Nuclear Physics News

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Going Beyond the Drip-Line with MoNA

J. A. Brown for the MoNA Collaboration

To cite this article: J. A. Brown for the MoNA Collaboration (2010) Going Beyond the Neutron Drip-Line with MoNA, Nuclear Physics News, 20:3, 23-26, DOI: 10.1080/10619127.2010.483401 To link to this article: http://dx.doi.org/10.1080/10619127.2010.483401

Published online: 08 Sep 2010.

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Download by: [Michigan State University] Date: 03 July 2017, At: 08:45 facilities and methods

Going Beyond the Neutron Drip-Line with MoNA

Physics of Interest Understanding the structure of very neutron-rich nuclei is critical to developing reliable and robust models of the nucleus. For nuclei near the neutron drip-line, where the neutron removal energy approaches zero, sev- eral features of nuclear structure are much more important than they are in stable nuclei. The neutron pairing energy becomes comparable to the neutron removal energy and nuclei with odd neutron numbers tend to be unbound while their even neighbors remain bound. Additionally, shell effects can be greatly altered by the absence of protons in the same valence orbitals occupied by the neu- trons. The lack of the stabilizing influ- Figure 1. The Modular Neutron Array. ence of protons in similar orbits allows for the tensor interaction between these weakly bound and the core reconstructed kinematically from the Undergraduate Student nucleons to alter the energies of single measured neutron and charged Involvement particle levels. These changes lead to fragment. Perhaps the most unique aspect of the disappearance of well-known With the exception of the lightest the MoNA collaboration is its pur- magic numbers and the appearance of nuclei, the study of nuclei beyond poseful and ubiquitous involvement new sub-shell closures that have the the drip-line is performed at projec- of undergraduate students. One of requisite properties of a magic number: tile fragmentation facilities, which the peculiarities of the U.S. educa- enhanced stability, a large excitation have the advantage that both the tional system is that there are many energy for the first excited state, and emitted neutron and the daughter small primarily undergraduate col- spherical shape. fragment move near beam velocity leges, which have a strong tradition Exploring the evolution of shell and only forward angles have to be of developing students, and these structure requires studying the covered by the detector systems. students eventually seek advanced excited states of neutron-rich nuclei The importance of the exploration degrees at rates greater than similar and at least the ground states of their of the neutron drip-line is evidenced students from Ph.D.- granting insti- neighboring nuclei with either one or by the development of detector tutions [5]. The MoNA collabora- two fewer or additional neutrons. arrays at all of the major projectile tion brings together ten of these Near the drip-line, the excited states fragmentation facilities, for example primarily undergraduate colleges, of even doubly magic nuclei become LAND at GSI [1], DEMON at with three graduate research institu- unbound to direct neutron emission, GANIL [2], NEUT-(A,B) at RIKEN tions. The construction of MoNA and nuclei just beyond the drip-line [3], and the Modular Neutron Array was funded by the U.S. National have neutron-unbound ground states. (MoNA) [4] at the National Super- Science Foundation and executed by Determining their excitation energy conducting Cyclotron Laboratory students at the undergraduate institu- experimentally requires neutron decay (NSCL) at Michigan State Univer- tions [6]. The construction of and spectroscopy in which the decay is sity (MSU). research with these detectors allows

Vol. 20, No. 3, 2010, Nuclear Physics News 23 facilities and methods

beam particles are measured with two tracking detectors in the beam- line. Timing scintillators near the exit of the A1900 and near the secondary target establish the speed of each incoming particle and its time of arrival at the target. The sweeper magnet separates the unreacted beam from the reaction products, which are identified with two position-sensitive charged-particle detectors and a ΔE-E system made of an ionization cham- ber and two plastic scintillators. A diagram of the experimental hall is Figure 2. Experimental set-up. shown in Figure 2. MoNA itself consists of 144 bars of fast plastic scintillator, which are each 200 × 10 × 10 cm in size. Each students to be involved in a large superconducting sweeper magnet that bar has a light guide and photomulti- project at a leading laboratory, yet bends the charged particles by 40°. plier tube on its ends. The position retain many of the benefits of their The position and angle of incoming and time of arrival of a neutron home institutions [7]. The assem- bled array located at the NSCL is shown in Figure 1 . 8

