- Email: [email protected]
A novel experimental technique of nuclear lifetime measurements
c!fGL -58 !3
Nuclear Instruments and Methods in Physics Research 3 95 ( 1995) 543-547
NIIIMI B
Beam Interactions with Materials l&Atoms
!!o
ELSEVIER
A novel experimental technique of nuclear lifetime measurements O.A. Yuminov a-*, S.Yu. Platonov a, D.O. Eremenko a, N.V. Eremin a, I.M. Egorova a, V.O. Kordyukevich”, O.V. Fotina a, F. Malaguti b, A. D' Arrigo ‘, G. Giardina ‘, A. Taccone ‘, G. Vannini d, A. Moroni e, R.A. Ricci f, L. Vannucci f a Instituteof Nuclear
Physics.
Moscow
h Istituto
Nazionale
di Fisica
Nucleare
and Dipartimento
di Fisica
dell’Universir&
Bologna.
Italy
’ Istituto
Nuzioncrle
di Fisica
Nuclenre
and Dipartimento
di Fisica
dell’Universit&
Messina,
Itn&
d Istiruto
Nazimule
di Fisica
’ Istituto ’ Istituto
Nazionale
Nucleare
Nazionale di Fisica
State
University
and Dipartimento
di Fisica Nucleare,
Nucleore, Lahoratori
Moscow
di Fisica Sezione Nazionali
119899.
Russia
del[‘Universitir,
di Miluno.
Trieste,
Italy
Italy
di Legnaro.
Padow
Italy
Received 11 August 1994; revised form received 11 November 1994
Abstract In the present paper a new experimental method to measure nuclear reaction time in the 10-‘5-10-“’ s region is presented. Measurements of the lifetimes of low-lying and long-lived states of 19F and *“Ne decaying via a-channel were carried out with the aim of checking the feasibility of the method. The results obtained in this way are compared with the lifetimes known from different techniques.
1. Introduction
2. Basic ideas of the method
Over the past years some methods have been developed to directly measure nuclear lifetimes. They allow one to study both the nuclear transformation mechanisms and the structure of interacting nuclei. Among these the time-of-Right [ I] and blocking techniques [ 2,3] are commonly used, and allow measurements of the time characteristics of nuclear processes at r 2 1O-‘o s and lO-‘9 5 r 2 lo-” s, respectively. To study the nuclear transformations in the 10-‘51OK’” s range the method based on the Doppler shift of yradiation lines is intensively used [4]. However, this method cannot be applied to investigate the decay of excited nuclei either in the region of strongly overlapping resonances or by emission of charged particles. The present paper suggests a new experimental “slowing down method” (SDM) , based on the phenomenon of energy loss of charged particles, able to directly measure nuclear lifetimes ranging from lo-l5 to lo-” s [ 51. After presenting the principle of the new method, we describe here the results of an experiment carried out with the aim of testing its feasibility and sensitivity. This has been done measuring known lifetimes of long-lived states in r9F and *“Ne that decay via a-channel.
Let us consider the interaction of a high-energy heavy ion with a target made up of two layers, a thin layer of light elements (first layer), and a thick layer of heavy elements (second layer). If the bombarding energy is sufficient for the fusion with nuclei of light elements, but not for the fusion with those of heavy elements, the nuclear reactions will occur only in the first layer. The compound nuclear system (CN), produced from the fusion of interacting nuclei in the first target layer, is subjected to great energy losses, when it moves in the second layer, and decays. If the energy loss of the CN is much larger than that of the decay product (DP), the energy that this last will have when escaping the target will depend on the depth X in the second layer where the CN has decayed, and the source of the DP is located. But X depends on the time I at which the CN has decayed, therefore the final energy distribution of the DPs will depend on the decay law of the CN. Following these simple ideas, we will now write the mathematical relations on which the SDM is based. The CN energy EC at depth X in the second layer can be written as X
E,(X) =&CO) -
J
$j
dx,
0
* Corresponding
0168-583X/95/W9.50
author, e-mail [email protected].
@
SSD10168-583X(94)00602-4
1995
Elsevier Science B.V. All rights reserved
and the time-of-flight
down to X as
(1)
544
O.A. Yuminov rt ol./N~d.
