Radiochimica Acta 46, 1 3 - 15 (1989)
© R. Oldenbourg Verlag, München 1989 - 0 0 3 3 - 8 2 3 0 / 8 9 S 3.00+0.00
Excitation Functions for the Reactions 7 Li(1 5IM,1 H)2 1 F, 6-7L i (1 5N , " H )2 0F and 9B e (1 5N , 2 a ) 1 6N
By Μ. E. COLIN, C. FRIEDLI and P. LERCH*, Institut d'Electrochimie et de Radiochimie, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
(Received November 9, 1987; revised January 22, 1988)
15Ν reactions/Lithium and beryllium target/Excitation function/ Yield
Abstract
The yield curves and the excitation functions for the
7L i (1 5N ,1H )2 IF ,6'7L i (l sN , ' - ' H ^ F a n d » B e (, sN , 2 a ), 6N
reactions were determined in the 10 to 30 MeV energy range. The highest yield was obtained with the reaction ®Be(15N, 2 α ) " N.
The maximum cross-section for this reaction is 370 ± 70 mb at 24 MeV.
Introduction
During the past years, charged particle activation has proved to be an outstanding analytical technique for deter-mining light elements [ 1 ]. The recent development of heavy ion beams has further increased the number of nuclear reactions available for that purpose. The poten-tial applicability of 7Li, 9Be, 10B, UB , and 1 80 ion
beams for the determination of light elements have been described in preceding papers [2 — 6]. The main advantages of the technique consist firstly in the linear relation be-tween the signal intensity and the concentration of the element to be determined, and secondly in the independ-ence of the signal from the chemical nature of the matrix. Contrarily to most of the other direct analytical methods applicable to the solid phase, this method allows the direct comparison of the activity produced in a standard with that of the sample to be analyzed. A very precise calibration technique, described by ISHII et al. [7], is next used to calculate the concentration of the trace element. To apply ISHIl's technique one needs, however, to know the activation curve of the nuclear reaction of interest. For this reason and also to define the best experimental conditions for the determination of lithium and beryllium, the activation curves and the excitation functions for the 7L i (l sN ,1H )2 1 F,
6 > 7Li(1 5N, 1>2H)20F and 9B e (I SN , 2 a )1 6N reactions
have been established.
nology in Ziirich. Enriched ammonia (98% 15NH3, US
Department of Energy) was used to produce the 1 s Ν ion
beam. The gas, mixed with hydrogen, was ionized using a tungsten filament at high temperature to produce
l sN H j ions.
The irradiation chamber , especially designed for deter-mining the yields of short lived radionuclides, has been described in a preceding paper [8]. A mobile target holder permits cyclic irradiations and activity measurements
without interruption of the vacuum (10- 5 mm Hg). The
beam current was monitored immediately after the meas-urement of the sample activity with a Faraday cup placed directly behind the target. The whole cycle (irradiation — activity measurement — beam monitoring) was electro-nically controlled and repeated for at least 10 cycles.
Detection
A coaxial Ge(Li) detector (ORTEC 8001 - 1 0 2 0 V, 2.0 keV resolution at 1.33 MeV) was used to detect the y radiation emitted by 16N, 2 0F and 2 1F , through a thin
plexglass window attached to the irradiation chamber. The detector was connected to a multichannel analyzer (CANBERRA 80). At the end of the last counting, the total spectrum, obtained by summing all the individual countings, was transferred to a floppy disc recorder (SCINETIFIC MICRO SYSTEM, D222) for further analysis with a PDP 11 / 23 computer. The absolute
de-tection efficiency for the 350keV γ-rays of 2 1F was
determined to be (1.65 ± 0.05) χ 10~2 in the counting
conditions, using a 1S2Eu standard source. For the 1634
keV γ-rays of 2 0F , this value was found to be
(2.6 ± 0.1) χ 10"3. In the case of I 6N, the detector's
efficiency was estimated to be (3.5 ± 0.5) χ 10~4 at
6128 keV, as suggested by SEYFARTH et al [9].
Targets
Experimental Irradiation
Beams o f1 5 Ν ions at various energies and in various
charge states were obtained from the HV EC-EN Tandem Van de Graaff accelerator of the Institute for Medium Energy Physics at the Swiss Federal Institute of
Tech-Beryllium was irradiated as pure metallic foils (Be, 99.8%, GOODFELLOW METALS Ltd.). The lithium targets consisted of p. a. LiF (99.9%, MERCK AG); the powder was dried and pressed in small discs of 13 mm diameter and 1—2 mm thickness. All targets were thicker than the range of the incident ions.
14 Μ. Ε. COLIN, C. FRIEDLI and P. LERCH
Q u a n t i t a t i o n
Due to the cyclic mode of activation and the summation of the consecutive spectra, the primary results were ob-tained as the total number of c o u n t s , ^ , detected in the γ-line of the radionuclide of interest during k cycles. The results are related to the yield of the nuclear reaction by the equation given by BATCHELOR et al. [10]:
Table 1. K i n e r a a t i c p a r a m e t e r s o f t h e r e a c t i o n s a n d characteristics of the products V = Nt\( 1 ) (1 —e c) -\tD k + 1 —e -\ktc 1 with (1 — e ~X A t) e
η = yield of the nuclear reaction (Bq s ion"1)
λ = decay constant (s~1)
t( = single irradiation time (s)
tc = time separating two consecutive irradiations (s)
φ = incident particle intensity (ion s"1)
e = absolute efficiency of the detector for the corresponding y energy
ξ = abundance of the detected γ-rays (11) μ = natural abundance of the target nucleus
At = single counting time (s)
tD = delay between the end of irradiation and the
beginning of counting (s)
The activation curve η(Ε) for a given reaction could then be obtained by reporting the yield as a function of the beam energy. This curve was used to determine the aver-age activation energy according to ISHII [7]. Its derivative allowed the calculation of the excitation function apply-ing the relation:
with σ = cross-section of the reaction (mb)
η = concentration of the target nuclei (at g "1)
dE
stopping power of the target material [12].
