRIBF) and a pure target of research-grade ⁴He–a difficult task as both beam and target species are naturally gaseous–were compared with normal kinematics data taken at the University of Notre Dame FN tandem using an alpha beam and melamine target enriched in ¹⁵N. This comparison was done for 15 energies spanning roughly 3.9–4.8 MeV in the center of mass, with the elastically-scattered particles detected in the SIlicon Detector ARray (SIDAR).

Not only were the yields consistent between the two techniques, but the use of the JENSA gas jet target allowed for extension of the scattering data down to much lower center-of-mass angles: as the target exhibits cylindrical symmetry, detectors can be placed all the way to 90 ° in the laboratory frame without the target or target frame material impeding the reaction products.

A distinct benefit to the use of gas jets for direct reaction studies is the ability to enable reactions between two gaseous elements to be studied in high precision. A proton beam of 30 MeV, produced by the HRIBF, impinged on the JENSA target operating with natural neon. Deuterons from the (p,d) reaction, populating states in ¹⁹Ne of astrophysical interest, were selected in the SIDAR detector array using standard energy loss techniques. An example spectrum is shown in Figure 2. In Figure 6 of Ref. 26, a similar spectrum is compared against the results of a test with a neon-implanted carbon foil. The reduction in background and improvement in the resolution due to JENSA is clear.

In the case of ¹⁸F(p, α )¹⁵O reaction, which is known to destroy ¹⁸F in novae, the spin and parity of a resonance at 6,288 keV in the compound nucleus ¹⁹Ne was the largest unknown. By using the ²⁰Ne(p,d)¹⁹Ne reaction to populate this level, with a pure neon gas target from JENSA, this uncertainty was removed. This resulted in a factor of 2.8 reduction in the uncertainty of detection of astronomical ¹⁸F due to the underlying nuclear reaction rate. These results, along with additional data from this measurement, were published by Bardayan et al. [27–29].

As with gas jets in previous decades, the relatively thin jet (with respect to energy loss) allows for precision particle spectroscopy from direct reaction studies. JENSA was used with a natural nitrogen jet to study the ¹⁴N(p,t)¹²N reaction, looking for potential new levels in ¹²N. Because the energy straggling of the incoming beam as well as the outgoing tritons through the jet was small, the resolution of the measurement was dominated by the resolution of the detectors (SIDAR in “lampshade” mode), and the width of broad, unbound levels in ¹²N was immediately apparent in the spectra.

Two potentially new levels were observed in this direct reaction measurement with JENSA, including a strongly-populated level at ∼ 4.5 MeV excitation energy with a width of approximately 500 keV. The results illuminated the ongoing need for spectroscopic information of light-mass, weakly-bound nuclei. These results were published by Chipps et al. [30].

In addition to the direct spectroscopy capability, decay particles–such as protons or alphas emitted by the product of the reaction–are able to escape the thin jet target and potentially be detected. In the case of ¹²N, protons corresponding to the p0 (to the ground state of ¹¹C), p1 (¹¹C E x = 2 MeV), and p2 (¹¹C E x = 4.3 MeV) decays, were observed in coincidence with the tritons of the reaction. Branching ratios for the decay channels as a function of energy can be extracted. This analysis was first done with JENSA data by Chipps et al. [31].

Taking advantage of the unique combination of a gas jet target with a facility able to deliver high-energy proton beams, the ²⁰Ne(p,t)¹⁸Ne reaction was studied, again at HRIBF. Due to the very high Q-value barrier for this reaction ( ∼ 20 MeV), a 37 MeV proton beam was utilized. At these energies and angles, the reaction proceeded partially through direct reactions, and partially through a multi-step process. Tritons from the (p,t) reaction were detected in the SIDAR array using standard energy loss techniques.

The spin and parity of the level at 6,150 keV, which appeared as a shoulder on top of the 6,297 + 6,353 doublet, has been contested, as has the width of this level: depending on the spin assignment, variations of up to a factor of 2.4 in the ¹⁴O( α ,p) reaction rate are possible. The JENSA data favor a reassignment of the levels in this triplet versus the adopted ordering in the literature. This work was the thesis project of UTK PhD student Thompson [32].

As before, the thin jet target allowed for decay particles to escape and be detected in coincidence with reaction tritons. This is shown in Figure 3. A determination of the branching ratio from the 6,150 keV level to the ground state of ¹⁷F will help to confirm whether the state contributes strongly to the ¹⁴O( α ,p) reaction rate in explosive proton-rich nucleosynthesis.

( α ,p) reactions of relevance to astrophysical environments such as novae and x-ray bursts, while known to proceed through levels in the compound nucleus and hence falling outside of the scope of this review, nevertheless demonstrate the opportunities for studying direct reactions with rare isotope beams and gas jets.

During its tenure in the reaccelerated (ReA3) hall at the new Facility for Rare Isotope Beams (FRIB), JENSA has been used to study several ( α ,p) reactions, including ¹⁴N, ³⁴Ar [33], ⁵⁶Ni, and ²⁶Al ( α ,p).

The ¹⁴N( α ,p)¹⁷O reaction was first observed by accident: Ernest Rutherford, measuring the scattering of alpha particles from other light particles, was surprised to discover that protons were being produced when alpha particles hit air molecules in his test chamber. Almost 100 years later, the measurement was repeated with JENSA. The protons from the reaction are clearly visible (Figure 4).

A spectroscopic measurement of the ³⁴Ar( α ,p)³⁷K reaction cross section was undertaken and published by Browne et al. [33]. The ⁵⁶Ni( α ,p)⁵⁹Cu and ²⁶Al( α ,p)²⁹Si reactions are under analysis. The latter measurement constitutes the first use of the JENSA gas jet target to study a reaction cross section into the Gamow window.