A cross section at stellar energies can be measured directly, by aiming a beam at a target and counting the reactions, or inferred from other reactions and from theory. Each way has its own reach and its own systematic limits.
Methods
Direct measurement underground
An accelerator in a laboratory deep underground fires a beam at a target, and detectors count the products of the reaction. The rock above stops most cosmic rays, so the detectors can see reactions that happen only a few times per day, at energies close to those inside stars.
For specialists
The observable is the reaction yield per incident projectile, measured with γ-ray or charged-particle detectors as a function of beam energy. The cross section, or the resonance strength for narrow resonances, follows from the yield, the target stoichiometry and thickness, the stopping power and the detection efficiency. Underground, the muon flux and the muon-induced γ-ray and neutron backgrounds are reduced by orders of magnitude, which lowers the minimum measurable yield. The remaining background comes from the environmental radioactivity of the rock and of the set-up, and from beam-induced reactions on target contaminants. Typical systematic limits are the beam current integration, the target thickness and its degradation under beam, the effective interaction energy in a thick or degrading target, the γ-ray detection efficiency including summing and angular distributions, and electron screening at the lowest energies.
Strengths
+Measures the reaction of interest directly, without a nuclear model.
+Reaches energies inside or near the Gamow window for many hydrogen-burning reactions.
+Low background allows long runs on very small cross sections.
Limits
−Only a few facilities exist, and beam time is limited.
−At the lowest energies the yield is still small and statistics dominate.
−Laboratory data include electron screening, which differs from that in a stellar plasma.
−Below the lowest measured energy an extrapolation, usually by R-matrix, is still needed.
The same kind of experiment in an ordinary laboratory above ground. It covers a much wider range of energies and beams, but cosmic rays limit how small a cross section can be measured.
For specialists
Yields are measured as underground, but the background from cosmic-ray muons and their secondaries limits the sensitivity for γ-ray energies up to several MeV unless active or passive shielding is used. Surface data usually fix the higher-energy part of the excitation function, the resonance energies and widths, and the γ-ray branchings and angular distributions that an R-matrix fit needs. They are often the main input for the extrapolation into the Gamow window. Systematic limits are the same as underground, with background subtraction added.
Strengths
+Wide choice of accelerators, beams and energies.
+Supplies angular distributions, branchings and the high-energy data that constrain R-matrix fits.
Limits
−Cosmic-ray background limits the lowest measurable cross section.
The roles of beam and target are swapped: a heavy beam hits a light target such as hydrogen or helium gas. The product nuclei fly forward with the beam, and a recoil separator picks them out and counts them.
For specialists
In a radiative capture A(a,γ)B studied as a(A,γ)B, the recoil B leaves the target inside a narrow forward cone. An electromagnetic separator suppresses the unreacted beam by many orders of magnitude, and the recoils are identified at the focal plane, often in coincidence with the capture γ-rays. The method measures the total capture cross section with no need for a γ-ray efficiency model, and it works with radioactive beams. Systematic limits are the separator acceptance and transmission, the charge-state distribution of the recoils, the beam suppression, and the gas-target density and beam normalization.
Strengths
+Gives the total cross section directly, independent of the γ-ray cascade.
+Works with short-lived beams that cannot be made into targets.
Limits
−The recoil cone must fit the separator acceptance, which restricts reactions and energies.
−Beam leaking through the separator limits the lowest measurable yield.
−Charge-state fractions must be measured or modelled.
When the reaction product is radioactive, the target is irradiated and then moved to a low-background counting station. The number of decays gives the number of reactions.
For specialists
The number of product nuclei is inferred from their decay radiation, measured offline, together with the half-life and the decay branching. The method gives the total cross section, summed over all γ-ray cascades and integrated over angle, which avoids the summing and angular corrections of prompt γ-ray spectroscopy. Long-lived products can also be counted atom by atom with accelerator mass spectrometry. Systematic limits are the decay data, the counting efficiency, the irradiation history and the loss of product nuclei from the target.