Insrr. and Meth. in Phu. Res. B 95 (1995) 543-517
where M, is the mass of the CN, and V,(X) its velocity at depth X in the second layer:
J
2&(X)
v,(X)=
-yg---. c
The time-decay law of the CN, fo(t). is connected to the spatial distribution of DP sources, f(X), by the obvious relation: f(x)
=
y
-16
I 0.00
fo(f(X))
2.00
I
I
T
4.00
6.00
8.00
(4)
K:.(X) In the case of exponential
I
I
I
10.00 12.00 14.03
DEPTH, pm Fig. 2. Time-of-flight of “F. produced in the 7Li( 160,a) reaction at 120 MeV bombarding energy, versus depth X within the stopping layer.
fa ( t), this becomes
and its probability density F(E) is related to the spatial density of DP sources, f(X), by the obvious relation r being the CN mean lifetime. In this way the spatial density of DP sources, f( X), can be calculated starting from the CN mean lifetime r and from its energy loss dE, (X) / dX. From now on, we will consider DPs that leave the target at zero angle relative to the beam, but the formalism can easily be extended to any other emission angle. For these DP, whose mass and centre-of-mass energy are Mt and Ea, the lab-system energy El at the starting depth X is simply obtained adding their centre-of-mass velocity to the CN velocity, viz.
E,(X)=Ec(X)~+Eo+2M, ‘
(6)
The corresponding energy E with which the DP leaves the second target layer, that has thickness 1, is
E(X)=,,(X)-ji-1
dx,
(7)
X
BEAM
Fig. I. Sketch of the experimental setup. The two-layer target was -0.13 /an ‘LiF evaporated on a 10 /an gold backing. The beam was I20 MeV oxygen ions. The AE-E telescopes were surface-barrier Si detectors, 20 and 2000 pm thick, and 20 mm in diameter.
F(E)
= ,dE;;;dX(’
(8)
where dE( X) / dX can be obtained from Eq. (7). The result is 1 f(X) F(E(X)) = jldE,(X)/dXI’
(9)
Therefore, calculation of Eqs. (7) and (8) for different depths X allows one to construct F( E), the energy spectrum of DPs leaving the target, and the result depends on the time-decay law fa( I). This means that measuring the energy spectrum can give information on the decay time. To illustrate the possibilities of the method. we will consider the a-decay of excited resonant states of “F, that are populated through the reaction ‘Li +lh 0 % rrF* 5. For this case we carried out calculations of the cr-particle spectra, emitted in the forward direction, following Eqs. ( l)(8) and according to the experimental conditions described below, see Fig. 1. In the calculations we varied the lifetime in the range from IO-” to lo-‘* s to study the sensitivity of the experimental method. The calculations of the energy loss of the CN and of the emitted cu-particles were carried out within the Bethe-Bloch formalism with allowance for the charge-exchange processes of the ion in the medium [ 61. Fig. 2 gives an idea of the lifetime range that can be studied by the present technique. There, we display the time-offlight of the compound system 19F as a function of the depth X within a gold stopping target, calculated for a 120 MeV energy of the bombarding oxygen ions. Figs. 3 and 4 display the spatial probability density of the emitting source “F as a function of the penetration depth X in the second layer, and the corresponding energy spectra of cY-particles leaving the thick target. As can be seen, they are strongly dependent on the 19F lifetime. The SDM shows a great sensitivity in
O.A. Yuminov et ai./Nucl.
54.5
Instr. and Meth. in Phys.Res. B 95 (1995)543-547
f f
0
40 d iii
E 0
8 4 0.00 2.00 4.00
6.00
8.00 10.00 12.00 14.00
DEPTH, pm Fig. 3. Spatial distribution for the a-emitting sources ( 19F* nuclei) vs. penetration depth in the stopping layer, calculated for the present experimental conditions. The curves correspond to different lifetimes of the 5.47 MeV state of 19F, viz. I: 5.0~10-‘~ s; 2: 1.0 x IO-l4 s; 3: 5.0 x IO-l4 s; 4: I .O x lO-‘3 s: 5: 1 .O x lo-l2 s. and for the thickness of the stopping layer 13 pm.
the time range from lo-” to lOpi2 s. The maxima of the a-particle spectra at low energy (see Fig. 4) are connected with the decay of the 19F nucleus outside the stopping target, and occur at high lifetime values. The magnitude of these maxima increases with nuclear lifetime, and allows us to extend the SDM up to lo-” s.
