Direct measurements in Nuclear Astrophysics: ERNA and LUNA
| Lucio Gialanella and Alessandra Guglielmetti (2013), Scholarpedia, 8(5):11959. | doi:10.4249/scholarpedia.11959 | revision #143450 [link to/cite this article] |
Nuclear reactions between charged particles play a crucial role in astrophysics since they determine and regulate the energy production and element nucleosynthesis in stars. As we will discuss later, Coulomb repulsion makes the fusion of nuclei very improbable at the typical stellar energies. As a consequence, these reactions are characterized by extremely low cross sections, which makes their study a challenge for experimentalists. Here two different and somewhat complementary experimental approaches are discussed. They are based in two laboratories in Italy and aim at the direct measurement of important reactions for stellar evolution and nucleosynthesis.
Nuclear reactions in stars and on earth
In a non-degenerate, non-relativistic stellar plasma, ion energy follows the Maxwell-Boltzmann distribution, which reaches a maximum at E=kT (where T is the star temperature and k the Maxwell-Boltzmann constant) and then decreases exponentially for increasing energy. Typical temperatures for stellar interiors in hydrostatic equilibrium range between 10\(^6\) to 10\(^8\) K, depending on the stellar mass and stellar phase, corresponding to kT values up to a few keV. This last quantity compares with the Coulomb potential energy barrier between interacting nuclei, ranging from 1 to a few tens of MeV, depending on the specific nuclei taken into consideration. Since the fraction of nuclei having the necessary energy to overcome the Coulomb barrier is negligible, reactions between charged particles in stars occur through tunneling of the Coulomb barrier, resulting in very small interaction cross sections. The famous physicist George Gamow, and independently Ronald Gurney and Edward Condon, first calculated the probability of penetrating the nuclear Coulomb barrier [1], which decreases exponentially for decreasing energy. The convolution of the energy distribution of nuclei in the stellar plasma and the tunneling probability through the Coulomb barrier determines a relatively narrow energy window, the “Gamow window”. This represents the effective energy window in which most charged particle induced nonresonant thermonuclear reactions occur. Such a concept breaks down under certain conditions if a nuclear reaction proceeds through narrow resonances at elevated temperatures [2] but this particular case is not considered here. In spite of the low cross section, the stellar rate is quite high because of the large stellar mass and density. For instance, in our Sun, the reaction rate of the p+p fusion reaction, which regulates the whole p-p chain and thus hydrogen burning, can be easily calculated dividing the Sun luminosity \( 2\cdot 10^{39} MeV/s \) by the energy released in the p-p chain (about 27 MeV) resulting in a reaction rate of about \(10^{38} s^{-1}\). On the contrary, in a laboratory, typical values for the beam intensity, the target density and the detector efficiency only allow us to obtain much smaller numbers: for the case above mentioned the rate would reduce to about 1 reaction every 100 million years, which hinders a direct measurement of this reaction cross section. The usual procedure, with a few exceptions discussed below, is to collect information over a broad energy range in order to gain a detailed understanding of the reaction mechanism and parameters, which allow an extrapolation to the astrophysical relevant energies. This procedure is not unambiguous, and, depending on the uncertainties on the model used, inaccurate and imprecise results may be obtained. Usually, thermonuclear cross sections are described in terms of their astrophysical S-factor which contains the pure nuclear behavior of the cross section and is related to the cross section itself by the simple relationship:
- \(S(E)=E\cdot\sigma(E)\cdot e^{2\pi\eta(E)}\)
where E is the center-of-mass energy and \(2\pi\eta(E)\) the Sommerfeld parameter describing the exponential-like energy dependence of Coulomb barrier tunneling [3, 4]: the energy dependence of the astrophysical S-factor is therefore free from the tunneling effect.