Strengths
+Total cross section, independent of the γ-ray cascade and angular distribution.
+Counting can be done in a low-background setting away from the beam.
Limits
−Needs a radioactive product with a suitable half-life.
Neutrons with a wide range of energies are produced in short bursts. The time a neutron takes to reach the sample tells its energy, so one experiment measures the cross section at many energies.
For specialists
At spallation or photoneutron sources the neutron energy is found from the flight time over a known path. Capture is measured with γ-ray detectors and charged-particle reactions with silicon or gas detectors. Maxwellian-averaged cross sections at stellar temperatures are then computed from the energy-dependent data. Systematic limits include the neutron flux shape, the sample mass, the detector response and backgrounds from scattered neutrons.
Strengths
+Covers many energies in one measurement.
+Applies to neutron captures and to neutron-induced reactions on radioactive samples.
Limits
−Requires a sample of the target nucleus, which is hard for short-lived nuclei.
−The neutron flux must be known over the whole energy range.
An indirect method in which the interesting nucleus is carried inside a larger one, the Trojan horse, at an energy above the Coulomb barrier. The reaction with the carried nucleus can then be studied at very low effective energies.
For specialists
A two-body reaction A + x → c + C at sub-Coulomb energy is studied through a three-body reaction A + a → c + C + s, where a has a large x + s cluster structure and s acts as a spectator. In quasi-free kinematics the measured three-body cross section is factorised into a kinematic factor, the momentum distribution of x inside a, and the half-off-energy-shell two-body cross section. The method gives the energy dependence of the bare-nucleus cross section without Coulomb suppression or electron screening; its absolute scale is normalized to direct data at higher energies. Systematic limits are the validity of the quasi-free factorisation, the momentum distribution adopted, the off-shell corrections and the normalization.
Strengths
+Reaches energies far below those accessible directly, free of electron screening.
+Applicable to charged-particle-induced and to neutron-induced reactions.
Limits
−Requires normalization to direct data.
−Depends on the quasi-free reaction model and its assumptions.
At very low energy, capture happens at large distances from the nucleus, where only the tail of the final-state wave function matters. A transfer reaction at higher energy can measure the size of that tail, which then gives the low-energy capture rate.
For specialists
The asymptotic normalization coefficient (ANC) fixes the amplitude of the bound-state wave function outside the nuclear interior. For peripheral transfer reactions, measured at energies where the cross section is large, the ANC can be extracted with much smaller dependence on the optical-model and single-particle parameters than a spectroscopic factor. The ANC then determines the external direct-capture contribution and the contribution of subthreshold states in an R-matrix calculation. Systematic limits are the optical potentials, the peripherality of the reaction and the reaction model (DWBA or coupled channels).
Strengths
+Gives low-energy direct capture and subthreshold-state contributions without measuring the capture itself.
+Uses reactions with large cross sections.
Limits
−Model dependent through the optical potentials and the reaction theory.
−Covers only the non-resonant and subthreshold parts of the rate.
Not a measurement of the reaction itself, but of one ingredient of it. How long an excited state of the product nucleus lives fixes how strongly it takes part in the capture. Very short lifetimes are measured from how much the emitted γ-rays are shifted by the Doppler effect while the recoiling nucleus slows down in the target.
For specialists
In the Doppler-shift attenuation method (DSAM) the nucleus is populated by a reaction that gives it a known recoil velocity, and the γ-ray energy is measured while the nucleus slows down in the target or backing. The attenuation of the full Doppler shift, compared with the stopping power, gives lifetimes from about 1 fs to 1 ps. The radiative width that follows enters an R-matrix fit of the capture, for example for a subthreshold state whose tail reaches into the Gamow window. Systematic limits are the stopping powers, which are known to a few per cent at best, and the sensitivity at the shortest lifetimes, where the attenuation is nearly complete.