3. Experiment The validity and sensitivity of the SDM have been tested by carrying out a measurement of the cu-spectra coming from the decay of some resonant states of i9F and 20Ne and slowed down in a Au target. For these states the experimental data on the lifetime, obtained by means of the method based on the Doppler shift of y-radiation lines, exist in the literature 171. The experiment was carried out at the XTU-Tandem of Legnaro Laboratories (Padova, Italy) with a 120 MeV beam of oxygen ions. Typical current value was -1 nA, and total running time around 10 hours. The scheme of the experiment is displayed in Fig. 1. The two-layer target consisted of -0.13 pm ‘LiF evaporated onto a 10 pm gold backing, that was used as the stopping layer. The ‘“F and *‘Ne excited nuclear states were populated using the direct reactions ‘Li( 160, (u) and ‘Li( ‘60,3 H) occurring in the first layer of the target. The cY-particles, decay products of the investigated resonant states, were detected by a AEi -El telescope consisting of two Si surface-barrier detectors, 20 pm and 2 mm thick, respectively, and 20 mm in diameter. This telescope was mounted at 11“ relative to the oxygen beam, and at a 13 cm distance from the target. The direct reaction products were detected by an analogous A&E2 telescope placed at 90” relative to the beam and
MeV Fig. 4. Calculated energy spectra of a-particles, produced in the a’-n channel, escaping from the target. The curves are labelled in a similar way as in Fig. 3.
at a 6 cm distance. The telescopes allowed good discrimination between cY-particles and other reaction products. Selection of the reaction channel was carried out taking the a-particles detected in telescope 1 in coincidence with the direct reaction products in telescope 2: a-particles select the low-lying excited states of 19F ( (Y’-_(ychannel), while ‘H select the low-lying excited states of *‘Ne (t-cx channel). Simple kinematical arguments allowed us to locate the excitation energy of the decaying nuclei. For exam le, to select P the decay products of the excited level 5.47( i
546 Table I Parameters of the investigated
resonance states for ‘“F
5.11 (1-j
(161t7)”
0. I 39
0.02”
5.34 ( I+)
-
lh
0.09h
(2.6 zt 0.7)”
0.963
o.ooa
s.47 (;+,
119”
N lh
0.5 I h
5.50 (j+)
(Jz!L I) keV”
-
0.3gh
5.43 ( i-
1
lh
’ Data of Ref. [ 7 I. ’ These estimations were produced on the basis of experimental crow section data on investigated reactions [ 81 with the use of the assumption: lb > I’,. ’ pi is the relative weight of the a-particle resonance states in the region Em < 28 MeV.
19.00 21.00 23.00 25.00 27.00 29M)
E,, MeV Fig. S. Energy spectra of n-particles, from the a’-a channel. escaping from the target. The dots are experimental values. The curves are calculations for a lifetime r=3.2 x IO-l4 s for the 5.47 MeV state of “F* (solid line), and for a very small ( I .O x IO- ” s) lifetime of all relevant states (dashed line). The experimental energy resolution was assumed to be 500 keV.
lifetime of the state at 5.50 MeV, z’, with r=4 keV [8] is concerned, it is significantly below the lowest sensitivity limit of the method. Therefore, we tried to reproduce the data varying only the lifetime of the 5.47 MeV state and fixing that of the 5.50 MeV one at a IO-‘” s value, which is much smaller than the lowest sensitivity limit of the method. Using a least-squares procedure, the best fit to the (Yparticle spectrum was achieved with the parameters shown in Table I and with the lifetime r=( 3.2 i 3.0) x 1OKI4 s for the “F state at 5.47 MeV. All calculations were performed taking into account the 500 keV energy resolution, that was
yields from the different
determined by the finite solid angle covered by the detectors. To test the effect of energy resolution on the SDM. we display in Fig. 7 different calculations of the energy spectrum in the cy’--LY channel, done assuming a small lifetime (7 < IO-” s) for all relevant states (as though the stopping layer were absent), and using different energy resolutions of 10. 100 and 300 keV. Therefore the curves of Fig. 7 do not contain any lifetime effect and, if compared with the data of Fig. 5, they demonstrate that the experimental a-particle spectra are really determined by the large lifetime of the nuclear states. Moreover, the spectral slopes are independent of the energy resolution, at least in first approximation. In the case of the t-a channel, it is possible to isolate the energy region E, < 22 MeV. where the yield is due only to decay of the *“Ne states at 5.62 MeV, 3- and at 5.78 MeV, 1- The resonant state parameters used in the calculation of
3 f 5
2
e 0
’
d z
0
z 0 $
-1
a
\
I
I
I
I
15.00
17.00
19.00
21.00
‘I
23.00
E,, MeV Fig. 6. Energy spectra of a-particles, from the t-a channel, escaping from the target. The dots are experimental values. The curves ax calculations for a lifetime 7=2.l x IO-” s for the 5.62 MeV state of 20Ne*(solid line). and for a very small (1.0 x IO-l6 s) lifetime of all relevant states (dashed line). Experimental energy resolution was assumed to be 500 keV.
