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The interplay between nuclear and atomic physics: Electron capture in the presence of a nuclear resonance
Nuclear Instruments and Methods in Physics Research B24/25 (1987) 89-93 North-Holland. Amsterdam
89
THE INTERPLAY BETWEEN NUCLEAR AND ATOMIC PHYSICS: ELECTRON CAPTURE IN THE PRESENCE OF A NUCLEAR RESONANCE O.K. BAKER *, C. STOLLER and W.E. ~EYERHOF Department
of
Physics, Stanford University, Stanford, CA 94305, USA
J.N. SCHEURER Cenrre d’Etudes Nu&aires, WniversitCde Bordeaux I, Institut National de Physique NuclPaire et des Particules, 33170 Grad&pan, France
The total probability, P(E,,, Stab), for electron capture from “C, 14N, and 20Ne by protons of energy Era+.,between 0.5 and 1.510 MeV and from 22Ne by protons of energy E,, between 1.500 and 1.525 MeV has been measured at scattering angles Blabof 30 ’ and 150*. Experimental evidence for the dynamic interference effect between atomic electrons and nuclear reactions is clearly seen in P(E,,, filab) in the presence of the elastic s-wave nuclear resonance 22Ne@, p)22Ne at 1.510 MeV. The data is compared with theoretical calculations by Amundsen and Jakubassa-Amundsen using the strong-potential Born Approximation.
1. Introduction
(1) i2C(p, p)12C at 0.462 MeV (r=
A fascinating field of study has emerged within the past few years in which it is possible to observe dynamic interference effects between atomic electrons and nuclear reactions in ion-atom collisions [1,2]. Normally, measurements of electron excitation, ionization, and capture are made in the absence of any nuclear scattering, while investigations of nuclear reactions between projectile and target nuclei are performed without regard to atomic electronic processes. Those special circumstances where the atomic processes are affected by nuclear reactions can potentially provide valuable information in both areas, not generally accessible by other means [3]. An experimental investigation of this interplay between nuclear and atomic physics can be made using a Van de Graaff accelerator of only a few MV terminal voltage. We have made such an investigation at Stanford using a single-ended 3 MV Van de Graaf accelerator. Specially, we have measured probabilities for electron capture by protons in the presence of proton induced nuclear resonances for several target systems [4]. We have found evidence that there is a nuclear resonance effect in electron capture [4-61. The data is in qualitative agreement with theoretical calculations [7,8]. In this article, we present a comparison of the measurements of the probability for electron capture by protons in the presence of a proton induced nuclear resonance. The target systems and the resonances studied were: * Present address: Los AIamos National Laboratory, Los Alamos, NM 87545, USA. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Pub~s~ng Division)
B.V.
35 lcev) [9], (2) 14N(p, p)14N at 1.058 MeV (r= 6 keV) [lo], (3) *‘Ne(p, p)22Ne at 1.510 MeV (r= 2.45 keV) [ll]. We begin with a brief theoretical discussion. Then a comparison with theoretical expectations is made.
2. Theory Amundsen and Jakubassa-Amundsen [8] have found that the criterion for a strong interplay between the atomic electrons and nuclear processes in K-shell electron capture is
where r is the width of the nuclear resonance, u is the projectile velocity, m is the electron mass, and Ez and Ez are the electron binding energies to the target and projectile, respectively. A=~E,T~-~E;~+$w2,
(2)
is the energy transferred
to the K-shell electron during capture. It is the criterion of eq. (1) which is to be examined in this article. Qualitatively, it has the following physical interpretation. If the width of the resonance is too large, that is to say if the nuclear delay time is too short, the electron does not “know” that there has been a resonance. Then there would be not interference between incoming and outgoing channels in the capture mechanism and no nuclear resonance effect seen in capture. Alternatively, if the width of the resonance is too small (if the nuclear delay time is too long), the electron “does not remember” the incoming I. ATOMIC PHYSICS / RELATED PHENOMENA
90
0. K. Baker et al. / Interpluv between nuclear and atomic physics
channel in the collision. There would be, in this case, an incoherent addition of incoming and outgoing channels in the collision and, again, no nuclear resonance effect on electron capture. Therefore, it is advantageous to look for nuclear resonance effects where the width of the nuclear resonance is of the order of the energy transferred to the active electron in the capture process. The question to be answered is, how sensitive is the electron capture mechanism in the presence of nuclear resonance scattering to the criterion of eq. (1). Our measurements provide an answer. The experimental setup and procedure is described next.