Strengths
+Constrains the width of a state that cannot be reached directly at low energy.
+Independent of the capture cross section and its normalization.
Limits
−Depends on stopping powers.
−Lifetimes below about a femtosecond are hard to resolve.
The time-reverse of a capture. A fast beam of the product nucleus passes close to a heavy target, and the strong electric field breaks it apart. From the break-up rate the capture cross section can be calculated.
For specialists
Baur, Bertulani and Rebel proposed the method in 1986. The virtual photons of the Coulomb field of a high-Z target induce the photodissociation B + γ → A + a, which is related to the capture A(a,γ)B by detailed balance. The dissociation cross section is large because of the high virtual-photon flux, and radioactive beams can be used. Systematic limits are the contributions of nuclear break-up, of higher multipoles (E2 in an E1-dominated case), of post-acceleration in the Coulomb field, and the reaction model used to extract the photodissociation cross section.
Strengths
+Large cross sections and the use of radioactive beams.
+Low relative energies between the fragments are accessible.
Limits
−Depends on the reaction model and on multipole separation.
−Nuclear and higher-order Coulomb effects must be subtracted.
Lasers compress and heat a small capsule of fuel until the nuclei fuse. The reactions then take place in a hot plasma, as they do in stars, rather than between a beam and a cold target.
For specialists
Reaction yields are measured from the emitted neutrons, charged particles or γ-rays, and the plasma temperature and density are inferred from other diagnostics. The method samples the thermally averaged reactivity directly and can test plasma effects such as screening. Systematic limits are the knowledge of the plasma conditions, which vary in space and time, and the separation of the reaction of interest from other reactions in the fuel. At the National Ignition Facility, Casey et al. measured the 2H(d,n)3He and 3H(t,2n)4He S-factors in implosions at temperatures of 2.1–5.4 keV and densities of 1.2–16 g cm−3, in agreement with accelerator data within the uncertainties.
Strengths
+Probes reactions in a thermal plasma.
+Allows studies of reactions between radioactive light nuclei such as tritium.
When the target of a neutron capture is too radioactive to make into a sample, the reaction can be studied backwards: γ-rays knock a neutron out of the product nucleus, and what that reveals about the nucleus is used to calculate the capture.
For specialists
Monoenergetic, polarized photon beams from laser Compton backscattering, such as HIγS at the Triangle Universities Nuclear Laboratory, measure the (γ,n) cross section of the capture product from the neutron separation energy upwards; with (γ,γ′) data below it they fix the γ-ray strength function, the main uncertainty of statistical-model capture cross sections. The result is a constrained calculation, not a measurement of the capture itself; the 85Kr branch point is an example (Raut et al. 2013).
Strengths
+Needs no radioactive sample of the capturing nucleus.
+Constrains the γ-ray strength, the weakest input of the statistical model.
Limits
−Gives the capture cross section only through a model.
−Probes the product nucleus, not the stellar reaction at stellar energies.
Some rates cannot be measured at all at stellar energies and are calculated instead. The weak proton–proton fusion that starts the Sun's energy production is the main example.
For specialists
For weak-interaction processes such as p + p → d + e+ + ν the cross section is too small for any laboratory measurement and is computed from nuclear theory. For radiative captures and resonant reactions, R-matrix and microscopic models extrapolate measured data to stellar energies. Electron capture rates in the solar plasma also come from theory.
Strengths
+The only source for some weak-interaction rates.
+Provides the energy dependence that links measured data to stellar energies.
Limits
−Accuracy depends on the nuclear model and its inputs.
Accelerators under rock, where the cosmic-ray background is low enough to measure the smallest cross sections.
1At the surface, cosmic rays make a steady rain of muons, which leave signals in any γ-ray detector.
2About 1400 m of rock (3800 m water equivalent) stop all but about one muon in a million.
3LUNA 400 kV: proton and helium beams for hydrogen burning, since the early 2000s.
