Fission of heavy hypernuclei formed in antiproton annihilation

13 Pages·2011·0.71 MB·English

PHYSICAL REVIEW C VOLUME 47, NUMBER 5 MAY 1993
Fission of heavy hypernuclei formed in antiproton annihilation
T. A. Armstrong, li& J.P. Bocquet, (2) G. Ericsson, ~s& T. Johansson, (s)
"
T. Krogulski, & & R. A. Lewis, ( ) F Malek ( ) t M. Maurel, ( ) E. Monnand, ( &
J.Mougey, (s& H. Nifenecker, &2) J.Passaneau, (i& P. Perrin, (s) S.M. Polikanov, (7)
M. Rey-Campagnolle, ( & t C. Ristori, ( G. A. Smith, l & and G. Tibell& )
~Pennsylvania State University, University Park, Pennsylvania 26808
Institut des Sciences Nucleaires, Institut National de Physique Nucleaire et de Physique des Particles,
Centre National de la Recherche Scientifique and University of Grenoble, CEDEX, France
~ lDepartment ofRadiation Sciences, Uppsala University, P.O. Box585, S751M-Uppsala, Sweden
~ l Warsaw University, Bialystok Branch, PL 1548$-Bi-alystok, Poland
DRFMC, Centre d'Etudes Nucleaires de Grenoble, Boite Postale 85X, 880/1 Grenoble, France
Continuous Electron Beam Accelerator Facility, Newport News, Uirginia 85'6'06'
Gesellschaft fur Schwerionenforschumg Darmstadt, Postfach 1105/1, D 6100D-armstadt, Germany
~Centre de Spectrometric Nucleaire et de Spectrometric de Masse, Institut National de Physique Nucleaire et de Physique
des Particles, Centre National de la Recherche Scientifique, F 01/05-Orsay, France
(Received 5 August 1992)
Heavy hypernuclei are produced in the annihilation of antiprotons in U. The delayed fission
ofheavy hypernuclei and hypernuclei offission fragments are observed by using the recoil-distance
method in combination with measurement of secondary electron multiplicity. The lifetime of hy-
pernuclei in the region of uranium is found to be (1.25+0.15) x 10 sec. It is observed that A
hyperons predominantly stick to the heavier fission fragments. The yield ofhypernuclei is found to
be (7.4+1.7) x 10 per stopped antiproton. No coincidences with K+ were found. Statistical and
systematic errors on the number ofevents expected do not rule out this possibility.
PACS number(s): 21.80.+a, 25.85.—w, 27.90.+b
I.INTRODUCTION makes it possible to employ, in the study of heavy hy-
pernuclei, the recoil-distance technique, earlier used in
In previous papers we reported results of a search for studies of short-lived fission isomers [4]. The character-
heavy hypernuclei produced in the annihilation of an- istic feature ofthis technique is the experimental geome-
tiprotons in ~ssU [1]and ~osBi [2]. The underlying idea try which provides a strong suppression ofprompt fission
ofthe experiment was that the nonmesonic decay of the fragments, with a detection efBciency for delayed fission
A hyperon in a heavy hypernucleus leads to an excitation of about 1%.
of the residual nucleus to an energy sufBcient to induce The delayed fission lifetimes observed in the annihila-
fission. Thus, the decay of heavy hypernuclei would be tion of antiprotons in U and 9Bi were found to be
observed as delayed fission with the lifetime of the hy- 0.1+&'zz nsec [1) and 0.25+o'io nsec [2], respectively. It
pernucleus. was assumed that the lifetimes measured are related to
The A hyperon can be produced in a secondary inter- groups of nuclides in the vicinity of U and Bi, respec-
action of K mesons after pN annihilation with residual tively.
nuclei. More exotic reactions, such as the direct produc- As seen from the accuracy of the quoted results, it
tion ofA hyperons on pairs ofnucleons, are also possible was impossible to conclude whether the probability of
[3] nonmesonic decay for these two groups of nuclei is the
The annihilation of antiprotons in nuclei is accompa- same or not. The shorter lifetime in the case of ura-
nied by the emission ofmesons and nucleons, giving the niurn may result from the fact that the recoil-distance
residual nucleus a rather high recoil momentum. This technique, although providing separation of delayed and
prompt fission, does not exclude completely the recording
ofhypernuclei offission fragments which are produced in
the prompt fission ofexcited hypernuclei. Because ofthe
high fissility ofnuclei in the uranium region, the number
'Present address: Gesellschaft fur Schwerionenforschumg ofdetected hypernuclei offission fragments can be rather
Darmstadt, Postfach 110541, D-6100 Darmstadt, Germany. large. As will be discussed below this may result in too
tPresent address: Institut de Physique, Institut National short a lifetime for delayed fission for the 238U target.
de Physique Nucleaire et de Physique des Particles, Centre Here we report results obtained with an improved ex-
National de la Recherche Scientifique and the University of perimental apparatus. It allowed us to impose additional
Lyon, Iyon, Prance. criteria for the separation of prompt-fission events and
tPresent address: CERN/PPE, CH-1211 Geneve, Swit- thus made it possible to separate the delayed fission,
zerland. caused by the decay of the A hyperon, from the hyper-
47 1957 1993 The American Physical Society
1958 T.A.ARMSTRONG et al. 47
nuclei of fission fragments. This was achieved by using
the fact that fission fragments cause the emission of a
large number ofsecondary electrons when they leave the (a
target surface [5]. In contrast, when a slow heavy recoil
is knocked out of the target, the number of secondary etayed fission ofa
avy hypernucleus
electrons is small.