3. Experimental setup The first attempts to measure a nuclear resonance effect in electron capture were made by HorsdahlPedersen et al. in 1982 using an electric separation of the protons which capture an electron (Ho) from those which do not capture an electron (Hf) 1121.We used a singles method of data acquisition conceptually similar to theirs, but employing a magnetic separation instead. The scattering apparatus which made up the main part of the experimental setup is shown in fig. 1. The incoming proton beam, supplied by the Van de Graaff accelerator, is denoted by H’. The beam traverses a gaseous target denoted by J. The beam is stopped in a Faraday cup made of aluminum in order to minimize backscattering. There are four (silicon) solid state surface barrier detectors, Bl and B2 positioned at 150” with respect to the incoming beam, and Fl and F2 positioned at 30*. These particular angles were chosen in order to facilitate determination of the nuclear resonance energy. The ratio of the number of counts detected in a backward detector divided by the number detected in a forward detector is nearly proportional to the nuclear resonance cross section (at the nuclear resonance energy). Detectors Bl and Fl were placed inside a region where a high magnetic field could be applied, as shown by the dotted line. The magnetic field could be turned on and off in each experimental run. With the field on, a separation of the two charge state HO, neutral hydrogen, and H.’ , protons, occurs. Protons which capture an electron from the target gas at the interaction region J become neutral hydrogen and proceed undeflected to the detectors inside the magnetic field region. Protons which fail to capture an electron during the bombardment of the target gas at J are deflected away from the detectors Bl and Fl as indicated by the arrows in fig. 1. The two detectors outside of the magnetic field region, F2 and B2 were equally sensitive to neutral and charged particles and serve for normalization of the beam current. Slits S served to define the size of the beams
Fig. 1. JZxperimental arrangement to measure electron capture probabilities at 30° and 150°. For explanations of symbols, see text. The broken line indicates the boundary of the pole pieces of the magnet.
scattered to the detectors. To shield the entering beam from the fringing magnetic field as much as possible, the beam was passed through a thick walled iron tube T. In order to determine the capture probability, we proceeded as follows. By turning the magnetic field on the probability for electron capture at 150”, P(E, @= 1500), was extracted by taking the ratio of the number of counts in detector Bl and the number of counts in B2 in an experimental run. This ratio is then divided by those same quantities measured with the magnetic field off. p(
150°)
E,
=
‘sr’s2)on, MB
where E is the bombarding and mu=
i
Bl B2
energy of the proton beam
1 off’
where the symbols Bl and 32 here stand for the counting rates in the detectors. Similarly for the forward angles, the probability for electron capture of 30°, P( E, 0 = 30°) is extracted
of counts
in detectors
from the corresponding ratio Fl and F2 along with the solid
angle ratio wF.
&f‘
4. Charge state separation using a magnetic field
In principle, the magnetic field could be increased as much as desired above the ~~~ necessary to achieve the separation of the Ho and H’ particles described above. In practice, the fringing field in the region of the primary beam needed to be minimized as much as
O.K. Baker et al. / Interplay between nuclear and atomicph.vsics
possible in order to reduce its bending before traversing J, so that the magnetic field was kept as close to this minimum as possible. To ascertain the minimum magnetic field necessary to achieve this separation, measurements were made of the number of captured particles in detectors Fl and Bl at a fixed inlet gas pressure and integrated charge in the Faraday cup as a function of magnetic field strength. When the number of particles counted in detectors Fl and Bl in this way remained constant as the magnetic field strength is increased or decreased, the proper separation was achieved. As the proton beam energy E was increased, the deflection magnet current was increased proportionally to El/‘.