-2 23.00
24.M)
25.cil
26.00
27.M)
28.00
E,, MeV Fig. 7. Calculated energy spectra of a-particles. from the a’-n channel, escaping from the target. All curves correspond to a very small (1.0X IO_‘6 s) lifetime of the relevant states. but to different energy resolutions: IO keV (solid line), 100 keV (short-dashed line), and 300 keV (long-dashed line)
O.A. Yuminov et al./Nd. Table
547
2
Parameters
of the investigated
5.62 (3-l 5.78
Ins/r. md Meth. in Phy. Res. Et 95 (1995) 543-547
( I -)
a Data of Ref.
states for 19F
(200 + 50)”
N Ih
0.7b
(28 * 3) ev
N Ih
0.3b
171.
h These estimations and with
resonance
were producedon
consideration
the basis of the assumption
for the statistical
weight
lb
>> f’v
of the concrete resonance
state. ’
p;
is the
resonance
relative
weight
of
states in the region
the cr-particle
yields
from
the different
Em < 22 MeV. 9.00
the cY-particle spectra are displayed in Table 2. As is seen from Table 2, the Iifetime of the 5.78 MeV state is much below the lowest sensitivity limit of the method, so it was fixed at 10-‘h s. Using a least-squares method, we obtained the best fit to experimental data using the lifetime r=( 2.1 i 1.8) x!O-i3 s for the 5.62 MeV state in ‘“Ne. Practically, this is the same value, (2OO~t50) x IO-‘” s, that was obtained by the Doppler-shift of y-radiation lines [ 71. Finally, to better show the effect of the energy resolution, we display in Figs. 8 and 9 the cu-particle spectra calculated with the lifetime values of Tables I and 2, and for different energy resolutions: IO, 100, and 500 keV. It is clear that better statistical accuracy and energy resolution are required to extract decay components with a relatively low cu-particle yield, like the 5.1 I and 5.34 MeV states of ‘“F. From the present calculations (see Fig. 8)) it appears that these components can be disentangled with an energy resolution better than 100 keV.
2
I
,
1
’ \ : \ ’ \ : i \ : \ \ : / / I
0
-1 _
-2
23.00
I
I
I 24.00
25.M)
26.00
17.00
Fig.
9. Calculated
escaping displayed line).
5.
energy
spectra
of n-particles.
from the target. The curves correspond in Table
100 keV
2, but with
(short-dashed
different line),
escaping displayed line),
from
the target. The
in Table
100 keV
spectra of o-particles,
I, but with
(short-dashed
from
curves correspond different line),
energy
to the 19F*
and 500 keV
and G. Sletten, Nucl.
A.F. Tulinov.
I
Rev. Mod.
IO keV
(long-dashed
line).
Phys. A 199 ( 1973)
Phys. 46
Usp. Phiz. Nauk.
( 1974)
87 ( 1965)
P.M. Endt and C. van der Leun, Nucl. S.Yu. Platonov and O.A.
parameters
IO keV
(long-dashed
channel. parameters (solid
line).
In the present paper a new experimental method to measure nuclear reaction time characteristics in the range from 1O-‘5 to IO-“’ s is presented and discussed. Measurements of the lifetimes of low-lying and long-lived states of ‘“F and ‘“Ne nuclei decaying via a-channel were carried out with the aim of testing the feasibility and sensitivity of the method. The results obtained here are compared with previous experimental data. In conclusion we emphasize that this experimental technique can be used to study the lifetime of low-lying excited states of light nuclei, decaying via charged particle emission. In addition, the method can be employed to investigate the fission time in the near-threshold energy region, the formation and decay of short-lived spontaneously fissionable isomers and of super-heavy nuclei. It is important to stress that in some cases it is impossible to obtain such information by means of traditional methods. Besides, the method can be used for measurements of energy losses of exotic radioactive heavy ions if their lifetime is known.
P. Limkilde
[he LI’-O~ channel.
resolutions:
the t-n
resolutions:
and 500 keV
(Moscow
Yuminov,
(solid
504.
129. 585
Phys. A 214 (1973)
Proc. XXth
All-Union
116. Meeting
of Charged Particles with Solids. Moscow,
State University,
Moscow,
K. Shima. T. Ishihara, T. Mikuma, energy
energy
from
to the *“Ne*
Conclusion
D.S. Gemmel.
E,, MeV 8. Calculated
25.00
E,, MeV
Physics of Interaction
Fig.
21.00
References
28.00
27.w
13.00
1989)
Nucl.
on 1990
p. 56.
Instr. and Meth. A 200
( 1982)
605. F. Ajzenberg-Selove. I. Tserruya,
Nucl.
Phys. A 300 ( 1978)
B. Rosner. K. Bethge, Nucl.
I
;A
Phys. A 235
( 1987) ( 1974) 75.