4The 3.5 MV machine of the Bellotti Ion Beam Facility (BIBF), inaugurated in 2023: proton, helium and carbon beams for helium and carbon burning.
5The beam hits a target; a detector around it counts the γ-rays or particles from the few reactions.
Schematic, not to scale.
LUNA 50 kV
LUNA
Laboratory
Laboratori Nazionali del Gran Sasso (INFN)
Location
Assergi, L'Aquila, Italy
Depth
about 1400 m of rock, 3800 metres water equivalent
Accelerator
50 kV ion accelerator with a windowless gas target
Years
described in 1994; first results on 3He(3He,2p)4He in 1996
The first accelerator built deep underground for nuclear astrophysics. It was a small pilot machine, installed under the Gran Sasso mountain to show that the rock overhead removes enough cosmic-ray background to measure fusion reactions at the energies found in the Sun.
For specialists
Compact 50 kV accelerator with a windowless, differentially pumped gas target, a beam calorimeter for the current measurement and silicon detector telescopes for the charged reaction products. It was designed for 3He(3He,2p)4He in the solar Gamow peak, which it reached down to 16.5 keV.
about 1400 m of rock, 3800 metres water equivalent
Accelerator
400 kV electrostatic accelerator (LUNA II)
Years
in operation from the early 2000s (facility paper 2003)
The second LUNA accelerator, installed at Gran Sasso after the 50 kV pilot machine. Its very stable proton beam has been used for most of the underground measurements of hydrogen burning.
For specialists
High-current 400 kV accelerator. Resonance scans gave an absolute proton energy known to about ±300 eV, an energy spread below 100 eV and a long-term drift of about 5 eV per hour. It has served both solid targets with HPGe detectors and a windowless gas target with a 4π BGO summing detector.
about 1400 m of rock, 3800 metres water equivalent
Accelerator
3.5 MV Singletron (High Voltage Engineering Europa), terminal voltage 0.3–3.5 MV, ECR ion source
Years
installed 2022; inaugurated October 2023
An underground accelerator laboratory at Gran Sasso with a 3.5 MV machine, named after Enrico Bellotti, the first director of the laboratory. Its proton, helium and carbon beams extend underground measurements from hydrogen burning to helium and carbon burning.
For specialists
The Singletron has a 10 GHz permanent-magnet ECR source and delivers about 1 mA of protons, 0.5 mA of helium and 0.15 mA of carbon. The design paper reports an energy stability better than 10−5 and a terminal-voltage ripple of 1.5 × 10−5. The facility is run as a user facility by the LNGS accelerator service, with beam time allocated through yearly calls.
about 2400 m of marble, 6720 metres water equivalent
Accelerator
400 kV high-current platform with ECR ion sources (H+, He+, He2+)
Years
first experiments underground in early 2021
The Jinping Underground laboratory for Nuclear Astrophysics, a high-current accelerator deep inside a mountain in south-west China. Like LUNA, it uses the rock above it to reduce the background from cosmic rays.
For specialists
The 400 kV platform was designed for 10 emA of H+ and He+ and 2 emA of He2+ from ECR sources. It ran for more than 2000 hours underground in early 2021, with BGO and neutron detectors, on reactions including 12C(α,γ)16O, 13C(α,n)16O, 25Mg(p,γ)26Al and 19F(p,α)16O. The muon flux measured at the laboratory is (3.53 ± 0.22 ± 0.07) × 10−10 cm−2 s−1.
JN Van de Graaff, about 150 kV to 1.1 MV, moved from Notre Dame
Years
first beam 2017, inaugurated July 2017
The Compact Accelerator System for Performing Astrophysical Research, an underground accelerator in the former Homestake gold mine, where Davis ran his solar neutrino experiment. It is used mainly for helium-induced reactions, including the stellar neutron sources.