91
4
3
0
5. Electron capture measurements using a gaseous target Since at the energies of interest here, the proton beam reaches an equilibrium charge state within the first few atomic layers of a solid target, it would be difficult to study electron capture in a single collision at the initial charge state of the beam, as this type of experiment proposes to do. Thus, it is advantageous to use a gaseous target to study electron capture probabilities by protons. However the disadvantages in using a gaseous target are the low density of atomic scatterers and the difficulty of confining the target spatially. In our study, we employed a gas jet from a hypodermic needle because a windowless gas cell is not easily built if scattered particles have to be detected. The aim of our procedure was to maximize the pressure of the target gas in the small interaction region (approximately 3 cm3) at J (fig. 1) and to minimize the pressure throughout the rest of the scattering chamber. We tested the capture probability at a number of target gas pressures in the chamber in order to insure that there was no secondary reactions such as secondary capture or ionization which would skew the data. No such reactions were seen. All the data presented was gathered at a target gas pressure in the chamber of 3 X 10e4 Torr. The counting rates were as low as 10 counts/h in detector Bl with magnetic field on, and as high as 2000 counts/h in Fl with field off. A typical spectrum gathered in this way is shown in fig. 2. The spectrum is from protons scattered from 2oNe at 0.5 MeV. This particular spectrum is from detector Bl with field on gathered over a period of about 2 h. The various features of the spectrum are indicated. By making a least squares fit to the background we could subtract out the background counts from the scattering peak in each spectrum and determine the true number of particles scattered from the target gas at the interaction region J. For each series of runs, the procedure was to scan over the resonance by turning the magnetic field off and measuring the solid angle ratios (4) and (6) as well as taking the ratio of the number of particles detected at a
1000
I100
I200
I300
1400
CHANNEL
Fig. 2. A typical spectrum of H+ +20Ne at 1.0 MeV. The various features indicated are as follows: A. Protons scattering from the “Ne target gas which capture an electron. B. Broad peak arising from scattering from the needle used to let the “Ne target gas into the scattering chamber. C. High energy scattering contribution from neutron producing nuclear reactions.
backward angle to those detected at a forward angle during a certain time interval or, equivalently, for a specified integrated proton current. Since at 30’ the nuclear resonance scattering is dwarfed by the coulomb scattering, this ratio is essentially proportional to the 150° nuclear resonance cross section at each energy. The magnetic field was then turned on, and the scan over the resonance was repeated to obtain the capture probabilities (3) and (5).
6. Results and discussion Presented in fig. 3a is the probability for electron capture from 12C (CH,) across the 35 keV elastic nuclear resonance at 0.462 MeV. The excitation function which is proportional to the nuclear resonance at that energy is shown in fig. 3b. Here A/T is about 0.017. As can be seen, within experimental error there is no nuclear resonance effect on the capture probability. The probability for electron capture by protons from 14N (N2) in the vicinity of the 6 keV elastic nuclear resonance at 1.058 MeV is presented in fig. 3c along with the excitation function proportional to the nuclear resonance at that energy in fig. 3d. Here a small effect is expected theoretically. The data gives evidence of a noticeable effect. In this system A/r is 0.17, an order of magnitude closer to unity than in the previous case. Next is presented the probability for electron capture by protons from 22Ne in fig. 4a. The probability was measured in the vicinity of the 2.5 keV elastic I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
O.K. Baker et al. / Interplay between nuclear and atomicph.vsics
92
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’
460 PROTON
a
1055 ENERGY
60
65
0
(keV)
Fig. 3. (a) Electron capture probability at @lab=150°, for p + CH4 collisions in the neighborhood of the 462 keV resonance. (b) The 150”/30” counting rate ratios proportional to the nuclear scattering cross section at this energy. (c) Electron capture probability at @tat,= 150°, for p + N2 collisions in the neighborhood of the 1058 keV resonance. (d) The corresponding counting rate ratios proportional to the nuclear scattering cross section at this energy.
resonance at 1.510 MeV shown in fig. 4b for a target gas mixture of 75’%**Ne and 25%*‘Ne. (The capture probability shown in fig. 4a is corrected for the presence of nonresonant *‘Ne.) For this system A/r is 0.7 (of the order of unity). Thus the criterion of eq. (1) is met and a large nuclear resonance effect on electron capture is expected. The data indicated that there is indeed a large effect on the capture probability in this case. The results of this comparison suggests that the criterion posed by eq. (1) does bear dramatically on the possibility of seeing a nuclear resonance effect in electron capture probabilities.