475
I.
Nuclear Instruments and Methods in Physics Research 3 95 ( 1995) 543-547
NIIIMI B
Beam Interactions with Materials l&Atoms
!!o
ELSEVIER
A novel experimental technique of nuclear lifetime measurements O.A. Yuminov a-*, S.Yu. Platonov a, D.O. Eremenko a, N.V. Eremin a, I.M. Egorova a, V.O. Kordyukevich”, O.V. Fotina a, F. Malaguti b, A. D' Arrigo ‘, G. Giardina ‘, A. Taccone ‘, G. Vannini d, A. Moroni e, R.A. Ricci f, L. Vannucci f a Instituteof Nuclear
Physics.
Moscow
h Istituto
Nazionale
di Fisica
Nucleare
and Dipartimento
di Fisica
dell’Universir&
Bologna.
Italy
’ Istituto
Nuzioncrle
di Fisica
Nuclenre
and Dipartimento
di Fisica
dell’Universit&
Messina,
Itn&
d Istiruto
Nazimule
di Fisica
’ Istituto ’ Istituto
Nazionale
Nucleare
Nazionale di Fisica
State
University
and Dipartimento
di Fisica Nucleare,
Nucleore, Lahoratori
Moscow
di Fisica Sezione Nazionali
119899.
Russia
del[‘Universitir,
di Miluno.
Trieste,
Italy
Italy
di Legnaro.
Padow
Italy
Received 11 August 1994; revised form received 11 November 1994
Abstract In the present paper a new experimental method to measure nuclear reaction time in the 10-‘5-10-“’ s region is presented. Measurements of the lifetimes of low-lying and long-lived states of 19F and *“Ne decaying via a-channel were carried out with the aim of checking the feasibility of the method. The results obtained in this way are compared with the lifetimes known from different techniques.
1. Introduction
2. Basic ideas of the method
Over the past years some methods have been developed to directly measure nuclear lifetimes. They allow one to study both the nuclear transformation mechanisms and the structure of interacting nuclei. Among these the time-of-Right [ I] and blocking techniques [ 2,3] are commonly used, and allow measurements of the time characteristics of nuclear processes at r 2 1O-‘o s and lO-‘9 5 r 2 lo-” s, respectively. To study the nuclear transformations in the 10-‘51OK’” s range the method based on the Doppler shift of yradiation lines is intensively used [4]. However, this method cannot be applied to investigate the decay of excited nuclei either in the region of strongly overlapping resonances or by emission of charged particles. The present paper suggests a new experimental “slowing down method” (SDM) , based on the phenomenon of energy loss of charged particles, able to directly measure nuclear lifetimes ranging from lo-l5 to lo-” s [ 51. After presenting the principle of the new method, we describe here the results of an experiment carried out with the aim of testing its feasibility and sensitivity. This has been done measuring known lifetimes of long-lived states in r9F and *“Ne that decay via a-channel.
Let us consider the interaction of a high-energy heavy ion with a target made up of two layers, a thin layer of light elements (first layer), and a thick layer of heavy elements (second layer). If the bombarding energy is sufficient for the fusion with nuclei of light elements, but not for the fusion with those of heavy elements, the nuclear reactions will occur only in the first layer. The compound nuclear system (CN), produced from the fusion of interacting nuclei in the first target layer, is subjected to great energy losses, when it moves in the second layer, and decays. If the energy loss of the CN is much larger than that of the decay product (DP), the energy that this last will have when escaping the target will depend on the depth X in the second layer where the CN has decayed, and the source of the DP is located. But X depends on the time I at which the CN has decayed, therefore the final energy distribution of the DPs will depend on the decay law of the CN. Following these simple ideas, we will now write the mathematical relations on which the SDM is based. The CN energy EC at depth X in the second layer can be written as X
E,(X) =&CO) -
J
$j
dx,
0
* Corresponding
0168-583X/95/W9.50
author, e-mail [email protected].
@
SSD10168-583X(94)00602-4
1995
Elsevier Science B.V. All rights reserved
and the time-of-flight
down to X as
(1)
544
O.A. Yuminov rt ol./N~d.
