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NEUTRON EMISSION B. Jonson A BETA-DELAYED TWO-NEUTRON AND TITREE -NEUTRON EMISSION B. Jonsona¡¡, H.A. Gusta£ssona\ P.G. Hansen^, P. Ho££a\ P.O. Larsson0), S. Mattssonc\ G. Nymana^, H.L. Ravna) and D. Schardta) a) CERN-ISOLDE, CERN, Geneva, Switzerland b) Institute of Physics, University of Aarhus, Aarhus, Denmark c) Department of Physics, Chalmers University of Technology, Göteborg, Sweden. Abstract Gamow2) as beta-delayed alpha emission. The first delayed neutrons were observed in 1939 in exper• We review experiments on 11 Li that have led to iments3) on fission products produced in neutron the observation of two new radioactive decay modes: bombardments of uranium. A new wave of interest in beta-delayed two- and three-neutron emission. The the field began in the early sixties1*"9), as may be 2n decay mode was also observed in 3 0,31,3 2Na and seen from Table 1, and more than 100 precursors for very recent results, reported for the first time delayed particle emission are now known. On-line here, show that it is detectable also in 100Rb with mass separation has become increasingly important p. /p = 0.027 ± 0.007. for the field and has promoted the process from *zn *n being a barely detectable exotic nuclear decay mode 1. Introduction to being an important tool in investigations of high- energy beta-decay. This evolution as well as the Nuclear beta-decay to particle-unbound states present state of the art are illustrated in contribu• 10 11 may give rise to beta-delayed particle emission. tions to the Cargèse Conference » ) and to this 12 13 The first example of this process was found in 1914 conference > ). by Rutherford and Wood1) in experiments on naturally occurring thorium. They observed that some of the In this paper we review the experiments on the alpha particles had much higher energies than the main two latest additions to Table 1: The beta-delayed spectrum, and this finding was later explained by two- and three- neutron precursors. The experiments have been carried out at the ISOLDE facility at CERN and at the on-line mass separator at the CERN PS. Table 1 A summary of the results is given in Table 2. Delayed particle decay modes 2. Experimental studies of beta-delayed two- and three-neutron emission Emitted particle Reference Year An inspection of nuclear mass tables14) shows Alphas Rutherford and Wood1) 1916 that beta-delayed emission of more than one neutron (and also of more than one proton) becomes energeti• Neutrons Roberts et al.3) 1939 cally possible very far from the region of beta sta• bility. Figure 1 shows, as an example, the nuclear Karnaukhov et al.4) chart of the lightest elements with the expected Protons 1963 Barton et al.5) precursors of beta-delayed two- and three-neutron emission indicated. One finds that an appreciable Fission Skobolev6) 1972 number of particle-stable nuclides are possible precursors. But in view of the fact that they Azuma et al.7) all lie on the border regions that are hard to reach, Two neutrons 1979 Dêtraz et al.8) it will be clear that the main experimental problems lie on the production side. For a detailed descrip• Three neutrons Azuma et al.9) 1980 tion of target-ion-source techniques, essential in this kind of work, the reader is referred to the Table 2 Experimental branching ratios for beta-delayed two-neutron and three-neutron emitters Isotope p P Ref. Pin P2n 3n (*) m (*) xlLi 95 ± 8 82 ± 7 3.9 ± 0.5 1.8 ± 0.2 9 30Na a) 26 ± 4 1.2 ± 0.23 - 8 31Na 43 ± 11 - 1.0 ± 0.4 - 8 32 Na 10 ± 4 5.1 ± 1.8 b) - 8 100 Rb - p2n/pn=2.7±0.7 - this work a) See Section 2.3. b) Weighted mean of neutron correlation and gamma measurement. - 265 - the probability p^n per beta decay ). In most cases only a few of the neutrons from one event are regis• tered and it is therefore convenient to define the rate of actual correlated events of s counts in the data string. If each multiplicity is characterized by an energy-independent average efficiency e¿, the rate of events with s neutrons detected is D 2n precursor ~~~| Rs - E Pints'1 eiîJ 1=S Hei 2n & 3n precursor [><] where (g) is the binomial coefficient. The quantities Rs are directly determined from a correlation analysis of the data and the ratios of the individual p^n va• lues may be derived by solving Eq. (1). Fig. 1 Predicted precursors of beta-delayed two- and three-neutron emission in the lightest elements. In the actual data analysis a search for correla• The mass data used are taken from the tables of 14 ted events was done