For specialists
A JN-type Van de Graaff that ran above ground at Notre Dame was moved to the 4850 ft level of SURF from 2015. It delivers proton and α beams of 200–250 μA on target. The CASPAR collaboration gives a muon flux of about 4.4 × 10−9 cm−2 s−1. CASPAR is run by Notre Dame, the South Dakota School of Mines and Technology and the Colorado School of Mines.
Felsenkeller underground laboratory (HZDR and TU Dresden)
Location
Dresden, Germany
Depth
45 m of rock
Accelerator
5 MV Pelletron (NEC 15SDH-2), with an external sputter ion source and an internal RF source
Years
inaugurated 4 July 2019
A shallow-underground accelerator laboratory in tunnels under a hill in Dresden. The rock above it, together with detectors that veto cosmic-ray muons, lowers the background enough for many reactions that are hard to measure at the surface.
For specialists
The rock reduces the muon flux by a factor of 40 and the neutron flux by a factor of 180 with respect to the surface. With an active muon veto the γ-ray background is about twice that of a deep underground site. The Pelletron was used before for 14C accelerator mass spectrometry in York. Typical beam currents are up to 30 μA for helium and carbon. The laboratory is run jointly by HZDR and TU Dresden.
10 MV FN tandem, 5 MV Sta. Ana single-ended Pelletron (2012), 3 MV St. Andre tandem (2017)
A university laboratory with several accelerators for nuclear astrophysics, used for capture and alpha-induced reactions. The St. George recoil separator is part of it.
A recoil separator built to study helium captures in inverse kinematics. A heavy beam hits a helium gas target, and the separator picks out the few product nuclei from the much more intense beam.
For specialists
St. George (Strong Gradient Electromagnetic Online Recoil separator for capture Gamma Ray Experiments) was designed for (α,γ) reactions in inverse kinematics with beams up to about A = 40, with an angular acceptance of 40 mrad and an energy acceptance of 7.4%.
Couder et al., Nucl. Instrum. Methods A 587, 35 (2008)
LENA, TUNL
Laboratory
Triangle Universities Nuclear Laboratory
Location
Durham, North Carolina, United States
Accelerator
230 kV ECR accelerator and 1 MV JN Van de Graaff; a 2 MV Singletron was added in the LENA II upgrade
The Laboratory for Experimental Nuclear Astrophysics, a low-energy, high-current accelerator facility at the surface. It uses detector arrays with active shielding against cosmic rays to measure weak proton-capture resonances.
The Detector of Recoils And Gammas Of Nuclear reactions, a recoil separator at the radioactive-beam facility of TRIUMF. It measures radiative captures in inverse kinematics, including captures on short-lived nuclei that cannot be made into targets.
For specialists
DRAGON has a windowless gas target surrounded by a γ-ray detector array, an electromagnetic recoil mass separator and focal-plane detectors. It was commissioned with stable and radioactive beams from ISAC; its first radioactive-beam study was 21Na(p,γ)22Mg.
Hutcheon et al., Nucl. Instrum. Methods A 498, 190 (2003)
ERNA recoil separator
Laboratory
CIRCE laboratory, Caserta (at Ruhr-Universität Bochum until 2009)
Location
Caserta, Italy
Accelerator
recoil separator, first on the Bochum 4 MV Dynamitron, since 2009 at CIRCE
The European Recoil separator for Nuclear Astrophysics, built to measure helium-capture reactions such as 12C(α,γ)16O in inverse kinematics by counting the product nuclei directly.
For specialists
ERNA was built at the 4 MV Dynamitron in Bochum to detect 16O recoils from 12C(α,γ)16O. Wien filters before and after the analysing magnet reduced the 16O contamination of the 12C beam from about 10−11 to below 2 × 10−29. In 2009 the separator moved to the CIRCE laboratory in Caserta, where a charge-state selection dipole was added directly after the target.
The laboratory where William Fowler and colleagues measured many of the reactions that power stars. Salpeter's work on the triple-alpha process (1951) and the measurements of the Hoyle state (1953, 1957) were done here.