One should mention that several methods have been developed in order to extract relevant nuclear structure information indirectly through the study of nuclear processes having a higher cross section. The presentation of these indirect methods is out of the scope of this contribution. Still, it is important to note that such methods provide very important information, whereas usually they are not capable, without direct measurements, to fix the reaction rate. A key point in direct measurements is the background contribution in the detectors, which determines the energy range where signals from the reaction can be detected with sufficient precision and accuracy. There are three main sources of background on Earth: cosmic rays, natural radioactivity, and target-beam induced background. The first one is composed of high energy, and consequently highly penetrating, charged particles (mostly muons) produced in the interaction of primary cosmic rays with the atmosphere. This background can be reduced using active shielding, i.e. surrounding the detection setup with detectors sensitive to cosmic rays and setting an anticoincidence condition on the detection of reaction products. Passive shielding are of limited utility since additional background from interactions of the cosmic rays with the shielding material overwhelms the positive effect of attenuation in the shielding itself. An exception is the extremely large thickness which can be achieved in an underground laboratory. This constitutes an advantage also for the reduction of natural radioactivity, mainly related to the radioactive series (\(^{238}U, ^{235}U and ^{232}Th\)), or cosmogenic radionuclides, like for example \(^{40}\)K, as will be discussed in the following section An interesting alternative is offered by recoil mass separators (RMS), where the nuclei produced in a fusion reaction are detected keeping the information of the reaction kinematics. That constrains the observation in a condition where neither cosmic rays nor natural radioactivity practically contribute to the background. These two different approaches have been adopted by the LUNA (Laboratory for Underground Nuclear Astrophysics) [1][5,6] and ERNA (European Recoil mass separator for Nuclear Astrophysics) [7,8,9 and references therein] collaborations, respectively. The two facilities are highly complementary and provide a comprehensive view of astrophysically relevant nuclear cross sections involving charged particles.
The LUNA experiment
The LUNA experiment is located in the Gran Sasso Underground Laboratory (LNGS) in Italy [2]. This is the largest underground laboratory in the world for experiments in particle physics, particle astrophysics and nuclear astrophysics. The underground facilities have been built at the side of the ten kilometers long highway tunnel crossing the Gran Sasso Mountain, between L'Aquila and Teramo, about 120 km far away from Rome. They consist of three large experimental halls, each about 100 m long, 20 m wide and 18 m high and service tunnels. The rock overburden of about 1400 m (3800 m water equivalent) reduces the muon component of the cosmic background by a factor of 10\(^6\); the neutron component by a factor of 10\(^3\); and the gamma component by a factor of 10 with respect to a laboratory on the Earth’s surface [10]. As a result, the gamma background above 3 MeV in an High Purity Germanium (HPGe) detector placed underground at LNGS is reduced by a factor of ~2500 with respect to the same detector placed above ground [11]. In addition, going underground enhances the effect of passive shielding particularly for lower energy gammas where the background is dominated by environmental radioactivity. Indeed, as previously outlined, a passive shield can be built around the detector also in a laboratory at the Earth’s surface. However, the shielding efficiency cannot be increased by further adding any more shield since the cosmic muons would interact with the added material, creating more background. This problem is of course dramatically reduced in an underground laboratory. The LUNA collaboration has installed two accelerators underground: a compact 50 kV “home-made” machine [12] and a commercial 400 kV one [13]. Common features of the two are the intense beam currents achievable, the long-term stability, and the precise energy determination. The first two features are essential to maximize the reaction rate, while the third is important in view of the exponential energy dependence of the cross section. The first machine was operative between 1992 and 2001, while the 400 kV machine started operations in the year 2000 and is still operating. Figure 1 represents an artistical prospect of the LNGS underground laboratory with the location of the 50 kV (LUNA 1) and 400 kV (LUNA 2) accelerators. A possible location for the future machine (LUNA MV) is also shown.
Figure 1: Artistical view of the LNGS underground laboratory
With the 50 kV machine, two key reactions of the p-p chain of Hydrogen burning were studied at the solar Gamow peak energies: the \(^3He(^3He,2p)^4He\) [14] and the \(d(p,\gamma)^3He\) [15]. Hydrogen burning is the first nuclear fusion process occurring in stars having the final result of transforming 4 protons into a \(^4\)He nucleus with a net energy release of about 27 MeV. It coincides with the longest stage of a star’s life (also known as its main sequence phase) and is responsible for the prodigious luminosity of the star itself. It mainly proceeds either through the p-p chain or through the more efficient CNO cycle. In addition, the screening effect, i.e. lowering of the Coulomb barrier for a nucleus surrounded by electrons with respect to a bare one, was investigated through the \(d(^3He, p)^4He\) reaction [16], again using the 50 kV accelerator. Starting from year 2000, the 400 kV machine became operative. Figure 2 shows the main tank of the accelerator and part of the two beam lines, one devoted to experiments using gas targets and the other one to experiments with solid targets.