7. Conclusion
The role of nuclear resonant scattering in an ion-atom collision where the K-shell electron is captured into a bound state of the projectile has proven to be a unique tool for studying the interplay between nuclear and atomic physics. We have, in this article, presented an experimental setup and procedure for making measurements of electron capture probabilities by protons impinging on a gaseous target using the singles method of data acquisition. This experimental
P
1o‘3
I .496
I .504
I.512
I.520
Fig. 4. (a) Electron capture probability at eta,, =150’, for p+‘*Ne collisions in the neigborhood of the 1510 keV resonance. (b) The corresponding counting rate ratios proportional to the nuclear scattering cross section at this energy. The curve shown is the calculation of the ratio of the nuclear scattering cross section at 150” to that at 30°.
method has been used successfully to obtain measurements of probabilities for electron capture by protons scattered to large angles and in the presence of a proton induced nuclear resonance for several target systems. This work was done using a Van de Graaff accelerator capable of producing a continuous beam of protons between 0.3 MeV and 2.0 MeV with intensities on the order of tens of PA on target. We have found qualitative agreement with theoretical calculation. This suggests that the ratio A/r of the energy transfer to the captured electron to the resonance width plays an important role in determining the magnitude of the nuclear resonance effect on electron capture.
O.K. Baker et al. / Interplay between nuclear and atomicphvsics
This work was supported in part by NSF grant PHY and by a NATO fellowship for J.N. Scheurer.
83-13676
References
Ul W.E. Meyerhof.
in: Electronic and Atomic Collisions, eds.. J. Eichler, I.V. Hertel and N. Stolterfoht (North-Holland, Amsterdam, 1983). VI R. Anholt, in: Atomic Inner-Shell Physics, ed., B. Grasemann (1985). [31J.S. Blair and R. Anholt, Phys. Rev. A25 (1982) 907. 141J.N. Scheurer, O.K. Baker and W.E. Meyerhof, J. Phys. B18 (1985) L85.
93
[S] O.K. Baker et al., to be published
(1987). [6] O.K. Baker, PhD. thesis (1987). [7] D.H.J. Amundsen and P.A. Amundsen, private communication (1985). [8] D.H.J. Amundsen and P.A. Amundsen, Nordita Preprint Nordita-84/13 1 (1984). [9] E.A. Milne, Phys. Rev. 93 (1954) 762. [lo] J.W. Olness, J. Vorona and H.W. Lewis, Phys. Rev. 112 (1958) 475. [ll]
G.A. Keyworth, P. Wilhjelm, G.C. Kyker, H.W. Newson and E.G. Bilpuch, Phys. Rev. 176 (1968) 1302. [12] E. Horsdahl-Pedersen and J.L. Rasmussen, J. Phys. B15 (1982) 4423.
I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
89
THE INTERPLAY BETWEEN NUCLEAR AND ATOMIC PHYSICS: ELECTRON CAPTURE IN THE PRESENCE OF A NUCLEAR RESONANCE O.K. BAKER *, C. STOLLER and W.E. ~EYERHOF Department
of
Physics, Stanford University, Stanford, CA 94305, USA
J.N. SCHEURER Cenrre d’Etudes Nu&aires, WniversitCde Bordeaux I, Institut National de Physique NuclPaire et des Particules, 33170 Grad&pan, France
The total probability, P(E,,, Stab), for electron capture from “C, 14N, and 20Ne by protons of energy Era+.,between 0.5 and 1.510 MeV and from 22Ne by protons of energy E,, between 1.500 and 1.525 MeV has been measured at scattering angles Blabof 30 ’ and 150*. Experimental evidence for the dynamic interference effect between atomic electrons and nuclear reactions is clearly seen in P(E,,, filab) in the presence of the elastic s-wave nuclear resonance 22Ne@, p)22Ne at 1.510 MeV. The data is compared with theoretical calculations by Amundsen and Jakubassa-Amundsen using the strong-potential Born Approximation.