A nuclear research institute with several accelerators. Its astrophysics group is known for activation measurements, in which the cross section is found by counting the decays of the radioactive product after the irradiation.
A university cyclotron laboratory where many transfer-reaction measurements of asymptotic normalization coefficients (ANCs) were made for capture reactions of astrophysical interest.
20 GeV/c proton beam from the CERN PS on a lead spallation target
Years
in operation since 2001
A neutron source at CERN in which protons from an accelerator strike a lead target and knock out neutrons with a wide range of energies. The energy of each neutron is found from its time of flight along a long beam line. It measures neutron captures for the s- and r-processes and neutron-induced reactions on light nuclei.
For specialists
Pulses of up to 8.5 × 1012 protons, 6 ns (rms) long, strike the lead target. The horizontal flight path to experimental area EAR1 is 185 m; a second area, EAR2, sits about 20 m above the target on a vertical flight path. A nitrogen-cooled third-generation target was installed during the CERN Long Shutdown 2 (2019–2021).
A large laser system built for inertial-confinement fusion. Its lasers compress small capsules of fuel into a hot, dense plasma, in which fusion reactions between light nuclei can be studied in conditions closer to those in stars than an accelerator beam on a cold target.
Dynamitron, Stuttgart
Laboratory
Institut für Strahlenphysik, Universität Stuttgart
Location
Stuttgart, Germany
Accelerator
Dynamitron
A university accelerator laboratory where 12C(α,γ)16O, among other helium-capture reactions, was measured with γ-ray detectors.
Kernforschungszentrum Karlsruhe (now Karlsruhe Institute of Technology)
Location
Karlsruhe, Germany
Accelerator
single-ended Van de Graaff (3.7 MV)
A Van de Graaff accelerator at the Karlsruhe nuclear research centre, used for measurements of 12C(α,γ)16O and, for decades, of the neutron captures of the s-process. Protons on a lithium target there make neutrons whose energies mimic those in a star, and many of the stellar capture rates in use today were measured with them.
For specialists
Protons of 1912 keV, just above the 7Li(p,n)7Be threshold, give a forward-collimated neutron spectrum that closely resembles a Maxwell–Boltzmann distribution at kT = 25 keV; a sample in the cone is activated, and the 7Be activity of the target gives the integrated flux (Beer and Käppeler 1980; Ratynski and Käppeler 1988). With a pulsed beam the same machine served time-of-flight measurements with the Karlsruhe 4π BaF2 array, 42 crystals forming a spherical shell of 10 cm inner radius and 15 cm thickness (Wisshak et al. 2006).
The tandem accelerator of the Triangle Universities Nuclear Laboratory, run by Duke University, the University of North Carolina and North Carolina State University. LENA, the low-energy laboratory, is part of the same institute.
Jožef Stefan Institute
Laboratory
Jožef Stefan Institute
Location
Ljubljana, Slovenia
A research institute in Ljubljana whose accelerator laboratory has measured proton captures on light nuclei at low energy.
Research Center for Nuclear Physics, Osaka University
Location
Ibaraki, Osaka, Japan
A university centre for nuclear physics with cyclotrons for nuclear structure and reactions. The OCEAN low-energy set-up measured 3He + 3He fusion here.
Laboratory for Laser Energetics, University of Rochester
Location
Rochester, New York, United States
A high-power laser that compresses small fuel capsules for inertial-confinement fusion research. Fusion of light nuclei in its implosions has been studied as a plasma counterpart of accelerator measurements.
CERN's facility for radioactive nuclei. Protons strike thick targets, and the short-lived nuclei produced are separated by mass and delivered as low-energy or post-accelerated beams.
Heavy Ion Accelerator Facility, ANU
Laboratory
Australian National University
Location
Canberra, Australia
Accelerator
tandem electrostatic accelerator
The heavy-ion accelerator laboratory of the Australian National University, used for nuclear structure and reaction studies.
