INVESTIGATION OF THE INTERMEDIATE NEUTRON CAPTURE ON UNSTABLE NUCLIDES BY MEANS OF THE ACTIVATION METHOD. Abstract

Transcription

1 INVESTIGATION OF THE INTERMEDIATE NEUTRON CAPTURE ON UNSTABLE NUCLIDES BY MEANS OF THE ACTIVATION METHOD V. Skorkin 1, P. Mastinu 2 1 Institute for Nuclear Research RAS, Moscow, Russia 2 Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy Abstract The statistical uncertain of the Maxwellian-averaged neutron capture cross sections easuring by the activation ethod was calculated for ore than 20 unstable nuclides fro strontiu to hafniu at the stellar theral energy of kt=30 kev. The saples of unstable nuclides ( μg ass) can be produced by ion iplantation of Radioactive Ion Bea into a graphite target. The unstable isotopes can be activated by irradiating neutrons with a Maxwellian-shaped neutron spectru fro a lithiu target. After the irradiation unstable nuclide activity can be easured with using two HPGe detectors or BriLanCe detectors. 1 Intradaction Accurate knowledge of neutron-nucleus reaction cross sections in the kev region is of priary iportance in studies of stellar nucleosynthesis and in applications of nuclear technology for deterines the aount of radionuclides in burnt fuel eleents of fast reactors. Proton-induced nuclear reactions deterine the nucleosynthesis of light eleents in a variety of stellar objects in the universe. Three different processes can be identified in a ass region of the cheical eleents heavier than iron, the slow (s) neutron capture process during stellar heliu burning, the rapid (r) neutron capture process, and the photo-dissociation (p) process, the latter two being presuably related to Supernova explosions. About half of the eleental abundances between Fe and Bi are produced by the s- process, which is characterized by relatively low neutron densities (10 11 neutron/c 2 s -1 ) and neutron capture ties, which are typically uch longer than the half-lives against beta-decay. Soe unstable nuclei act as branching points in the reaction path of the s- process (Fig.1). Fig. 1. The s-process path in the Xe-Cs-Ba region, which includes the iportant branching point 134 Cs. This branching deterines the isotopic ratio of the isotopes 134 Ba and 136 Ba [1]. 1

2 The neutron capture cross section data are the fundaental ingredient for the calculation of the stellar reaction rates. The abundances of the produced isotopes are inversely proportional to their neutron capture cross sections. The relevant cross sections can be easured directly using radioactive ion beas. 2 Experiental procedure The stellar s-process odels require Maxwellian -averaged capture cross sections (MACS) of unstable nuclei of over a teperature range fro kt=1 kev to kt=100 kev. Measureents of the MACS usually carried out at Tie of Flight facilities (TOF) with ass saple to the illigras. The accurate MACS can be obtained by the neutron activation analysis (NAA) providing neutrons with the stellar spectru. This technique offers the high sensitivity, which is essential since easureents on radioactive isotopes have to be carried out with very sall saples ( 1 μg). The saples of unstable nuclides ( atos) can be produced by ion iplantation of Radioactive Ion Bea (RIB) into a graphite target. The RIB (10 10 s -1 intensity) will be available at SPES (Selective Production of Exotic Species) project at Laboratori Nazionali di Legnaro, Italy [2]. The project based on a superconductive accelerator coplex included Priary Linac accelerating protons to an energy of 5 MeV with a current of 20 A c.w. and Secondary Linac that will operate in pulsed ode at a repetition rate of 50 HZ and a duty cycle of 1% with an average current of 0.2 A. The priary proton bea used for production of fission fragents (10 13 s -1 ) fro a uraniu like-target by eans of a neutron converter. Neutron rich ion species are extracted, selected, further ionized at high charge state, isotopically purified and then accelerated through a Secondary Linac at energies up to 15 MeV/A. In this project, neutron generation is proposed to be carried out by dropping an intensive proton bea onto lithiu target using 7 Li(p,n) 7 Be threshold reaction and a good approxiation to a Maxwellian-shaped neutron spectru (MSNS) at 30 kev can be obtained by shaping the proton bea energy (Fig. 2). (1) Fig. 2. The MSNS of the epitheral neutrons fro the lithiu target [3]. 2