Since coming on-line in 2003, 16 experiments have been conducted with MoNA, and 85 undergraduates 7 from 15 institutions have been actively involved with MoNA. The 6 students are co-authors in peer- reviewed journals, which include three papers with undergraduate stu- 5 dents as first author [8–10]. In addi-

tion, the students have presented over ) [MeV]

+ 4 70 talks and posters at professional meetings.

E(2 3 Device Description Besides MoNA itself the experi- 2 mental set-up at the NSCL consists of several additional detectors to measure all parameters necessary to 1 reconstruct the decay events. The secondary beam produced by the 0 Coupled Cyclotron Facility at the NSCL and separated by the A1900 14O 16O 18O 20O 22O 24O fragment separator impinges on a sec- ondary target in front of a large gap Figure 3. Energy of first 2+ states of even oxygen isotopes.

24 Nuclear Physics News, Vol. 20, No. 3, 2010 facilities and methods

striking the bar are computed from the Recent Results isotopes has been pushed beyond the difference and average of the times, drip-line [19]. The isotopes were Doubly Magic 24 respectively, registered by the elec- O produced in the one-proton removal tronics processing the signal from The research of the MoNA collab- reaction 1H(14Be,12,13Li) and the each tube. oration has concentrated on the light ground-state of 12Li was observed to neutron-rich mass region. Especially The detector array has an intrinsic be an s-wave resonance with a decay efficiency of approximately 70% for the appearance of new sub-shell clo- energy spectrum as shown in Figure 4a. = = neutrons with energies typically sures for N 14 [11–13] and N 16 The MoNA collaboration performed [14, 15] in the oxygen region were produced in experiments at the cur- an experiment where 12Li was popu- studied. The last particle-stable iso- rent NSCL beam energies of 60–120 24 lated in a different reaction, namely MeV/u. Unfortunately, a limitation of tope of oxygen is O with 16 neu- the two-proton removal reaction trons while the fluorine isotopes the current set-up is the geometric 9Be(14B,12Li). In addition to the = efficiency of MoNA at higher decay extend to at least N 22, which pro- ground state, two excited states were energies, which drops to 50% at vides some evidence for a double observed for the first time [9] as shell closure at 24O. Yet the most 1.5 MeV. 24 shown in Figure 4b. The s-wave The invariant mass of the neu- compelling evidence for O being parameters were taken from Ref. [19] tron-emitting nucleus is calculated doubly magic comes from the obser- (dashed line) and the decay energies from the reconstructed four-momenta vation of a high lying first excited of the two excited states were 250 ± of the daughter and neutron; state, which was reconstructed from 20 keV (dot-dashed) and 555 ± 20 keV an invariant mass measurement with 222=++ − (dashed), corresponding to excitation mmmfn2(cos). EEpp fnfnq MoNA. The excitation energy was energies in 12Li of 130 ± 25 keV and ± The decay energy is then calculated from determined to be 4.72 0.11 MeV, 435 ± 25 keV, respectively. This dem- =− − which shows a large increase relative the masses; Emmmdecay f n . onstrates the selectivity of different to the first excited states of the neigh- nucleon removal reactions and the boring lighter oxygen isotopes as

need to study these neutron-rich shown in Figure 3. This work formed isotopes with a variety of probes and the basis of Dr. Hoffman’s thesis reactions. work, for which he was recently awarded the APS Division of Nuclear Physics’ 2010 Dissertation Award in Prospects MoNA has been very productive a) LAND/GSI Nuclear Physics. Hoffman’s results combined with the recent momentum in the exploration of neutron distribution measurement of the one- unbound states and nuclei; however, neutron removal reaction demonstrat- as mentioned earlier, the ability to