Insrr. and Meth. in Phu. Res. B 95 (1995) 543-517
where M, is the mass of the CN, and V,(X) its velocity at depth X in the second layer:
J
2&(X)
v,(X)=
-yg---. c
The time-decay law of the CN, fo(t). is connected to the spatial distribution of DP sources, f(X), by the obvious relation: f(x)
=
y
-16
I 0.00
fo(f(X))
2.00
I
I
T
4.00
6.00
8.00
(4)
K:.(X) In the case of exponential
I
I
I
10.00 12.00 14.03
DEPTH, pm Fig. 2. Time-of-flight of “F. produced in the 7Li( 160,a) reaction at 120 MeV bombarding energy, versus depth X within the stopping layer.
fa ( t), this becomes
and its probability density F(E) is related to the spatial density of DP sources, f(X), by the obvious relation r being the CN mean lifetime. In this way the spatial density of DP sources, f( X), can be calculated starting from the CN mean lifetime r and from its energy loss dE, (X) / dX. From now on, we will consider DPs that leave the target at zero angle relative to the beam, but the formalism can easily be extended to any other emission angle. For these DP, whose mass and centre-of-mass energy are Mt and Ea, the lab-system energy El at the starting depth X is simply obtained adding their centre-of-mass velocity to the CN velocity, viz.
E,(X)=Ec(X)~+Eo+2M, ‘
(6)
The corresponding energy E with which the DP leaves the second target layer, that has thickness 1, is
E(X)=,,(X)-ji-1
dx,
(7)
X
BEAM
Fig. I. Sketch of the experimental setup. The two-layer target was -0.13 /an ‘LiF evaporated on a 10 /an gold backing. The beam was I20 MeV oxygen ions. The AE-E telescopes were surface-barrier Si detectors, 20 and 2000 pm thick, and 20 mm in diameter.
F(E)
= ,dE;;;dX(’
(8)
where dE( X) / dX can be obtained from Eq. (7). The result is 1 f(X) F(E(X)) = jldE,(X)/dXI’
(9)
Therefore, calculation of Eqs. (7) and (8) for different depths X allows one to construct F( E), the energy spectrum of DPs leaving the target, and the result depends on the time-decay law fa( I). This means that measuring the energy spectrum can give information on the decay time. To illustrate the possibilities of the method. we will consider the a-decay of excited resonant states of “F, that are populated through the reaction ‘Li +lh 0 % rrF* 5. For this case we carried out calculations of the cr-particle spectra, emitted in the forward direction, following Eqs. ( l)(8) and according to the experimental conditions described below, see Fig. 1. In the calculations we varied the lifetime in the range from IO-” to lo-‘* s to study the sensitivity of the experimental method. The calculations of the energy loss of the CN and of the emitted cu-particles were carried out within the Bethe-Bloch formalism with allowance for the charge-exchange processes of the ion in the medium [ 61. Fig. 2 gives an idea of the lifetime range that can be studied by the present technique. There, we display the time-offlight of the compound system 19F as a function of the depth X within a gold stopping target, calculated for a 120 MeV energy of the bombarding oxygen ions. Figs. 3 and 4 display the spatial probability density of the emitting source “F as a function of the penetration depth X in the second layer, and the corresponding energy spectra of cY-particles leaving the thick target. As can be seen, they are strongly dependent on the 19F lifetime. The SDM shows a great sensitivity in
O.A. Yuminov et ai./Nucl.
54.5
Instr. and Meth. in Phys.Res. B 95 (1995)543-547
f f
0
40 d iii
E 0
8 4 0.00 2.00 4.00
6.00
8.00 10.00 12.00 14.00
DEPTH, pm Fig. 3. Spatial distribution for the a-emitting sources ( 19F* nuclei) vs. penetration depth in the stopping layer, calculated for the present experimental conditions. The curves correspond to different lifetimes of the 5.47 MeV state of 19F, viz. I: 5.0~10-‘~ s; 2: 1.0 x IO-l4 s; 3: 5.0 x IO-l4 s; 4: I .O x lO-‘3 s: 5: 1 .O x lo-l2 s. and for the thickness of the stopping layer 13 pm.
the time range from lo-” to lOpi2 s. The maxima of the a-particle spectra at low energy (see Fig. 4) are connected with the decay of the 19F nucleus outside the stopping target, and occur at high lifetime values. The magnitude of these maxima increases with nuclear lifetime, and allows us to extend the SDM up to lo-” s.