3 The onochroatic proton bea can be shaped by eans of a 34 g/c 2 thickness rotating carbon foil, reducing and spreading its 5 MeV energy to ± 0.09 MeV. The epitheral neutrons with MSNS at kt=30 for experient can be produced by a part of protons (4.4%) with the energy above the reaction threshold energy (1881 kev). The saple, contained unstable isotopes can be activated by irradiating neutrons with the flux of the order of neutron/c 2 s. In the second project neutrons with axiu energy board of 400 kev are produced by spallation reactions induced by a pulsed, µs wide and repetition rate up to 50 Hz, 209 MeV/c proton Linac at INR RAS bea with up to 12 A current per pulse, ipinging on a tungsten or lead target. Spallation reaction results in eission of fast neutron flux up to of n/s fro the target. A syste of converters and filters odify a wide energy spectru to the MSNS at a Spectroeter for neutrons slowing down in lead (LNS-100) and an Irradiation facility (RADEX). Density of epitheral neutrons flux with energy fro 1 kev to 100 kev in the irradiated channels can reach up to n c 2 s 1. The saples containing unstable nuclides can be obtained fro RFNC VNIIEF [4]. 3 Measureent procedure After the irradiation the saples containing unstable nuclides can be accoodated close in front of two high-purity geraniu HPGe or BriLanCe (LaBr 3 :Ce) detectors to easure a induced gaa activity of activated nuclides (Fig. 3). Fig. 3. Setup of the HPGe detectors. The close geoetry requires an exact and reproducible positioning of saple and detectors, which was achieved with a special saple holder. This characteristics of the HPGe, BrilanCe detectors are given in Table 1. The background should be reduced by a lead shielding. The neutron capture cross section of the unstable isotopes can be easured relative to that of gold ( 198 Au, Half-life 2.7 d, gaa kev energy). The extrapolation stellar cross section of gold is yields 582± 9 b at stellar teperature kt=30 kev. 3

4 Table.1. Characteristics of HPGe (2 3 ), BrilanCe B380) (3 3 ) and NaI(Tl) )(3 3 ) detectors. Energy of γ- rays, kev 122( 57 Co) 356( 113 Sn) 662( 137 Cs) 882( 54 Mn) 1332( 60 Co) 2615 Detector HPGe Resolution 1.0keV 1.2keV 1.3keV 1.5keV 1.7keV Efficiency (c ax = s -1 ) B380(LaBr 3 ) Resolution 6.6% 3.8% 2.9% 2.6% 2.1% 1.6% Efficiency (c ax = s -1 ) NaI(Tl) Resolution 8.9% 9.1% 7.0% 6.4% 5.4% 4.5% Efficiency c ax The axiu count rate for this detector. 4 Calculating statistical uncertain. The intensity Radioactive Ion Bea in a graphite target was of s -1. The irradiating neutron flux is of neutron/c 2 s. The gaa activity of the activated X A+1 nuclide is easured. The γ- background count rate of activity of the unstable X A nuclides shoud be less then the axiu count rate c ax for this detector (HPGe or BriLanCe (LaBr 3 :Ce). The instruental, cosic and Copton background fro the higher gaa level are taken into account. The calculate statistical uncertain of the MACS easuring by the activation ethod are given in Table 2. Tab. 2 The calculate statistical uncertain of the MACS easuring by the activation ethod. Iplanted Iplanted The ass Tie of Activated Tie of unstable nuclide tae of nuclide irradiation nuclide easureent X A X A X A+1 statistics uncertain 90 Sr 14 day 1.8 μg 12 h 91 Sr β 91 Y 1 d (τ 1/2 = 29 y) atos (τ 1/2 = 10 h) 2% 94 Nb 1 day 1.3 μg 14 day 95 Nb β 95 Mo 40 d (τ 1/2 = y) atos (τ 1/2 = 10 h) 2% 99 Tc 14 day 2.0 μg 3 in 100 Tc β 95 Ru 1 in (τ 1/2 = y) atos (τ 1/2 = 16 s) 1% 104 Ru 14 day 2.1 μg 36 h 105 Ru β 105 Pd 3 day (τ 1/2 = 45 s) atos (τ 1/2 = 35 h) 5% 4