The data on hypernuclei of fission fragments can be
used in the analysis of the prompt fission of excited hy-
pernuclei. We can understand the appearance of hyper-
nuclei offission fragments by using rather simple assump- ~Target on
~backing
tions concerning the nonmesonic decay of a A hyperon. Shadowed region I
The possibility ofexplaining two diferent types ofevents r//// /// ////////////'/'/'AV/// ////// 1///'I
using the same assumption on the nonmesonic decay of
hypernuclei would be an additional argument in favor of
the hypernuclear nature ofthe observed eKect.
Furthermore, a detector was implemented to record (b)
K+ mesons in coincidence with fission fragments. The Decay ofhypernucleus
observation ofkaons in coincidence with events identified ,~offission fragment
/
as the prompt or delayed fission ofhypernuclei would be
a direct confirmation ofthis identification.
In Sec.Ithe experimental setup is described. In Sec.II
the experimental results are presented. In Sec. III we
present the results ofthe data analysis performed on the Fission fragment
assumption that the effect observed is related to the de- detectors
Shadowed region
cay of hypernuclei, and in Sec. IV the results obtained r//////////////////// 8///////'/'/'///////'///////////
are discussed.
II.EXPERIMENTAL SETUP FIG. 1. Schematic representation of the recoil-distance
method as applied to the study of(a) delayed fission ofheavy
A. Principle and general description hypernuclei, and (b) decay ofhypernuclei offission fragments.
Both types ofevents can provide hits in the shadowed region
(hatched) ofthe detectors while prompt fission cannot.
Figure 1(a) shows schematically how delayed-fission
fragments are recorded by the recoil-distance method. In
the case ofprompt fission which occurs inside the target,
the fission fragments cannot hit that part ofthe detector neglected. In a nonmesonic decay on a free nucleon the
which is shadowed by the target backing (the hatched nucleons produced would have momenta of 400 MeV/c.
part). The delayed fission occurs in vacuum, at a certain When this decay occurs inside the nucleus, the Fermi
distance from the target plane. This distance is deter- motion of nucleons, as well as the rescattering of the
mined by the velocity and lifetime of the recoiling nu- nucleons produced, should be taken into account. It is
cleus. In this case the fragments can hit the shadowed then plausible to expect that the distribution ofthe mo-
area of the detector. Thus, the signature of delayed fis- mentum transfer to the fragments is broad and extends
sion would be that a fission fragment is recorded in this to several hundreds of MeV/c. Consequently, although
part ofthe detector. the momentum transfer from the A decay is appreciably
In principle, the scheme shown in Fig. 1(a) provides lower than the momentum of a fission fragment (about
reliable suppression ofthe prompt-fission fragments at a 4000 MeV/c), it is still sufficiently high to deflect the
level of at least 10 6. However, the situation turns out initial trajectory ofthe fragment. As a result, some frag-
to be somewhat diferent when it is used in the study of ments could hit the shadowed area of the detector near
hypernuclei. There, in addition to hypernuclei of heavy the target plane [Fig. 1(b)].
elements, there are also hypernuclei of fission fragments To distinguish between the delayed fission ofheavy hy-
produced as the result of the prompt fission of excited pernuclei and the decay of hypernuclei of fission frag-
hypernuclei. Since the hypernuclei of fission fragments ments, we made use of the following fact: While the
are emitted from the target, they cannot hit the shad- delayed fission of heavy hypernuclei occurs in vacuum,
owed area ofthe detector. However this is true only ifwe the hypernuclei of prompt-fission fragments are emitted
ignore the decay ofthe A hyperon in a moving fragment. from the target. In the latter case some tens ofslow sec-
Because of the relatively high velocity of fission frag- ondary electrons are emitted as aresult ofthe interaction
ments (about 10io mm/sec) the mean distance the frag- ofthe fragments with the surface layer of the target. A
ments travel before the A hyperon decays is about 2 mm, slow recoiling nucleus leaves the target with appreciably
comparable to the size of the target. Since in the re- lower velocity than fission fragments and will thus pro-
gion of the fission-fragment masses the nonmesonic de- duce fewer secondary electrons. Therefore, by measuring
cay predominates, we may expect that in this decay the the number of low energy secondary electrons one can
fission fragment receives a momentum which cannot be distinguish between these types ofevents. This was done
47 FISSION OF HEAVY HYPERNUCLEI FORMED IN. .. 1959
using an electron detector placed in front of the target They were filled with isobutane at a pressure of 7 Torr,
and operating in coincidence with the fission-fragment and separated from the volume ofthe reaction chamber
detectors. Those fragments which were recorded in the by 2 p,m thick Mylar foils. The position resolution ofthe
shadowed area ofthe detectors and accompanied by the chambers along the beam direction and in the perpendic-
emission ofalarge number ofelectrons were attributed to ular direction was found to be 1.0 and 1.8 mm, respec-
the hypernuclei of fission fragments, and the rest of the tively. In addition, recording independently fast anode
shadow events attributed to delayed fission. The thresh- pulses, we obtained a timing signal needed for time-of-
old of the electron detector could be set to such a low fiight (TOF) measurements and the amplitude of anode
level that practically all delayed fission events were ac- pulses and thus the specific ionization of the detected
companied by a small but measurable signal from the particles. Combining these data for each event, it was
detector. possible to discriminate fission fragments from any back-
In Fig. 2we show the scheme ofthe whole experimental ground due to light particles from p annihilation.