3. Experiment The validity and sensitivity of the SDM have been tested by carrying out a measurement of the cu-spectra coming from the decay of some resonant states of i9F and 20Ne and slowed down in a Au target. For these states the experimental data on the lifetime, obtained by means of the method based on the Doppler shift of y-radiation lines, exist in the literature 171. The experiment was carried out at the XTU-Tandem of Legnaro Laboratories (Padova, Italy) with a 120 MeV beam of oxygen ions. Typical current value was -1 nA, and total running time around 10 hours. The scheme of the experiment is displayed in Fig. 1. The two-layer target consisted of -0.13 pm ‘LiF evaporated onto a 10 pm gold backing, that was used as the stopping layer. The ‘“F and *‘Ne excited nuclear states were populated using the direct reactions ‘Li( 160, (u) and ‘Li( ‘60,3 H) occurring in the first layer of the target. The cY-particles, decay products of the investigated resonant states, were detected by a AEi -El telescope consisting of two Si surface-barrier detectors, 20 pm and 2 mm thick, respectively, and 20 mm in diameter. This telescope was mounted at 11“ relative to the oxygen beam, and at a 13 cm distance from the target. The direct reaction products were detected by an analogous A&E2 telescope placed at 90” relative to the beam and
MeV Fig. 4. Calculated energy spectra of a-particles, produced in the a’-n channel, escaping from the target. The curves are labelled in a similar way as in Fig. 3.
at a 6 cm distance. The telescopes allowed good discrimination between cY-particles and other reaction products. Selection of the reaction channel was carried out taking the a-particles detected in telescope 1 in coincidence with the direct reaction products in telescope 2: a-particles select the low-lying excited states of 19F ( (Y’-_(ychannel), while ‘H select the low-lying excited states of *‘Ne (t-cx channel). Simple kinematical arguments allowed us to locate the excitation energy of the decaying nuclei. For exam le, to select P the decay products of the excited level 5.47( i
546 Table I Parameters of the investigated
resonance states for ‘“F
5.11 (1-j
(161t7)”
0. I 39
0.02”
5.34 ( I+)
-
lh
0.09h
(2.6 zt 0.7)”
0.963
o.ooa
s.47 (;+,
119”
N lh
0.5 I h
5.50 (j+)
(Jz!L I) keV”
-
0.3gh
5.43 ( i-
1
lh
’ Data of Ref. [ 7 I. ’ These estimations were produced on the basis of experimental crow section data on investigated reactions [ 81 with the use of the assumption: lb > I’,. ’ pi is the relative weight of the a-particle resonance states in the region Em < 28 MeV.
19.00 21.00 23.00 25.00 27.00 29M)
E,, MeV Fig. S. Energy spectra of n-particles, from the a’-a channel. escaping from the target. The dots are experimental values. The curves are calculations for a lifetime r=3.2 x IO-l4 s for the 5.47 MeV state of “F* (solid line), and for a very small ( I .O x IO- ” s) lifetime of all relevant states (dashed line). The experimental energy resolution was assumed to be 500 keV.
lifetime of the state at 5.50 MeV, z’, with r=4 keV [8] is concerned, it is significantly below the lowest sensitivity limit of the method. Therefore, we tried to reproduce the data varying only the lifetime of the 5.47 MeV state and fixing that of the 5.50 MeV one at a IO-‘” s value, which is much smaller than the lowest sensitivity limit of the method. Using a least-squares procedure, the best fit to the (Yparticle spectrum was achieved with the parameters shown in Table I and with the lifetime r=( 3.2 i 3.0) x 1OKI4 s for the “F state at 5.47 MeV. All calculations were performed taking into account the 500 keV energy resolution, that was
yields from the different
determined by the finite solid angle covered by the detectors. To test the effect of energy resolution on the SDM. we display in Fig. 7 different calculations of the energy spectrum in the cy’--LY channel, done assuming a small lifetime (7 < IO-” s) for all relevant states (as though the stopping layer were absent), and using different energy resolutions of 10. 100 and 300 keV. Therefore the curves of Fig. 7 do not contain any lifetime effect and, if compared with the data of Fig. 5, they demonstrate that the experimental a-particle spectra are really determined by the large lifetime of the nuclear states. Moreover, the spectral slopes are independent of the energy resolution, at least in first approximation. In the case of the t-a channel, it is possible to isolate the energy region E, < 22 MeV. where the yield is due only to decay of the *“Ne states at 5.62 MeV, 3- and at 5.78 MeV, 1- The resonant state parameters used in the calculation of
3 f 5
2
e 0
’
d z
0
z 0 $
-1
a
\
I
I
I
I
15.00
17.00
19.00
21.00
‘I
23.00
E,, MeV Fig. 6. Energy spectra of a-particles, from the t-a channel, escaping from the target. The dots are experimental values. The curves ax calculations for a lifetime 7=2.l x IO-” s for the 5.62 MeV state of 20Ne*(solid line). and for a very small (1.0 x IO-l6 s) lifetime of all relevant states (dashed line). Experimental energy resolution was assumed to be 500 keV.
