Trial for the long neutron counter TETRA using 96,97Rb radioactive sources

The TETRA long neutron counter is operated at ALTO ISOL facility behind the PARRNe mass separator. TETRA has been proven to be a unique instrument for measurements of β-decay properties of short-lived neutron-rich nuclei having applications for the nuclear structure and/or astrophysical r-process calculations. A proper calibration of TETRA can allow validation of the experimental procedure used for determination of β-delayed one-neutron emission probabilities (P1n). It requires the use of a well-known β-neutron decaying radioactive source which can be only produced and measured on-line due to its short half-life. Thus, the present paper reports on measurements of P1n and T1/2 of 96,97Rb nuclei using TETRA . The results obtained are in a good agreement with the values available in the literature. This proves that the developed techniques can be applied to unknown P1n and T1/2 of neutron-rich species.


β-delayed neutron emission
Nowadays the interest in the phenomenon of β-delayed (multi) neutron emission is arising but the mechanism which drives this process in neutron-rich nuclei is still not understood. However, β-delayed neutrons play a significant role for the astrophysical calculations and the nuclear reactor physics. Today many nuclei are being revisited for detailed examination of this phenomenon. Modern detection systems such as TETRA [1,2] or BEDO [3] installed at ALTO ISOL facility [4] make it possible to measure β-decay properties of nuclei far away from the line of β-stability.
Many experiments performed at TETRA aimed at measurements of β-decay properties of the ground-state of neutron-rich medium-mass nuclei. The recent results obtained using TETRA/BEDO revealed the population of low-lying Gamow-Teller states in the β-decay of 83 Ga [5] and high energy γ-ray emission [6] which can be interpreted as a competition between γ-ray and neutron emission beyond the neutron separation energy threshold. Measurements of half-life (T 1/2 ) and β-delayed (multi) neutron emission probability (P xn ) are very sensitive to the parameters of the experimental setup. The performance of a neutron-detector can be characterized using standard neutron calibration (spontaneous-fission) sources, for example 252 Cf. However, the energy range of β-delayed neutrons is different from that of prompt fission neutrons emitted by 252 Cf. The response of TETRA to 252 Cf source emissions is well understood by extensive simulations validated by measurements [7]. But 252 Cf source, obviously, does not allow the validation of the data-analysis protocol for a β-decay experiment. Therefore, in order to test the experimental setup and the analysis procedure we performed two experiments using short-lived radioactive sources created at the centre of TETRA: 96 Rb and 97 Rb in the first and second runs respectively. The T 1/2 and P 1n values measured in the present experiment using previously adopted methods [1,5] are in fair agreement with the values available in the literature.

