Direct measurements of nuclear reactions at stellar energies face two walls: extremely small cross sections and cosmic-ray background. Underground laboratories such as Jinping suppress the background, but the target problem remains. Conventional gas cells are sealed with foil that saps energy from the beam and blurs it. Windowless targets solve this, but at milliampere currents new difficulties arise: the gas must be pumped away and recycled quickly, and the beam heats the gas, altering its density. Alongside this work lie other windowless target developments, such as the LUNA recirculating systems and the JENSA gas jets, but all of them are designed for lower currents or lack a full set of in-beam diagnostics.
The engineers assembled a system with three stages of differential pumping: the first uses Roots pumps, the second and third use turbomolecular pumps. Gas from the target is pumped out, purified through condensation and zeolite adsorbers, and returned. Pressure in the chamber is maintained by an MKS 250 controller. To measure beam flux, they built a copper calorimeter with four heaters and three thermocouples; it operates in power compensation mode, that is, it keeps a constant temperature by reducing heater power when the beam is on. Target thickness is monitored by a movable tube that scans along the beam and catches particles that have been elastically scattered twice: first by the gas, then by a thin carbon foil. Gas flow calculations were performed with a model that includes wall slip for low pressures.
At pressures of 1, 2, and 3 mbar, the system maintained stability within 1% for eight hours, and the pressure on the accelerator side stayed around 10⁻⁴ Pa. The gas flow model, at a flow rate of 1052 cubic cm/min, predicted a pressure nonuniformity along the beam of no more than 1.6%. The calorimeter was tested with the heaters: the model reproduced both the transient behavior and the steady-state temperatures. It turned out that the sensitivity to the beam and to the heaters differs by 6.2%, and this correction must be taken into account. With a proton beam of 300–310 keV, they measured the ¹⁴N(p,γ)¹⁵O reaction and saw how the resonance moves along the target. The ¹²C(p,γ)¹³N reaction showed that beam heating reduces gas density: the heating factor decreases with increasing power, down to 0.75 at 800 mW/cm.
This work differs from neighboring efforts in that, for the first time, a windowless target for milliampere currents combines three things: stable gas circulation, a calorimeter independent of the beam's charge state, and a position-sensitive thickness monitor. Previously, such systems either operated at lower currents or lacked full diagnostics under the beam. Now one can directly measure beam flux and local gas density during irradiation, which is critical for accurate cross sections.
The system is ready for measurements with isotopic gases such as ³He and enriched ²²Ne. Closed-loop circulation will save expensive gases, and the calorimeter and thickness monitor will provide the missing data for absolute normalization. In the future, this will allow going to lower energies in the reactions ³He(α,γ)⁷Be and ²²Ne(α,n)²⁵Mg, which are important for solar neutrinos and neutron sources in massive stars.
The results will affect nuclear astrophysics, especially measurements of reactions involving light nuclei. Accelerator technology will gain an example of reliable operation of windowless targets at high currents.
The next step is to calibrate the calorimeter with a real beam and compare its readings with the thickness monitor. Then one can proceed to measurements of ³He(α,γ)⁷Be in the underground laboratory.
For nuclear astrophysics, the key unsolved problem is how to measure cross sections at energies below the Coulomb barrier, where the signal drowns in background. This work closes part of the problem by providing a tool for precise control of target and beam, but it does not solve the background from the target and detector themselves.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper describes a windowless gas target for underground nuclear astrophysics, designed for beams in the milliampere range. The system includes differential pumping, closed-loop gas circulation, a calorimeter for beam flux measurement, and a target thickness monitor based on elastic scattering. Tests demonstrated pressure stability and revealed the effect of beam-induced heating on gas density.
Where its strength is
For the first time, a windowless target for milliampere currents combines stable gas circulation, a calorimeter independent of the beam charge state, and a position-sensitive thickness monitor, enabling direct measurement of flux and local gas density during irradiation.
Where it could go next
Worth trying
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.51, against a usual 0.64. It looks like few have drilled nearby.
The calorimeter measures beam power to within tenths of a watt, even though the beam itself carries hundreds of watts. That is like noticing that a kettle has cooled by half a degree when you add a spoonful of cold water.