Research program at MAX-lab

The research program at the upgraded tagging facility was initially discussed at an international workshop in 1997 [1] and after that in connection with the Program Advisory and Scientific Advisory Committee meetings at MAX-lab. At present the accepted proposals cover the following topics.

A. Light nuclei
At the pre-upgrade facility the 4He(g,n) reaction was investigated in the energy range 25 to 42 MeV [2] and 50 to 71 MeV [3] using two segmented neutron detectors [4] which resulted in seven-point angular distributions. The differential cross sections [5] show a resonant behavior peaking at a photon energy of 28 MeV, in good agreement with newer calculations. Also the photodisintegration of 3He was investigated in the energy range 14 to 32 MeV at 90 deg with Si-detector telescopes [6]. The 90 deg differential cross section for the two-body break-up channel was compared to theoretical predictions in order to study the influence of three-nucleon-forces. A prototype He gas scintillator active target was used to study the total absorption cross section in 4He. A new active target has been constructed and new measurements, including detection of neutrons and photons, are in progress at the upgraded tagging facility [7]. These measurements are largely motivated by new ab initio calculations based on the method of Lorentz-Integral-Transforms. This is also the motivation for a new set of measurements of the total photoabsorption cross section for 6Li and 7Li using the attenuation method [8].

B. Compton scattering
At the pre-upgrade facility Compton scattering was measured with nominal 10in x 10in NaI(Tl) detectors. At that time four such spectrometers were available. Most of the differential cross sections were measured with scattering angles 45, 60, 90 and 135 deg. Studies of 4He, 12C, 16O, 40Ca and 208Pb were mostly done in a parasitic mode downstream other experiments. The Göttingen group was very active during this period [9]. The last experiment was Compton scattering on deuterium [10] with the goal to extract the electric and magnetic polarizabilities of the neutron [11]. The differential cross section was measured in two 10 MeV wide energy bins (average energies 55 and 66 MeV) at three scattering angles 45, 125 and 135 deg. The results are in good agreement with previous measurements at University of Illinois. At the upgraded facility Compton scattering on deuterium has been measured in the energy range 66 to 97 MeV with the intention to go higher in photon energy. This requires high energy resolution since the break up channel in deuterium is at 2,2 MeV. Three very large NaI(Tl) spectrometers are presently available at MAX-lab; BUNI, CATS and DIANA, all with excellent energy resolutions [12]. For these new measurements on deuterium the same cryogenic target as pre-upgrade is used.

C. Studies of the (g,pi) reaction
A major consequence of the energy upgrade of the tagging facility is the possibility to investigate the (g,pi) reaction at and close to the pion production threshold. At present the investigations are concentrated on the (g,pi+) reaction in H, 2H, C, and a few heavier nuclei [13]. The existing data set for the proton is limited to measurements close to threshold and above 180 MeV. The goal is to improve this situation to facilitate comparisons with theoretical calculations with e.g. Chiral Perturbation Theory. Three different detector systems are used, a set of dE - E scintillators [14], range telescopes [15], and a Si-strip dE - CsI(Tl) E telescope [16]. The Ge6 [17] detector set-up has been tested in combination with Si-strip detectors. These detectors were primarily brought to MAX-lab to study halo nuclei in e.g. the 6Li(g,pi+)6He reaction, however, these detectors are available also for other studies.

D. Polarized photons
Within the I3HP HadronPhysics Program a coherent bremsstrahlung facility has been designed with the emphasis on the production of polarized photons below 100 MeV. A goniometer system has been built with a diamond radiator and first measurements show clear evidence of the production of polarized photons with about 40% polarization. This program is a collaboration with the groups from Kharkov and Glasgow [18].

E. Investigations of knockout reactions
At the pre-upgrade facility a large part of the program was investigations of knockout reactions mainly in the quasideuteron region with photon energies above 30-40 MeV. Besides the light nuclei already mentioned the (g,p) reaction was studied for 6Li, 10B, 12C, 14N, 16O, 27Al, 40Ca, 51V and 208Pb [19]. The emphasis of these experiments and the investigations of the (g,n) reaction in 6Li, 12C and 16O [19], and the (g,pn) reaction in 6Li and 16O [19], was the question of the importance of meson exchange currents. The analyses showed that the quasideteron mechanism is the dominating process in the energy range above the Giant Dipole Resonance and below 80 MeV. Sofar these reactions have not been investigated at the upgraded facility. The 208Pb(g,p) reaction is studied as possible method to study deeply bound pionic atoms [20].

F. Investigations of detector responses to photons
The response of different scintillators to electrons and photons started already at the pre-upgrade facility, however, these investigations have been a more frequent topic at the upgraded facility. Part of the Swedish interest in the FAIR facility is the detector PANDA which will be equipped with a large electromagnetic calorimeter consisting of PWO scintillators. Groups from Uppsala, Stockholm and Lund are studying the response of such detectors using tagged photons from 10 to 120 MeV [21].

References

[1] Proc. of the MAX-lab Workshop on the Nuclear Physics Programme with Real Photons below 200 MeV, Lund March 10 - 12, 1997.
[2] B. Nilsson, Thesis, Lund University 2003.
[3] D.A. Sims et al., Phys. Lett. B 442 (1998) 43.
[4] J.R.M. Annand, B-E. Andersson, I. Akkurt, B. Nilsson, Nucl. Instr. and Meth. A400(1997)344.
[5] B. Nilsson wt al., Phys. Rev. C75 (2007) 014007.
[6] M. Karlsson, Thesis, Lund University 2005.
[7] J.R.M. Annand et al. link.
[8] P. Grabmayr et al. link.
[9] M. Ludwig et al., Phys. Lett. B274 (1992); K. Fuhrberg et al., Nucl. Phys. A548 (1992) 579
[10] K. Fuhrberg et al., Nucl. Phys. A591 (1995); D. Häger et al., Nucl. Phys. A595 (1995); S. Proff et al., Nucl. Phys. A646 (1999) 67.
[11] M. Lundin, Thesis, Lund University 2002; M. Lundin et al., Phys. Rev. Lett. 90 (2003) 192501.
[12] The Compton@MAX-lab Collaboration, link
[13] link
[14] K.G. Fissum et al., Phys. Rev. C53 (1996) 1278.
[15] P. Golubev et al., Nucl. Phys. A806 (2008) 216.
[16] L. Isaksson and J. Brudvik, private communication.
[17] D. Branford et al. link
[18] V. Ganenko et al. link
[19] See the list of publications and theses at the present homepage.
[20] L. Isaksson et al. link
[21] P-E. Tegner et al. link