5 116 Cd 14 day 2.3 μg 3 h 117 Cd β 117 In 3 h (τ 1/2 = y) atos (τ 1/2 = 2.5 h) 1% 114 In 1 day 1.6 μg 5 h 115 In β 115 Sn 5 h (τ 1/2 = 50 d) atos (τ 1/2 = y) 1% 115 In 14 day 2.3 μg 15 s 116 In β 116 Sn 15 s (τ 1/2 = y) (τ 1/2 = 16 s) 3% 124 Sb 1 day at. 12 day 125 Sb β 115 Tl 1 y (τ 1/2 = 60 d) (τ 1/2 = 2.7 y) 3% 129 I 14 day at. 12 h 130 I β 130 Xe 12 h (τ 1/2 = 10 7 y) (τ 1/2 = 2.7 y) 1% 134 Cs 6 h at. 1 h 135 Cs β 135 Ba 3 h (τ 1/2 = 2 y) (τ 1/2 = 10 6 y) 2% 135 Cs 14 day at. 14 h 136 Cs β 136 Ba 1 d (τ 1/2 = 10 6 y) (τ 1/2 = 13 d) 1% 136 Cs 14 day at. 14 h 137 Cs β 137 Ba 14 d (τ 1/2 = 13 d) (τ 1/2 = 30 y) 4% 137 Cs 14 day at. 1 h 138 Cs β 138 Ba 1 d (τ 1/2 = 30 y) (τ 1/2 = 33 in) 1% 143 Ce 14 day at. 14 h 144 Ce β 144 Pr 300 d (τ 1/2 = 33 h) (τ 1/2 = 285 d) 5% 144 Ce 14 day at. 5 in 145 Ce β 145 Pr 5 in (τ 1/2 = 285 d) (τ 1/2 = 3 in) 2% 143 Pr 14 day at. 30 in 144 Pr β 144 Nd 30 in (τ 1/2 = 14 d) (τ 1/2 = 17 in) 1% 149 P 3 day at. 3 h 150 P β 150 S 3 h (τ 1/2 = 53 h) (τ 1/2 = 2 h) 2% 160 Tb 6h at. 7 d 161 Tb β 161 Dy 7 d (τ 1/2 = 72 d) (τ 1/2 = 6.8 d) 3% 161 Tb 6h at. 20 in 162 Tb β 162 Dy 20 in (τ 1/2 = 6.8 d) (τ 1/2 = 7.6 in) 1% 166 Ho 1 day at. 3 h 167 Ho β 167 Er 3 h (τ 1/2 = 27 h) (τ 1/2 = 3 h) 1% 181 Hf 14 day at. 12 day Hf β 182 Ta 12 day