Figure 2: The 400 kV LUNA accelerator
The first reaction investigated with such a machine was the \(^{14}N(p,\gamma)^{15}O\), the slowest reaction of the CNO cycle governing its energy and neutrino production. It was studied using two different and complementary approaches: a solid target coupled with an High Purity Germanium detector allowed us to measure single transitions and determine branching ratios [17,18]; a windowless gas target coupled with an high efficiency Bismuth Germanate Detector (BGO) allowed us to determine the total cross section at lower energies than any other experiment [19], down to 70 keV in the center of mass system. It is remarkable to note that at the lowest energy of 70 keV, a cross section of 0.24 pb was measured, corresponding to an event rate of only 11 counts per day! The results obtained with both techniques were approximately a factor of 2 lower than the existing extrapolation [20] from previous data [21, 22] at very low energy. As a consequence, the CNO neutrino yield in the Sun is reduced by about a factor of 2. Moreover, the \(^{14}N(p,\gamma)^{15}O\) reaction in core hydrogen burning affects the age of globular clusters, i.e. conglomerates of 10\(^4\) to 10\(^6\) gravitationally bound stars among the oldest stars in our galaxy. The luminosity of the turnoff point in the Hertzsprung-Russell diagram of a globular cluster, namely the point at which the main sequence turns toward cooler and brighter stars, is used to determine the age of the cluster [23] and to derive a lower limit on the age of the universe. For a star at the turnoff point, hydrogen shell burning is powered by the CNO cycle and therefore the \(^{14}N(p,\gamma)^{15}O\) cross section plays a crucial role in the age determination: the higher the cross section , the younger the globular cluster’s age for a given turn-off luminosity. The LUNA measurement allowed to increase the age of globular clusters by 0.7 to 1 billion years [24], thus raising the lower limit on the age of the Universe up to 14 billion years. The results obtained by LUNA with solid [17] and gas target [19] in terms of astrophysical S factor as a function of energy are reported in Figure 3, together with the results obtained by LENA [25], Lamb and Hester [21] and Schröder [22] and together with NACRE [20] and LUNA [18] extrapolations.
Figure 3: Astrophysical S factor as a function of energy for the \(^{14}N(p,\gamma)^{15}O\) reaction
Then, another key process of the p-p chain, the \(^3He(^4He,\gamma)^7Be\) reaction was measured. This constitutes a very important parameter for the solar model prediction of the \(^7Be\) and \(^8B\) neutrino fluxes which are then compared with experimental values measured for example by SNO [26], SuperKamiokande [27] or Borexino [28]. Moreover, such a reaction is also important for the \(^7Be\) production during the Big Bang Nucleosynthesis (BBN). At LUNA, it has been measured using two complementary techniques: the detection of the prompt gammas directly emitted after the impinging of an intense \(^4\)He beam on a gaseous \(^3\)He target [29] and the collection of the \(^7Be\) atoms followed by an off-line detection of their \(\gamma\) activity (activation technique) [30]: the results obtained with the two techniques are in very good agreement, as shown in Figure 4. The energy region covered by LUNA is above the Gamow peak for the Sun but well within the Gamow peak for the BBN. Our results clearly rule out the \(^3He(^4He,\gamma)^7Be\) cross section as a possible source of the discrepancy between the predicted primordial \(^7Li\) abundance and its much lower observed value. The LUNA results along with other high precision measurements [31, 32, 8] (including the ERNA results) were considered in the recent compilation “Solar fusion II” [33] to derive the best astrophysical S-factor for the \(^3He(^4He,\gamma)^7Be\) reaction. The value obtained allows for an uncertainty reduction in the solar neutrino fluxes from 7.5\(\%\) to 4.3\(\%\) in the case of neutrinos coming from \(^8B\) beta decay and from 8\(\%\) to 4.5\(\%\) in the case of neutrino coming from \(^7Be\) beta decay [34].
Figure 4: Astrophysical S factor as a function of energy for the \(^3He(^4He,\gamma)^7Be\) reaction: the LUNA activation results are shown as filled red triangles [30] while the prompt \(\gamma\) ones are shown as empty red triangles [29]. The other data points refer to recent measurements of the same reaction [31,32,8].