1. Introduction
(1) i2C(p, p)12C at 0.462 MeV (r=
A fascinating field of study has emerged within the past few years in which it is possible to observe dynamic interference effects between atomic electrons and nuclear reactions in ion-atom collisions [1,2]. Normally, measurements of electron excitation, ionization, and capture are made in the absence of any nuclear scattering, while investigations of nuclear reactions between projectile and target nuclei are performed without regard to atomic electronic processes. Those special circumstances where the atomic processes are affected by nuclear reactions can potentially provide valuable information in both areas, not generally accessible by other means [3]. An experimental investigation of this interplay between nuclear and atomic physics can be made using a Van de Graaff accelerator of only a few MV terminal voltage. We have made such an investigation at Stanford using a single-ended 3 MV Van de Graaf accelerator. Specially, we have measured probabilities for electron capture by protons in the presence of proton induced nuclear resonances for several target systems [4]. We have found evidence that there is a nuclear resonance effect in electron capture [4-61. The data is in qualitative agreement with theoretical calculations [7,8]. In this article, we present a comparison of the measurements of the probability for electron capture by protons in the presence of a proton induced nuclear resonance. The target systems and the resonances studied were: * Present address: Los AIamos National Laboratory, Los Alamos, NM 87545, USA. 0168-583X/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Pub~s~ng Division)
B.V.
35 lcev) [9], (2) 14N(p, p)14N at 1.058 MeV (r= 6 keV) [lo], (3) *‘Ne(p, p)22Ne at 1.510 MeV (r= 2.45 keV) [ll]. We begin with a brief theoretical discussion. Then a comparison with theoretical expectations is made.
2. Theory Amundsen and Jakubassa-Amundsen [8] have found that the criterion for a strong interplay between the atomic electrons and nuclear processes in K-shell electron capture is
where r is the width of the nuclear resonance, u is the projectile velocity, m is the electron mass, and Ez and Ez are the electron binding energies to the target and projectile, respectively. A=~E,T~-~E;~+$w2,
(2)
is the energy transferred
to the K-shell electron during capture. It is the criterion of eq. (1) which is to be examined in this article. Qualitatively, it has the following physical interpretation. If the width of the resonance is too large, that is to say if the nuclear delay time is too short, the electron does not “know” that there has been a resonance. Then there would be not interference between incoming and outgoing channels in the capture mechanism and no nuclear resonance effect seen in capture. Alternatively, if the width of the resonance is too small (if the nuclear delay time is too long), the electron “does not remember” the incoming I. ATOMIC PHYSICS / RELATED PHENOMENA
90
0. K. Baker et al. / Interpluv between nuclear and atomic physics
channel in the collision. There would be, in this case, an incoherent addition of incoming and outgoing channels in the collision and, again, no nuclear resonance effect on electron capture. Therefore, it is advantageous to look for nuclear resonance effects where the width of the nuclear resonance is of the order of the energy transferred to the active electron in the capture process. The question to be answered is, how sensitive is the electron capture mechanism in the presence of nuclear resonance scattering to the criterion of eq. (1). Our measurements provide an answer. The experimental setup and procedure is described next.
3. Experimental setup The first attempts to measure a nuclear resonance effect in electron capture were made by HorsdahlPedersen et al. in 1982 using an electric separation of the protons which capture an electron (Ho) from those which do not capture an electron (Hf) 1121.We used a singles method of data acquisition conceptually similar to theirs, but employing a magnetic separation instead. The scattering apparatus which made up the main part of the experimental setup is shown in fig. 1. The incoming proton beam, supplied by the Van de Graaff accelerator, is denoted by H’. The beam traverses a gaseous target denoted by J. The beam is stopped in a Faraday cup made of aluminum in order to minimize backscattering. There are four (silicon) solid state surface barrier detectors, Bl and B2 positioned at 150” with respect to the incoming beam, and Fl and F2 positioned at 30*. These particular angles were chosen in order to facilitate determination of the nuclear resonance energy. The ratio of the number of counts detected in a backward detector divided by the number detected in a forward detector is nearly proportional to the nuclear resonance cross section (at the nuclear resonance energy). Detectors Bl and Fl were placed inside a region where a high magnetic field could be applied, as shown by the dotted line. The magnetic field could be turned on and off in each experimental run. With the field on, a separation of the two charge state HO, neutral hydrogen, and H.’ , protons, occurs. Protons which capture an electron from the target gas at the interaction region J become neutral hydrogen and proceed undeflected to the detectors inside the magnetic field region. Protons which fail to capture an electron during the bombardment of the target gas at J are deflected away from the detectors Bl and Fl as indicated by the arrows in fig. 1. The two detectors outside of the magnetic field region, F2 and B2 were equally sensitive to neutral and charged particles and serve for normalization of the beam current. Slits S served to define the size of the beams
Fig. 1. JZxperimental arrangement to measure electron capture probabilities at 30° and 150°. For explanations of symbols, see text. The broken line indicates the boundary of the pole pieces of the magnet.