Results and discussion
Reaction Radionuclide produced
Q (MeV) Ec (MeV) T\n (s) Ey γ-Rays (keV) intensities ,L i (, iN ,,H ) " F 7.7 14.5 4.35 350 0.70 ® L i (l 5N , Ή )ί 0Ρ ' L i (l sN ,!H ) " F 6.9 1.8 16.5 14.0 11.4 1634 1122 611 1.00 Ρ Ρ ' B e (l 5N , 2 4H e ) " N 0.8 15.2 7.13 6128 5617 5106 0.69 Ρ Ρ 1 0 " 10" 10" 10" Y (Bq.s.ion) (MeV) 10 20 30
Fig. 1. Yield curves of t h e reactions a) 9B e (1 5N , 2 α ) " N,
b) 6 , 7L i (1 5N , 1 , 1H )2 0F , c) 7L i ( ' » N ,1H )1 1F . The nuclear reactions investigated are listed in Table 1,
together with their β-value and Coulomb barrier (Ec).
All the reactions are exoenergetic and thus their threshold energy is zero. Beryllium has one single natural isotope
and therefore the 1 6 Ν activities measured at various
inci-dent beam energies are directly related to the reaction
cross-sections. Lithium occurs naturally as 6 Li (7.5%)
and 7 Li (92.5%). While 2 1F can only be produced by the
reaction with 7L i ,2 0F might be formed by the 1 SN
bom-bardment of both 6Li and 7Li. In this latter case, the 2 0F activity measurements yield the determination of an over-all activation curve and cross-section values.
The characteristics of the radionuclides produced are also given in Table 1. The cyclic activation set-up allowed the accurate measurements of the production yields of 2 0F ,2 1F , and I 6N i n spite of their short half-lives [13],
The experimental yield curves of the nuclear reactions investigated, obtained by reporting the yield as a
func-tion of the incident beam energy, are presented in Figure 1. In order to facilitate their comparison, the yield scale is logarithmic. The error on individual yields has been estimated to be 5%, taking into account the counting statistics and the error on the various parameters involved in the cyclic mode of irradiation and activity measure-ment. The 9B e (1 5N , 2 a ) 1 6N reaction is characterized by relatively high yields whatever the beam energy. Hence the sensitivity could be excellent for analytical applica-tions. The yield curves of both 9B e (l sN , 2 a )1 6N and
7Li (1 S N, p)21 F reactions reach a plateau at 30MeV
1S Ν while the 2 0 F yield continues to increase beyond
that limit.
For each reaction, the experimental activation curves were refined using a computer program [14], They were expressed as a polynomial function of the 4th order by a least square fitting. The derivative of these expressions,
Excitation F u n c t i o n s for the Reactions 7L i ( '5N , ' H ^ ' F , , , 7L i (, sN) , , 1H ) " F and ' B e ( 'SN , 2 α ), 6Ν 15 too 300 200" 100-a (mb) (MeV) 10 20 30
Fig. 2. Excitation functions of the reactions a) ' B e (l sN , 2 a )1 6N , b) ', 7L i (1 5N , 1 , 2H )1 0F , c) 7L i ( '5N ,1H )2 1F .
of the corresponding excitation function presented in Figure 2. The errors on the cross-sections were estimated to be about 20% due to the fitting and the derivative calculations.
The examination of Figure 2 and Table 2, which re-cords the maximum cross-sections with the corresponding beam energies, confirms the excellent yield of the
9B e (l sN , 2 a ) 1 6N reaction. At this point, it is worth noticing the similitude of this reaction with
9B e (1 80 , 2 α )1 90 (owing the same target nucleus and the same nature and number of particles ejected) which has also relatively high cross-sections [15]. On the other hand, the cross-section of the reaction 7L i (1 5N , p )2 1F remains low for the whole range of energy studied; the maximum value barely reaches 3 0 mb at 2 0 MeV I SN , while the excitation functions of the two other reactions show maximum values for higher beam energies.
Table 2. Maximum cross-sections
Reaction Energy Cross-section
(MeV) (mb)
7L i (l sN ,1 H )J ,F 2 0 3 0 ± 6
s'7L i (1 5N , I>,H ) " F 25 1 6 0 ± 3 0
' B e (l sN , 2 4H e )l <N 24 3 4 0 ± 7 0
Conclusion
The activation curves and the excitation functions for the nuclear reactions induced by bombarding lithium and
beryllium with energetic 1 1Ν ions have been determined. The short half-life of the radionuclides produced and the relatively high cross-sections should allow the fast, sensi-tive, nondestructive and simultaneous determination of traces of lithium and beryllium [16].
Acknowledgement
The assistance of the operation personnel of the J andern Van de Graaff accelerator in Zürich is gratefully acknowl-edged.
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