More recently, the LUNA collaboration has been engaged in the measurement of a few other important (p, \(\gamma\)) fusion reactions belonging to the CNO, MgAl and NeNa cycles of hydrogen burning. These cycles are important for second-generation stars with central temperatures and masses higher than those of our Sun. Due to their higher Coulomb barriers, the reactions involved are relatively unimportant for energy generation, although being essential for the nucleosynthesis of elements with mass number higher than 20. Low-energy resonances (or the low-energy part of the direct capture component) which are inaccessible in a laboratory at the Earth’s surface could be measurable underground. Some reactions have already been measured above ground, but an underground reinvestigation could substantially improve our knowledge of the related reaction rate in the different astrophysical scenarios responsible, for example, for the abundances of the isotopes which are filling the universe. Details on all these measurements and their astrophysical implications can be found in specific publications reported in the LUNA experiment web-site [3]. This experimental program is foreseen to go on for the next two-three years at the 400 kV machine. Meanwhile, a more ambitious program is growing up. This consists in the installation of a 3.5 MV machine underground (LUNA-MV) to study key reactions of the helium burning which are relevant in stars at higher temperatures translating into higher energies. In particular, the \(^{12}C(^4He,\gamma)^{16}O\) reaction, the “Holy Grail” of nuclear astrophysics [35]; the (\(\alpha\),n) reactions on \(^{13}\)C and \(^{22}\)Ne, which provide the neutron sources for the s-process (a process responsible for the synthesis of about half of all heavy elements beyond Fe); and other (\(\alpha,\gamma\)) reactions on \(^{14}\)N, \(^{15}\)N and \(^{18}\)O relevant for the He-burning stage of stellar evolution are foreseen. This ambitious and long-lasting program has already received the scientific approval of the LNGS Scientific Committee and has been selected as "special project" by the Italian Research Ministry. The so-called “B node” at LNGS (see figure 1) has been identified as the best possible place to install the accelerator underground, due to its size and distance from the other LNGS experiments. A detailed technical study of the site preparation including floor sealing, construction of a rough experimental hall with all the necessary services and structures is being performed and should be completed by the end of year 2012. A key issue here is the necessity of an effective neutron shielding both toward the rest of the LNGS laboratory and toward the internal rock walls where water uptake positions of the Teramo aqueduct are present, since a relatively small neutron production is intrinsic in the reactions to be studied. This will be realized with two thick borated concrete doors (1 m frame + 1 m wall), a 10 cm HDPE (5\(\%\) Li) cover of the rock “walls” and a 20 cm concrete pavement. A simple picture of the site with indication of doors, frames and HDPE panels constituting the shielding is reported in Figure 6. Monte Carlo GEANT 4 simulations have been performed using the maximum foreseen neutron production rate (2000 n/s) and the maximum neutron energy (5.6 MeV): the results are very encouraging since the neutron flux is only 1\(\%\) of the natural LNGS one just outside the shielding, as requested by LNGS management. The LUNA MV project is again highly complementary to the ERNA program as, for instance, the \(^{12}C(^4He,\gamma)^{16}O\) reaction will be studied (and has already been studied by the ERNA collaboration in the past [36]) by both experiments allowing to reach the more comprehensive view of the physics and astrophysical impact behind it.
Figure 6: The LUNA MV site and its shielding
The ERNA experiment
Since few decades a new experimental technique has been exploited to determine nuclear reaction cross sections, based on the direct detection of the nuclei produced in the nuclear reactions by means of a Recoil Mass Separator (RMS). In particular, this approach is suited to study nuclear reactions where the two interacting nuclei merge producing a single nucleus and emitting one or more gamma rays, depending on the details of the process. These reactions are generally referred to as radiative capture reactions. Figure 7 shows the working principle of a RMS for the case of \(^4He(^{12}C,\gamma)^{16}O\).
In a radiative capture reaction, the fusion of projectile and target nuclei produces a nucleus which recoils in the laboratory system with an average momentum \(p_{r0}\) equal to that of the projectile. In fact, gamma-ray emission determines a change of the momentum of the recoils depending on the gamma ray emission angle. As a result, the trajectories of the recoils lay within a cone centered on the beam axis with an opening angle \(\theta_{max}=arctan(E_{\gamma}/(cp_{r0}))\), where \(E_{\gamma}\) represents the gamma-ray energy and \({c}\) the speed of light. Correspondingly, the momentum of the recoils varies in a momentum range equal to \(2\cdot E_{\gamma}/c\) around \(p_{r0}\).
Figure 7: Schematic working principle of a RMS.
In order to make the direct detection of the recoil ions possible, the reaction is initiated in inverse kinematics, thus maximizing \(p_{r0}\) and minimizing \(\theta_{max}\). If a thin enough target is used, the kinematically forward focused recoils emerge from the target together with the beam nuclei, whose number exceeds that of recoils by about 10 to 17 orders of magnitude for typical cross sections of astrophysical interest. Therefore an ion optical system consisting of elements such as magnetic dipoles, electrostatic analyzers or cross field Wien filters plus some focusing elements is needed to suppress the beam and transport the recoils to an end detector. Since these systems are sensitive to the charge of ions, one out of all charge states in which the recoils emerge from the target must be selected. The finite suppression factor still allows some beam ions to reach the final detector, therefore the possibility of ion identification is desirable.