ds/dE (arb. un.) ds/dE (arb. ing that the neutrons in the 24O ground study states with larger decay ener- gies (~1MeV) is currently limited by state occupy the 2s1/2 orbital [16] 80 firmly establishes 24O as a doubly the geometric acceptance. This limita- tion will be mitigated in the near 60 b) MoNA/NSCL magic nucleus [17]. future with the recent NSF funding of 40 the Large multi-Institutional Scintila- Counts Going Beyond 11Li: 12Li tor Array (LISA). LISA will essen- 20 Since the first report of an tially double the size of the array, extended matter radius (11Li [18]), the which will greatly expand the angu- 0 0.0 0.5 1.0 1.5 2.0 heaviest bound lithium isotope has Decay Energy (MeV) lar coverage available, allowing the been one of the most thoroughly stud- exploration of states with higher Figure 4. Comparison of 11Li + n ied halo nuclei. With the recent inves- decay energies, and permitting data. a) Adapted from Aksyutina et al. tigation of 12Li and 13Li the study experiments with more complex [19] and b) from Hall et al. [9]. of even more neutron rich lithium multiple neutron emitting processes.

Vol. 20, No. 3, 2010, Nuclear Physics News 25 facilities and methods

Another enhanced capability is the 4. B. Luther et al., Nucl. Instr. Meth. addition of the gamma-ray spectrom- A505 (2003) 33. eter CAESAR (CAESium iodide 5. NSF InfoBrief 08-311 (2008) 2. http:// ARray). This combination allows www.nsf.gov/statistics/infbrief/nsf08311/ nsf08311.pdf experimenters to distinguish reac- 6. T. Feder, Physics Today 58(3) (2005) 25. tions leading to ground states of the 7. R. Howes et al., Am. J. Phys. 73(2) daughter from those leaving the (2005) 122. daughter in a bound excited state. 8. D. H. Denby et al., Phys. Rev. C 78 In the longer term, MoNA and (2008) 044303. LISA are ideally suited for even 9. C. C. Hall et al., Phys. Rev. C 81 more neutron-rich beams that will (2010) 021302(R). 10. P. J. Voss et al., J. College Teaching become available with the Facility for and Learning 5(2) (2008) 37. Rare Isotope Beams (FRIB), which is 11. A. Schiller et al., Phys. Rev. Lett. 99 currently being designed at MSU. It (2007) 112501. will extend the study of nuclei 12. N. Frank et al., Nucl. Phys. A 813 beyond the drip-line past oxygen. We (2008) 199. expect to see our undergraduate stu- 13. M. Strongman et al., Phys. Rev. C 80 J. A. BROWN FOR THE MONA dents working on these cutting edge (2009) 021302. COLLABORATION 14. C. R. Hoffman et al., Phys. Rev. Lett. experiments in nuclear physics and The MoNA Collaboration consists of 100 (2008) 152502. researchers from Augustana College, with our collaboration for many 15. C. R. Hoffman et al., Phys. Lett. B 672 Central Michigan University, years to come. (2009) 17. Concordia College, Florida State 16. R. Kanungo et al., Phys. Rev. Lett. 102 University, , Hope References (2009) 152501. College, Indiana University at South 1. Th. Blaich et al., Nucl. Instrum. Meth. 17. R. Janssens, Nature 459 (2009) A 314 (1992) 136. 1069. Bend, Michigan State University, 2. I. Tilquin et al., Nucl. Instrum. Meth. 18. I. Tanihata et al., Phys. Rev. Lett. 55 , Rhodes A 365 (1995) 446. (1985) 2676. College, Wabash College, 3. T. Nakamura et al., Phys. Rev. Lett. 96 19. Yu. Aksyutina et al., Phys. Lett. B 666 Western Michigan University, and (2006) 252502. (2008) 430. Westmont College

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