-2 23.00
24.M)
25.cil
26.00
27.M)
28.00
E,, MeV Fig. 7. Calculated energy spectra of a-particles. from the a’-n channel, escaping from the target. All curves correspond to a very small (1.0X IO_‘6 s) lifetime of the relevant states. but to different energy resolutions: IO keV (solid line), 100 keV (short-dashed line), and 300 keV (long-dashed line)
O.A. Yuminov et al./Nd. Table
547
2
Parameters
of the investigated
5.62 (3-l 5.78
Ins/r. md Meth. in Phy. Res. Et 95 (1995) 543-547
( I -)
a Data of Ref.
states for 19F
(200 + 50)”
N Ih
0.7b
(28 * 3) ev
N Ih
0.3b
171.
h These estimations and with
resonance
were producedon
consideration
the basis of the assumption
for the statistical
weight
lb
>> f’v
of the concrete resonance
state. ’
p;
is the
resonance
relative
weight
of
states in the region
the cr-particle
yields
from
the different
Em < 22 MeV. 9.00
the cY-particle spectra are displayed in Table 2. As is seen from Table 2, the Iifetime of the 5.78 MeV state is much below the lowest sensitivity limit of the method, so it was fixed at 10-‘h s. Using a least-squares method, we obtained the best fit to experimental data using the lifetime r=( 2.1 i 1.8) x!O-i3 s for the 5.62 MeV state in ‘“Ne. Practically, this is the same value, (2OO~t50) x IO-‘” s, that was obtained by the Doppler-shift of y-radiation lines [ 71. Finally, to better show the effect of the energy resolution, we display in Figs. 8 and 9 the cu-particle spectra calculated with the lifetime values of Tables I and 2, and for different energy resolutions: IO, 100, and 500 keV. It is clear that better statistical accuracy and energy resolution are required to extract decay components with a relatively low cu-particle yield, like the 5.1 I and 5.34 MeV states of ‘“F. From the present calculations (see Fig. 8)) it appears that these components can be disentangled with an energy resolution better than 100 keV.
2
I
,
1
’ \ : \ ’ \ : i \ : \ \ : / / I
0
-1 _
-2
23.00
I
I
I 24.00
25.M)
26.00
17.00
Fig.
9. Calculated
escaping displayed line).
5.
energy
spectra
of n-particles.
from the target. The curves correspond in Table
100 keV
2, but with
(short-dashed
different line),
escaping displayed line),
from
the target. The
in Table
100 keV
spectra of o-particles,
I, but with
(short-dashed
from
curves correspond different line),
energy
to the 19F*
and 500 keV
and G. Sletten, Nucl.
A.F. Tulinov.
I
Rev. Mod.
IO keV
(long-dashed
line).
Phys. A 199 ( 1973)
Phys. 46
Usp. Phiz. Nauk.
( 1974)
87 ( 1965)
P.M. Endt and C. van der Leun, Nucl. S.Yu. Platonov and O.A.
parameters
IO keV
(long-dashed
channel. parameters (solid
line).
In the present paper a new experimental method to measure nuclear reaction time characteristics in the range from 1O-‘5 to IO-“’ s is presented and discussed. Measurements of the lifetimes of low-lying and long-lived states of ‘“F and ‘“Ne nuclei decaying via a-channel were carried out with the aim of testing the feasibility and sensitivity of the method. The results obtained here are compared with previous experimental data. In conclusion we emphasize that this experimental technique can be used to study the lifetime of low-lying excited states of light nuclei, decaying via charged particle emission. In addition, the method can be employed to investigate the fission time in the near-threshold energy region, the formation and decay of short-lived spontaneously fissionable isomers and of super-heavy nuclei. It is important to stress that in some cases it is impossible to obtain such information by means of traditional methods. Besides, the method can be used for measurements of energy losses of exotic radioactive heavy ions if their lifetime is known.
P. Limkilde
[he LI’-O~ channel.
resolutions:
the t-n
resolutions:
and 500 keV
(Moscow
Yuminov,
(solid
504.
129. 585
Phys. A 214 (1973)
Proc. XXth
All-Union
116. Meeting
of Charged Particles with Solids. Moscow,
State University,
Moscow,
K. Shima. T. Ishihara, T. Mikuma, energy
energy
from
to the *“Ne*
Conclusion
D.S. Gemmel.
E,, MeV 8. Calculated
25.00
E,, MeV
Physics of Interaction
Fig.
21.00
References
28.00
27.w
13.00
1989)
Nucl.
on 1990
p. 56.
Instr. and Meth. A 200
( 1982)
605. F. Ajzenberg-Selove. I. Tserruya,
Nucl.
Phys. A 300 ( 1978)
B. Rosner. K. Bethge, Nucl.
I
;A
Phys. A 235
( 1987) ( 1974) 75.
475
I.