The observed yield Y is related to the total reaction cross section \(\sigma\) by the following expression:
\[\tag{1}
Y=\sigma N_p N_t \epsilon T \phi(q_r)\
\]
where \(N_p\) is the number of projectiles impinging on the target and \(N_t\) is the number of target nuclei per unit target area perpendicular to the beam axis. \(\epsilon\) is the detection efficiency of the end detector, while \(T\) and \(\phi_{q_r}\) are, respectively, the transmission and the charge state probability for the selected charge state \(q_r\).
Obviously the accuracy and precision of cross section values extracted from the observed yield depend on the detailed knowledge of all quantities in the equation above. In most direct experimental approaches to cross section measurements quantities like \(N_p\), \(N_t\), and \(\epsilon\) play a role. Their impact on the cross section measurements is straightforward. The case of \(\phi_{q_r}\) and \(T\) is somewhat different, due to the fact that these parameters are particular to RMS. A detailed discussion of these aspects can be found in [37].
In the past decades a few RMS facilities were realized. A pioneering work was done at Caltech [38], where \(^4He(^{12}C,\gamma)^{16}O\) was measured using a recoil mass separator and a NaI(Tl) detection setup. Here the insufficient beam suppression required a coincidence condition between gamma-rays and recoils, thus reducing the advantages of the use of a recoil mass separator. Later started the NABONA (NAples BOchum Nuclear Astrophysics) collaboration [39] between the Institut für Experimentalphysik III of the Ruhr-Universität Bochum, the INFN-Sezione di Napoli and Dipartimento di Scienze Fisiche of the University of Naples Federico II, and for some experiments, the ATOMKI, Debrecen. The aim of this collaboration was to measure the cross section of \(^1H(^7Be,\gamma)^8B\) at the Tandem Laboratory of the University of Naples Federico II, Italy. This experiment was performed using a \(^7Be\) beam and a windowless hydrogen gas target in combination with a recoil separator with sufficient beam suppression and acceptance to detect the recoils without the need of the coincidence condition with gamma-rays [40]. As a follow-up of the NABONA collaboration, a new collaboration between the same groups started, whose aim was to study \(^4He(^{12}C,\gamma)^{16}O\) [4,6] and \(^4He(^3He,\gamma)^7Be\) [8] using a new recoil separator installed at the 4MV Dynamitron Tandem Laboratorium (DTL) of the Ruhr-Universität Bochum, named ERNA (European Recoil separator for Nuclear Astrophysics). At the same time a recoil mass separator named DRAGON was designed and installed at TRIUMF, where several reactions have been studied involving both radioactive and stable beams [41-45]. Other similar systems have been developed or are in course of development at different laboratories in Japan [46] and USA [47,48].
The ERNA collaboration completed a first experimental program at the Dynamitron Tandem Laboratorium of the Ruhr-Universität Bochum, Germany between 2000 and 2008.
In 2009 the separator has been dismantled and moved to the CIRCE (Center for Isotopic Research on Cultural and environmental heritage) laboratory of the Second University of Naples and INNOVA, Caserta, Italy.
Figure 8 shows the layout of ERNA at the DTL.
The details of the experimental setup have been described elsewhere [49,50,51]. Briefly, the ion beam emerging from the 4MV Dynamitron tandem accelerator was focused by a quadrupole doublet, filtered by a 52° analyzing magnet, and guided into the 75° beam line of ERNA by a switching magnet (these elements are not shown in Figure 8). A quadrupole doublet (MQD2) after the switching magnet was used to focus the beam on the gas target. For the purpose of beam purification, there is one Wien filter (WF1) before the analyzing magnet and one (WF2) between MQD2 and the gas target. The windowless gas target can be operated either in flow-mode or in recirculation. In the first case the target gas flows just once through the system, while in the second case a recirculation system allows to recover the gas, which is re-injected in the target system after passing a zeolith cryogenic filter for cleaning [52]. Depending on the pressure in the inner target cell, the target thickness can be as high as about \(4•10^{17}atoms/cm^2\). The number of projectiles is determined through the detection of the elastic scattering yield in two collimated silicon detectors located in the target chamber at 75° from the beam axis. When the gas target is operated in flow mode, a post-target stripping system can be used [53]. It consists of a thin inert gas layer (e.g. Ar) after the target cell, which allows the recoil ions to reach, independently of the reaction coordinate, the same charge state distribution, that can be accurately measured in order to determine the probability of recoil ion charge state \(q_r\) selected in the separator. The use of a post stripper allows to perform cross section measurements detecting recoils in one single charge state. Alternatively, the reaction yield must be measured for all relevant recoil charge states.
