3NN ADOPTED LEVELS:UNOBSERVED 15NDS 202205 3NN H TYP=FUL$AUT=J.E. Purcell{+#}, C.G. Sheu{+*}$CIT=NDS 130 1 (2015)$ 3NN2 H CUT=30-Jun-2015$ 3NN Q 3NN c The experimental evidence reviewed in all three mass 3 evaluations 3NN2c (1975Fi08,1987Ti07,2010Pu04) is mostly against the existence of a 3NN3c bound state consisting of three neutrons. Some evidence of a possible 3NN4c three neutron resonance has been reported, but it has generally been 3NN5c weak and not reproducible or more likely due instead to two neutron 3NN6c final state interactions. See (1968Ka35) for a summary of the 3NN7c experimental situation concerning possible {+3}n systems as of 1968. 3NN8c A theoretical study reported in (1996Cs02,1998Cs02) using the complex 3NN9c scaling method predicts a J|p=3/2+ three neutron resonance at 14 MeV 3NNac with a width of 13 MeV. 3NN c A listing of the reactions in which attempts were made to observe bound 3NN2c or resonant three neutron states follows. 3NN c 3NN c {+3}H(n,p){+3}n: 3NN2c This reaction has been studied at several neutron energies. A possible 3NN3c {+3}n bound state with binding energy around 1 MeV seen in the 0|' 3NN4c proton spectrum was reported by (1965Aj03) but was not confirmed in the 3NN5c subsequent similar experiments reported in (1966Th10,1967Fu09,1968De03) 3NN c 3NN c {+3}H({+3}H,{+3}He){+3}n,{+3}He({+3}H,{+3}He){+3}H: 3NN2c Studies of these reactions with E({+3}H)=22.25 MeV are reported in 3NN3c (1968Oh04). With the {+3}H target, there was no indication of a bound 3NN4c {+3}n system in the outgoing {+3}He spectrum, but there was a 3NN5c suggestion of a possible broad resonance at around 1-1.5 MeV excitation 3NN6c in the {+3}n system. With the {+3}He target, there was no evidence in 3NN7c the outgoing {+3}He spectrum of a resonance in the {+3}H system 3NN8c corresponding to the analogue of a bound {+3}n state. 3NN c 3NN c {+3}H(|p{+-},|g){+3}n: 3NN2c Studies of this reaction with stopped pions have been reported in 3NN3c (1976Bi05,1979Bi13,1980Mi12). There was no evidence seen in the |g 3NN4c spectrum of {+3}n bound or resonance states. A shift in the outgoing 3NN5c |g spectrum possibly indicating resonant {+3}n structure is adequately 3NN6c explained as due to pair-wise two neutron final state interaction 3NN7c (1980Mi12). 3NN c 3NN c {+3}He(|p{+-},|p{++}){+3}n: 3NN2c This double charge exchange reaction has been studied with E(|p{+-}) 3NN3c from 65-295 MeV 3NN4c (1970Sp05,1974Sp08,1986St09,1997Yu01,1999Gr01,1999Gr31). A 3NN5c resonance-like structure reported in (1970Sp05,1974Sp08,1986St09) has 3NN6c been explained as most likely due to the final state interaction of two 3NN7c outgoing neutrons (1970Ph05). As reported in (1999Gr01), a search for 3NN8c either bound or resonant {+3}n states was unsuccessful. The primary 3NN9c interest of the results reported in (1997Yu01,1999Gr31) was in the 3NNac double charge exchange mechanism, not on possible three neutron states. 3NN c 3NN c {+4}He(|p{+-},p){+3}n: 3NN2c One study of this reaction is reported in (1968Ka35) with 3NN3c E(|p{+-})=140 MeV who report a possible resonance feature in the 3NN4c proton spectrum at an excitation energy in the 3-neutron system of 3NN5c about 15 MeV. Studies of this reaction are also reported in 3NN6c (1970Zi05,1973Ba70,1981Ce01) with stopped pions and in 3NN7c (1977Ja15,1981Ka41,1981Ka43,1981Mc09,1983Ka14) with E(|p{+-}) ranging 3NN8c from 50-475 MeV. There was no mention of {+3}n states being observed in 3NN9c any of these experiments. However, the focus of these experiments was 3NNac on pion-nucleon interaction and not on possible three neutron states. A 3NNbc study of the reaction {+7}Li(|p{+-},{+4}He){+3}n with stopped |p{+-} 3NNcc is reported in (1977Ba47,1993Am09), but there is no mention of 3NNdc possible {+3}n states. 3NN c 3NN c Others: 3NN2c The following reactions using heavy ions were studied in unsuccessful 3NN3c attempts to observe {+3}n states: 3NN c {+14}N(|p{+-},{+3}n){+8}|b{++}{+3}H with stopped |p{+-} (1972Ag01) 3NN c {+7}Li(n,{+3}n){+5}Li, E(n)=14-19 MeV (1968Fu06,1970Ko24) 3NN c {+2}H({+14}C,{+13}N){+3}n, E({+14}C)=336 MeV (1995Bo10) 3NN c {+3}H({+7}Li,{+7}Be){+3}n and {+7}Li({+7}Li,{+11}C){+3}n, 3NN2c E({+7}Li)=65-82 MeV (1974Ce06,1987Al10,2005Al15) 3NN c {+7}Li({+11}B,{+15}O){+3}n, E({+11}B)=52-88 MeV 3NN2c (1986Be44,1986Be54,1987Bo40,1988Be02) 3NN c 3NN c A study of the breakup of {+14}Be by a carbon target, E({+14}Be)=490 3NN2c MeV, in which attempts to produce and detect neutron clusters is 3NN3c reported in (2002Ma21,2005Ma97). No {+3}n clusters were observed, but 3NN4c there was a possible hint of a few {+4}n clusters. 3NN c 3NN c On the theoretical side, an early study of the {+3}n system using 3NN2c Faddeev methods is reported in (1979Li19) and a shell model type 3NN3c calculation is reported in (1980Be22). These articles also reference 3NN4c earlier {+3}n system calculations. A calculation reported in (1996Cs02) 3NN5c using the complex scaling method obtained a resonance in the {+3}n 3NN6c system with J|p=3/2+ and an energy of 14 MeV and a width of 13 MeV. A 3NN7c study is reported in (1997So27) using the hyperspherical method with 3NN8c simplified two particle interactions of J|p=1/2- {+3}n states, which 3NN9c they consider to be the most likely spin-and-parity value of such 3NNac states. 3NN c 3NN c Three recent studies looking for possible bound or resonant {+3}n 3NN2c states are reported in (1999Wi08,2002He25,2005La11). As expressed in 3NN3c (2005La11), modern nucleon-nucleon interactions and calculational 3NN4c techniques do not predict any observable {+3}n bound states or 3NN5c resonances.