in a defined time interval 6 fol• Wapstra and Bos -'. lowing an initial event. If (q - 1) neutrons are found within this time interval, the event is regis• tered as q-fold. With the mean residence time for an event in the counter X"*1 the detection probability per unit time for an individual neutron belonging to the event is Xe"^. The rate of true q-fold events found with such an analysis technique is review paper by Ravn15). In the following we limit ourselves to a description of the experimental tech• R r niques used for the detection of the radiation. s s, (2) s=q 2.1 Time-correlation measurements where r,5 q is the probability that exactly (q events 2.1.1 Data-taking technique and strategy of out of s - 1 possible are falling insiid< e the chosen time window data evaluation. The simultaneous emission of more than one neutron emitted after a beta decay to an excited nuclear state has been verified7"9) in neu• tron coincidence measurements. In these experiments (q I Î) Í1 - e-Ae)q_1 e_A9(S"q) (3) the neutrons were detected with 3He proportional s.q counters embedded in a paraffin-wax moderator. Such The measured rate of q-fold events is with this a detector equipped with 12 neutron counters had an r strategy Mq = + M^ ), where Mtf) is the rate of efficiency of en = 20 ± 1% . The beam of radioactivity random events. The random rate is- dependent on the was directed to a collector situated at the centre of rate of registered neutrons and the corrections are the paraffin block. The residence time for a neutron calculated as combinations of events with lower mul• emitted from this position was determined from 3n 9 tiplicity. The general formulae for the random rate coincidences on Li. The time distribution was found become very complicated and we shall here only give to be exponential with a mean of X""1 = 89 ± 1 us. those of interest in this work. In the case of This time is long compared to the response time of doubles there is only one term given by the combina• the 3He detectors and it was therefore possible to tion of two single events, (l)2, connect all of them in parallel. In the string of neutron signals from the detector, neutron coinci• dences appear as time-correlated counts separated in 2 Rl6 M2W = R 6 e" time with a distribution characteristic of the resi• (4) dence time. The arrival times of the individual neutrons were recorded by feeding the neutron signals In the case of triples there are two types of into a CAB microprocessor, which was used as a conti• random contributions. The first is a pure random nuous time-to-digital converter recording the times triple event, that is of type (l)3 which has the of all the signals from the twelve 3He tubes. The form arrival times were recorded "on the fly" with a pre• cision of 1 us. The correlation analysis was then -Ri9 (5) performed in playback mode from magnetic tape. The main problem in the analysis of the time- correlated data is to find the true rate of double There are then three possible ways of combining and triple events in the presence of random correla• a single event with a registered double event: tions. As an example one may take 11 Li, which is a i) a double registered first and then one single, precursor of both two- and three-neutron emission: ii) a single first, and iii) the single event regis• the random triple rate originates in this case in tered in the time interval between the two events forming the double. The three terms are two sources : i) three single events combined to make a pure random triple event, and ii) a real double event detected together with a random single •xe •Rl0 j-j 21 (i) RiR26 e (6) event. We shall in the following outline the main xe formulae for the rates of true and random events and also describe the strategy chosen in the analysis of our data. Assume that we have a source with a total beta k) Most formulae given here are not restricted to disintegration rate D and that it decays through two- and three-neutron events but are generally channels involving the emission of i neutrons with valid for any multiplicity. - 266 - -xe) the delay time in the target15) and the rate fluctua• Rl6 (1 - e" (ii) R!R20 e (7) tions due to the macrostructure of the proton beam xe may then be neglected. The long-term variations in the data rate TX^ were corrected for by computing Dn, •Ri0 d - e"xe) _ -xe (8) (iii) RiR26 e" Dn, and D¿ for each buffer of about 1400 counts and xe e by averaging over the whole time for the experiment.
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