Figure 8: ERNA at DTL. AP=Aperture; FC=Faraday Cup; ST=Steerer; SS=Slit System; MQT=Magnetic Quadrupole Triplet; MQS=Magnetic Quadrupole Singlet; MQD=Magnetic Quadrupole Dipole; WF=Wien Filter
After the gas target, the separator consists sequentially of the following elements: a magnetic quadrupole triplet (MQT), a Wien filter (WF3), a magnetic quadrupole singlet (MQS1), a 60° dipole magnet, a magnetic quadrupole doublet (MQD4), a Wien filter (WF4), and finally a detector for the recoils. Additionally, several steerers (ST), Faraday cups (FC), slit systems (SS), and apertures (AP) are installed along the beam line for setting-up and monitoring purposes. Typical beam suppression factors range from \(10^{-10}\) to \(10^{-12}\), that is effectively improved by about 3 orders of magnitude using a detector for the recoils able to discriminate them from the leaky projectile beam ions. Different detectors are available for the detection of recoils: a \(\Delta E-E\) ionization chamber telescope and a TOF-E (Time-Of-Flight vs Energy) setup.
Figure 9: Sample matrices for \(^4He(^3He,\gamma)^7Be\) obtained with (a) the \(\Delta E-E\) detector, (b) the TOF-E detector (Courtesy A. Di Leva). See text for details.
In the first case, the approximately inverse proportionality between the energy loss per unit length and the ion kinetic energy in the MeV energy range is exploited, being the the proportionality constant equal to \(MZ^2\), where \(M\) and \(Z\) are, respectively, the ion mass and charge (see e.g. [54] for details). In the second case the time an ion requires to travel a known length in the last section of the separator is measured together with its kinetic energy, thus allowing for a mass identification. Figure 9 shows sample matrices obtained with these two techniques for \(^3He(^4He,\gamma)^7Be\) [8]. ERNA is also equipped with a gamma-ray detection setup at the gas target. The coincidence condition with the detection of recoils allows to obtain nearly background-free spectra. In fact, a direct estimate of the residual background in the spectrum is possible, when random coincidence events with the "leaky" ions are considered and scaled by the ratio of the number of recoils to the number of "leaky" ions. The results obtained for \(^{12}C(^4He,\gamma)^{16}O\) [9] are shown in Figure 10.
Figure 10: Gamma-ray spectrum from \(^4He(^{12}C,\gamma)^{16}O\). Black line: coincidence spectrum with both \(^{16}O\) recoils and "leaky" \(^{12}C\) ions. Red line: coincidence spectrum with \(^{16}O\) recoils ("true" coincidence). Grey line: coincidence spectrum with "leaky" \(^{12}C\) ions (random coincidence) scaled down to the observed \(^{16}O\) recoils to estimate background (Courtesy D. Schuermann). See text for details.
A considerable limitation to the performances of ERNA was determined by the overfocussing in the magnetic quadrupole triplet of beam ions in higher charge states than the one selected for the recoils. Due to that, a considerable fraction of the beam hit the plates of the first velocity filter, producing an intense leaky beam and effectively limiting the use of ERNA at low energy. A solution to this problem is the addition of a dipole magnet which selects a single charge state for both recoil and beam ions entering the first lens. The design of this Charge State Selection Magnet (CSSM) is rather complex, since its effective length must be kept as short as necessary in order not to increase the distance of the lens to the target too much. Due to the short length, a significant fraction of the magnetic field strength is in the fringe field which needs to be accurately tailored. This solution implemented now that ERNA has been moved to the Italian laboratory CIRCE [55]. Figure 11 shows the layout of ERNA at CIRCE.
Figure 11: ERNA at CIRCE. After the CSSM the layout is the same as in Figure 8, starting from MQT, with the exception of the 60° magnet which was rotated to fit in the laboratory.
In 2012 the re-installation has been completed and a new experimental program started, including \(^{1}H(^{7}Be,\gamma)^{8}B\), \(^4He(^{12}C,\gamma)^{16}O\), \(^4He(^{14,15}N,\gamma)^{18,19}F\), \(^4He(^{16}O,\gamma)^{20}Ne\), and other reactions of interest in Nuclear Astrophysics.
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