scattered to the detectors. To shield the entering beam from the fringing magnetic field as much as possible, the beam was passed through a thick walled iron tube T. In order to determine the capture probability, we proceeded as follows. By turning the magnetic field on the probability for electron capture at 150”, P(E, @= 1500), was extracted by taking the ratio of the number of counts in detector Bl and the number of counts in B2 in an experimental run. This ratio is then divided by those same quantities measured with the magnetic field off. p(
150°)
E,
=
‘sr’s2)on, MB
where E is the bombarding and mu=
i
Bl B2
energy of the proton beam
1 off’
where the symbols Bl and 32 here stand for the counting rates in the detectors. Similarly for the forward angles, the probability for electron capture of 30°, P( E, 0 = 30°) is extracted
of counts
in detectors
from the corresponding ratio Fl and F2 along with the solid
angle ratio wF.
&f‘
4. Charge state separation using a magnetic field
In principle, the magnetic field could be increased as much as desired above the ~~~ necessary to achieve the separation of the Ho and H’ particles described above. In practice, the fringing field in the region of the primary beam needed to be minimized as much as
O.K. Baker et al. / Interplay between nuclear and atomicph.vsics
possible in order to reduce its bending before traversing J, so that the magnetic field was kept as close to this minimum as possible. To ascertain the minimum magnetic field necessary to achieve this separation, measurements were made of the number of captured particles in detectors Fl and Bl at a fixed inlet gas pressure and integrated charge in the Faraday cup as a function of magnetic field strength. When the number of particles counted in detectors Fl and Bl in this way remained constant as the magnetic field strength is increased or decreased, the proper separation was achieved. As the proton beam energy E was increased, the deflection magnet current was increased proportionally to El/‘.
91
4
3
0
5. Electron capture measurements using a gaseous target Since at the energies of interest here, the proton beam reaches an equilibrium charge state within the first few atomic layers of a solid target, it would be difficult to study electron capture in a single collision at the initial charge state of the beam, as this type of experiment proposes to do. Thus, it is advantageous to use a gaseous target to study electron capture probabilities by protons. However the disadvantages in using a gaseous target are the low density of atomic scatterers and the difficulty of confining the target spatially. In our study, we employed a gas jet from a hypodermic needle because a windowless gas cell is not easily built if scattered particles have to be detected. The aim of our procedure was to maximize the pressure of the target gas in the small interaction region (approximately 3 cm3) at J (fig. 1) and to minimize the pressure throughout the rest of the scattering chamber. We tested the capture probability at a number of target gas pressures in the chamber in order to insure that there was no secondary reactions such as secondary capture or ionization which would skew the data. No such reactions were seen. All the data presented was gathered at a target gas pressure in the chamber of 3 X 10e4 Torr. The counting rates were as low as 10 counts/h in detector Bl with magnetic field on, and as high as 2000 counts/h in Fl with field off. A typical spectrum gathered in this way is shown in fig. 2. The spectrum is from protons scattered from 2oNe at 0.5 MeV. This particular spectrum is from detector Bl with field on gathered over a period of about 2 h. The various features of the spectrum are indicated. By making a least squares fit to the background we could subtract out the background counts from the scattering peak in each spectrum and determine the true number of particles scattered from the target gas at the interaction region J. For each series of runs, the procedure was to scan over the resonance by turning the magnetic field off and measuring the solid angle ratios (4) and (6) as well as taking the ratio of the number of particles detected at a
1000
I100
I200
I300
1400
CHANNEL
Fig. 2. A typical spectrum of H+ +20Ne at 1.0 MeV. The various features indicated are as follows: A. Protons scattering from the “Ne target gas which capture an electron. B. Broad peak arising from scattering from the needle used to let the “Ne target gas into the scattering chamber. C. High energy scattering contribution from neutron producing nuclear reactions.
backward angle to those detected at a forward angle during a certain time interval or, equivalently, for a specified integrated proton current. Since at 30’ the nuclear resonance scattering is dwarfed by the coulomb scattering, this ratio is essentially proportional to the 150° nuclear resonance cross section at each energy. The magnetic field was then turned on, and the scan over the resonance was repeated to obtain the capture probabilities (3) and (5).