Center for Isotopic Research on Cultural and Environmental heritage (Università della Campania and INNOVA)
Location
Caserta, Italy
An accelerator laboratory in Caserta built for isotope analysis. Since 2009 it has also hosted the ERNA recoil separator for helium-capture measurements.
Institut de Physique Nucléaire d'Orsay (IPN Orsay)
Location
Orsay, France
An accelerator at Orsay that hosts STELLA, a station built to measure the fusion of light nuclei such as carbon with carbon at very low energies, where the reaction is extremely rare.
For specialists
STELLA combines a silicon array for the charged particles with LaBr3 detectors of the UK FATIMA array for the γ-rays, with nanosecond timing that separates protons from α particles, and a rotating target that withstands beam currents above 10 µA. It is designed for cross sections down to tens of picobarns.
Centre d'Études Nucléaires de Bordeaux Gradignan (CNRS/IN2P3)
Location
Gradignan, Bordeaux, France
Accelerator
Van de Graaff
A nuclear physics laboratory near Bordeaux. Its Van de Graaff accelerator was used for one of the direct measurements of 7Be(p,γ)8B with a radioactive target.
RIKEN (today the RIKEN Nishina Center for Accelerator-Based Science)
Location
Wako, Saitama, Japan
A Japanese national research institute whose accelerators make beams of short-lived nuclei by fragmenting heavier ones in flight. The first Coulomb-dissociation measurement of 8B was done here with the RIPS fragment separator.
A heavy-ion accelerator laboratory. Its fragment separator delivers fast beams of short-lived nuclei, which were used to measure the Coulomb dissociation of 8B and the break-up of 6Li.
pulsed electron linear accelerator; measurement stations 10 to 400 m from the neutron source
A neutron time-of-flight facility of the European Commission. Short bursts of electrons make bursts of neutrons of every energy from thermal to about 20 MeV, and the time each neutron takes to reach a sample gives its energy. It measures neutron cross sections for nuclear data and for astrophysics, 197Au(n,γ) among them.
For specialists
The electron pulses are compressed to about 1 ns; up to ten experiments run at once on flight paths between 10 and 400 m, covering neutron energies from 10 meV to 20 MeV.
Los Alamos Neutron Science Center, Los Alamos National Laboratory
Location
Los Alamos, New Mexico, United States
Accelerator
800 MeV protons on a tungsten spallation target (Lujan Center); DANCE on a 20 m flight path
The Detector for Advanced Neutron Capture Experiments: a ball of barium fluoride crystals that surrounds a tiny sample and catches nearly every γ-ray emitted when it captures a neutron. Because the sample can weigh as little as a milligram, it can be radioactive, which is what branch points of the s-process need.
For specialists
Up to 160 BaF2 crystals form a 4π shell of about 17 cm inner radius and 15 cm thickness; samples down to about 1 mg, or with activities up to about 1 Ci, can be measured. Neutron energies come from the time of flight.
Van de Graaff for 7Li(p,n) activations; a high-current RFQ–IH proton linac (about 2 MeV) under construction
The Frankfurt Neutron Source, designed to be the most intense source of neutrons at the energies found in stars, about a thousand times more intense than the Karlsruhe source in that energy range. Like Karlsruhe, it makes the neutrons from protons on lithium.
superconducting proton linac (SARAF) on a liquid-lithium target (LiLiT), 1.9 MeV, 1–2 mA
A proton accelerator whose milliampere beam strikes a flowing film of liquid lithium, making neutrons with a spectrum close to that of a star at 30 keV, more than ten times more intensely than earlier lithium targets. Samples are activated and their radioactivity counted.
For specialists
The thick-target 7Li(p,n)7Be spectrum at 1.92 MeV is quasi-Maxwellian with kT ≈ 30 keV; 3–5 × 1010 n/s were reached in the first experiments, which measured the 30 keV MACS of 94Zr and 96Zr (Tessler et al. 2015).