6 (τ 1/2 = 44 d) (τ 1/2 = y) 2% Forulas for calculating: A naber of atos of iplanted X A isotope befor irradiating: N 0 = I 0 T 0 1/2 (1- exp( t ipl / T 0 1/2)/0.693 The γ and β- activity of the iplanted X A isotope befor irradiating: A 0 γ = I 0 (1- exp( t ipl / T 0 1/2) I 0 γ A 0 β = I 0 (1- exp( t ipl / T 0 1/2) I 0 β I 0 - intensity of the ion iplantation (10 10 p/s) t ipl tie of the iplantation T 0 1/2 half-life of iplantated ions I 0 γ - intensity of the gaa rays fro the decay of the one iplantated X A ion I 0 β - intensity of the beta particles fro the decay of the one iplantated X A ion The γ and β- activity of the X A+1 isotope after irradiating neutrons: if T 0 1/2 T 1 1/2: A 1 γ = I 1 γ I 1 σ N 0 (exp( t irrad /T 0 1/2) - exp( t irrad /T 1 1/2)) T 0 1/2 /( T 0 1/2 - T 1 1/2) A 1 β= I 1 β I 1 σ N 0 (exp( t irrad /T 0 1/2) - exp( t irrad /T 1 1/2)) T 0 1/2 /( T 0 1/2 - T 1 1/2) if T 0 1/2 T 1 1/2: A 1 γ = I 1 γ I 1 σ N 0 exp( t irrad /T 1 1/2) t irrad /T 1 1/2 A 1 β= I 1 γ I 1 σ N 0 exp( t irrad /T 1 1/2) t irrad /T 1 1/2 I 1 - intensity of the neutrons ( n/c 2 s) t irrad tie of the irradiation T 1 1/2 half-life of the X A+1 isotope ato I 1 γ - intensity of the gaa rays fro the decay of the one X A+1 isotope ato I 1 β - intensity of the beta particles fro the decay of the one X A+1 isotope ato σ - the extrapolation Maxwellian-averaged cross sections of the X A isotope at the kt=30 kev. The γ and β- activity of the iplanted X A isotope after cooling, befor easureent: A 0c γ = A 0 γ exp(-0.693(t irrad + t cool )/T 0 1/2) t cool tie of cooling A 0c β= A 0 β exp( t irrad + t cool )/T 0 1/2) This activity is background. This count rate should be less then the axiu count rate for the detector c ax. The counting statistics of the registered γ and β- activity of the decayed atos of X A+1 isotope in the tie of easureent: t cool tie of cooling C 1 γ =Ω ε γ A 1 γ exp( t cool /T 1 1/2) (1-exp( t eas /T 1 1/2)) T 1 1/2/0.693 C 1 β =Ω ε β A 1 β exp( t cool /T 1 1/2) (1-exp( t eas /T 1 1/2)) T 1 1/2/0.693 Ω=Ω 1 +Ω 2 geoetric efficiency of the detectors is 2 /16R 2 at large distance of R between easured saple and detector (is deaeter of ) should be placed plastic to absorb electron of the β-decay. ε= ε 1 =ε 2 ; ε γ = efficiency FEPE of the detector; ε β (E 1 β - E b )/ E 1 β, E b - threshold energy ( E 0 β axiu) A counting statistics of the registration gaa rays in the coincidence ode is: C γ γ cc = C 1 γ Ω ε 6

7 The registered background of the γ and β- activity of the decayed atos of X A isotope in the tie of easureent: C f γ = Ω ε f γ I f γ N 0 = Ω ε f γ A 0c γ T 0 1/2/0.693 C f β = Ω ε f β I f β N 0 = Ω ε f β A 0c β T 0 1/2/0.693 ε f γ ΔE γ /E 0 γ ε f β (E 1 β - E 0 β)/ E 1 β ΔE- energy resolution of the detector τ cc - resoulion tie of set up of two detector (τ cc =1ns) I f γ intensity of bacgraund gaa rays In the coincidence ode count rate of the γ and β -background fro X A decay is reduce to: C f cc = 2 C 0 γ A γ 0 τ cc Ω ε f γ C f cc = 2 C 0 β A γ 0 τ cc Ω ε f β References 1. N. Patronis, S. Dababneh, et al. Phys. Rev. C 69 (2004) F. Graegna, A. Andrighetto, et al. Acta Phys. Polon. B 38 (2007) W. Ratynski and F. Käppeler, Phys. Rev. C 37, (1988) Alekseev A.A., Bergan A.A. et al. ISINN-12, Dubna (2004)