Measurements of β-delayed neutron emission using the TETRA setup
To produce neutron-rich nuclei, the UC x target placed in a Ta oven heated up to >2000 • C was exposed to the primary 50 MeV electron beam delivered by the ALTO linear accelerator [4]. The average beam current on the target was 10 µA. For this experiment, the oven was connected to a W tube used to selectively surface ionize the fission products. The beam was then accelerated to 30 keV. TETRA was operated behind the mass separator PARRNe whose resolution (δm/m=1300) was high enough to provide isobaric selection between A = 96 and A = 97. The other members of the isobaric chain (Sr, Y) have higher ionization potential and thus their production rate can be neglected in the experiment.
The β-decay station installed in the ALTO experimental hall was used in the neutron detection mode [1,3]. The detailed description of the setup is provided in ref. [1]: the surface-ionized mass separated beams were collected on an Al-coated mylar tape at the centre of the long-neutron counter TETRA creating a radioactive source. The collection point was surrounded by a plastic scintillator for β detection. A High Purity Ge detector was used for γ detection. One cycle of data taking consisted of a short background measurement (T bg ) followed by an irradiation time (T beam ) when the beam was impinging on the tape, and a decay time (T dec ) when the beam was deviated. Therefore, the data acquisition system recorded neutron, β and γ activities from the source during T m =T bg +T beam +T dec . Then the tape was moved by two meters to transport the source outside the detection system. The time settings and the number of completed cycles for A = 96, 97 are listed in table 1. Table 1. Tape-cycle parameters used for the A = 96, 97 settings: T bg is the background counting time before beam collection, T beam is the duration of the beam collection and T dec is the beam-off source decay counting time, in ms. N cycles is the total number of tape cycles for each mass setting. T 1/2 , P 1n , φ are the values obtained from the analysis of the βand neutron-activity curves for the A = 96, 97 settings.  (4) The resulting βand neutron activity curves for A = 96, 97 mass separator settings accumulated over N cycles are plotted in figure 1. All neutrons detected for a selected mass of rubidium were attributed either to the background or to β-neutron decay of rubidium isotopes. Even if β-delayed neutron emission is energetically allowed, there is no experimental evidence of β-delayed neutron emission of 96,97 Sr so far. The simultaneous fit of the grow-in and decay patterns of accumulated neutron curves leads to half-lives of 96,97 Rb as reported in table 1, where the uncertainty is the uncertainty from the fit.
The effective efficiency of the β detector ( β ) and the TETRA array ( n ) was derived from coincidence γ-ray spectra recorded for A=96 and A=97 mass separator settings plotted in figure 2. The efficiencies were obtained from the observed ratios of an area of i th peak in singles (S i γ ), β gated (S i γβ ), and β-neutron gated (S γβ i n ) γ-ray spectra. The relative intensities of the observed transitions are summarized in tables 2 and 3. The β was derived as a weighted average of S i γβ /S i γ ratios measured for the i-th transition. Due to lack of statistics in the β-n γ-gated spectrum n was found using only the most strong transitions at 352 keV and 815 keV in 95 Sr and 96 Sr respectively as reported in tables 2 and 3. All the γ activities recorded were identified and no isobaric contaminants were observed within our detection limits. Moreover, no evidence for contamination from surface ionized 96,97 Sr isotopes was found in the analysis of the activity curves. Indeed, such a contamination is unlikely due to the higher ionization potential of Sr. To extract P 1n values and the production rates φ we used the method reported previously [1]. The method is based on the system of Bateman equations describing decay of a given radioactive source. In this method the P 1n and φ values are determined as roots of a corresponding system of Bateman equations (2.1).  λ (A, Z+1) , . . ., λ (A−1, Z+2) respectively. The activity of the mother nuclei A (A, Z) (t) at a t-moment, populated by the beam, depends on both the decay constant λ (A, Z) and the intensity of the beam, φ: The system of the equations (2.1) was solved for the A=96 and A=97 collected datasets. Whereas λ (A, Z) was fixed to the value obtained from the fit of neutron activities, the λ (A, Z+1) , . . ., λ (A−1, Z+2) were fixed to their tabulated values. The obtained P 1n and φ for 96 Rb and 97 Rb are reported in table 1. A constant average production yield was assumed. The errors on P 1n and φ were mostly dominated by associated statistical errors but also by uncertainties on β and neutron efficiencies and uncertainties on half-lives of Sr and Y daughters as given in the literature. Once P 1n and φ were derived the contributions due to the decay of the parent nucleus and its daughters to the β-activity curve accumulated after N cycles were determined. The results for A=95 and A=96 mass separator settings are shown. Figure 3 shows T 1/2 plotted versus P 1n values for 96 Rb (left) and 97 Rb (right). The cited references correspond to reported experiments in which these two quantities were measured simultaneously. The T 1/2 and P 1n values presently obtained are in a remarkable agreement with the existing systematics.

Conclusions
The TETRA long neutron counter is a unique device which is currently used at the ALTO ISOL facility to measure global β-decay properties of neutron-rich nuclei. The β-decay station operated in neutron detection mode allows the simultaneous detection of β, γ and neutron emissions from a radioactive sources formed by the accumulation of the beam at the centre of the detection system. The performance of the setup was characterized using pure radioactive sources of 96,97 Rb produced from the photo-fission of 238 U. The sources were carefully chosen because their β-decay properties are known and thus represent well-known reference cases to test neutron detectors and validate the data analysis procedure. The half-lives and the probabilities of β-delayed neutron emission for 96,97 Rb measured in the experiment are in good agreement with the previously reported values. We plan to apply the described procedure to derive unknown values of P 1n of different nuclei with large N/Z ratio will be produced at ALTO ISOL facility.