6. Results and discussion Presented in fig. 3a is the probability for electron capture from 12C (CH,) across the 35 keV elastic nuclear resonance at 0.462 MeV. The excitation function which is proportional to the nuclear resonance at that energy is shown in fig. 3b. Here A/T is about 0.017. As can be seen, within experimental error there is no nuclear resonance effect on the capture probability. The probability for electron capture by protons from 14N (N2) in the vicinity of the 6 keV elastic nuclear resonance at 1.058 MeV is presented in fig. 3c along with the excitation function proportional to the nuclear resonance at that energy in fig. 3d. Here a small effect is expected theoretically. The data gives evidence of a noticeable effect. In this system A/r is 0.17, an order of magnitude closer to unity than in the previous case. Next is presented the probability for electron capture by protons from 22Ne in fig. 4a. The probability was measured in the vicinity of the 2.5 keV elastic I. ATOMIC
PHYSICS
/ RELATED
PHENOMENA
O.K. Baker et al. / Interplay between nuclear and atomicph.vsics
92
PC 150 (IO”
a
1
ID.8
I3.4
2‘
(I-
0
f
b
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164
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- 1.496
I.504
1.512
I.520
s? 5
o-’
420
’
460 PROTON
a
1055 ENERGY
60
65
0
(keV)
Fig. 3. (a) Electron capture probability at @lab=150°, for p + CH4 collisions in the neighborhood of the 462 keV resonance. (b) The 150”/30” counting rate ratios proportional to the nuclear scattering cross section at this energy. (c) Electron capture probability at @tat,= 150°, for p + N2 collisions in the neighborhood of the 1058 keV resonance. (d) The corresponding counting rate ratios proportional to the nuclear scattering cross section at this energy.
resonance at 1.510 MeV shown in fig. 4b for a target gas mixture of 75’%**Ne and 25%*‘Ne. (The capture probability shown in fig. 4a is corrected for the presence of nonresonant *‘Ne.) For this system A/r is 0.7 (of the order of unity). Thus the criterion of eq. (1) is met and a large nuclear resonance effect on electron capture is expected. The data indicated that there is indeed a large effect on the capture probability in this case. The results of this comparison suggests that the criterion posed by eq. (1) does bear dramatically on the possibility of seeing a nuclear resonance effect in electron capture probabilities.
7. Conclusion
The role of nuclear resonant scattering in an ion-atom collision where the K-shell electron is captured into a bound state of the projectile has proven to be a unique tool for studying the interplay between nuclear and atomic physics. We have, in this article, presented an experimental setup and procedure for making measurements of electron capture probabilities by protons impinging on a gaseous target using the singles method of data acquisition. This experimental
P
1o‘3
I .496
I .504
I.512
I.520
Fig. 4. (a) Electron capture probability at eta,, =150’, for p+‘*Ne collisions in the neigborhood of the 1510 keV resonance. (b) The corresponding counting rate ratios proportional to the nuclear scattering cross section at this energy. The curve shown is the calculation of the ratio of the nuclear scattering cross section at 150” to that at 30°.
method has been used successfully to obtain measurements of probabilities for electron capture by protons scattered to large angles and in the presence of a proton induced nuclear resonance for several target systems. This work was done using a Van de Graaff accelerator capable of producing a continuous beam of protons between 0.3 MeV and 2.0 MeV with intensities on the order of tens of PA on target. We have found qualitative agreement with theoretical calculation. This suggests that the ratio A/r of the energy transfer to the captured electron to the resonance width plays an important role in determining the magnitude of the nuclear resonance effect on electron capture.
O.K. Baker et al. / Interplay between nuclear and atomicphvsics
This work was supported in part by NSF grant PHY and by a NATO fellowship for J.N. Scheurer.
83-13676
References
Ul W.E. Meyerhof.
in: Electronic and Atomic Collisions, eds.. J. Eichler, I.V. Hertel and N. Stolterfoht (North-Holland, Amsterdam, 1983). VI R. Anholt, in: Atomic Inner-Shell Physics, ed., B. Grasemann (1985). [31J.S. Blair and R. Anholt, Phys. Rev. A25 (1982) 907. 141J.N. Scheurer, O.K. Baker and W.E. Meyerhof, J. Phys. B18 (1985) L85.
93
[S] O.K. Baker et al., to be published
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I. ATOMIC
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