setup. The antiproton beam passed successively through
the 0.1mm Bewindow ofthe beam tube, a 3x7mmz slit C. Target arrangement
in a scintillator used as an active collimator, and through
a variable Mylar degrader. It was further slowed down in The 2ssU target was deposited, by ion implantation,
the plastic scintillator which served both as target back- on a 0.2 mm thick plastic scintillator backing, as a spot
ing and beam detector. Some antiprotons were stopped 2 x 5 mmz of 0.2 mg/cmz thickness. The target surface
in the U target deposited on this scintillator, where they was fiat within 1pm. Backings with two different widths,
were captured to form antiprotonic atoms of uranium. 2.5 and 8.0 mm, were used. The scintillator provided the
The subsequent annihilation of antiprotons can induce zero-time signal required in the TOF measurement. An
many different processes. Some ofthese may lead to the additional adjustable degrader, placed before the scintil-
formation ofhypernuclei —ofresidual excited nuclei which lator, optimized the stopping rate of antiprotons in the
either underg—o prompt or after the emission ofneutrons uranium target.
and mesons delayed fission. In both cases the fission
fragments are detected in coincidence by a pair ofparal-
D. Secondary-electron detector
lel plate avalanche counters (PPAC's) placed perpendic-
ularly to the target plane at a distance of 2?0 mm. The
electron detector was positioned in front ofthe target at A microchannel plate electron multiplier (MCP) was
a distance of about 5 mm. The front face of the kaon used to detect secondary electrons produced in the in-
detector was at 45 cm from the target, behind one ofthe teraction of fission fragments and recoil nuclei with the
PPAC's subtending 16%ofthe full solid angle. target. The pulse height of the signal from the MCP
detector is proportional to the number ofsecondary elec-
trons. The mean value of this number for fission frag-
B.Fission-fragment detectors ments is approximately 180, and an order of magnitude
lower for recoiling hypernuclei. The electrons were accel-
The two PPAC's, 190x290mmz each, consisted oftwo erated and focused by an electric field of5kv between the
planes of horizontal and vertical wires placed between target and the entrance ofthe Mcp detector. Microchan-
two outer cathode planes and one central anode plane. nel detectors demand a vacuum of 10 Torr to perform
in a stable way. The leakage of isobutane through the
windows of the fission counters did not permit such a
vacuum in the whole volume ofthe reaction chamber. It
lA'1////1//XX/rVr'1//1 could be sustained locally around the target and MCP
I
by an additional cryogenic pump. Nevertheless, in the
PPAC1
course of the experiment, the MCP detector had to be
Scintillator with ahole i Scintillator backing replaced to ensure a proper response.
Target
~L l/ E.K+ meson detector
P
In the present experiment a kaon range telescope
/ (KRT) was also used. A detailed description ofthe KRT
// Electron is given in Ref. [6]. It is a rectangular parallelepiped
Variable detector 100 x 100 x 50 cm, composed of 100 cells constructed
degrader
~ PPAC2 with alternating plates ofcopper and acrylic scintillator.
It provides coarse tracking in three dimensions as well as
I1/ZX/&VX/1/ZYXXX/Xl I energy loss and timing information. Both the scintillator
and the copper serve as moderators, contributing roughly
equally to the energy loss. The stopped K+ mesons de-
FIG. 2. Scheme ofthe experimental setup (not to scale). cay in the KRT (K+~p+v or z.+pro) with a mean life
PPAC is the parallel plate avalanche counter; KRT is the of 12 nsec. The signals from the cells are identified in
Kaon range telescope. two groups, the first one corresponding to the stopped
1960 T.A. ARMSTRONG et al. 47
K meson and the second one to the pion or muon from located in the same region as the one obtained in the
its decay.(The(momentum range of the detected kaons 5 Cf calibration. The shadow events, however, are di-
was 250 P~ 750 MeV/c, the upper limit being deter- vided into two groups. One of them is centered around
mined by the thickness of the KRT, and the lower limit channel 1000and the other group is located below chan-
by attenuation in material between the target and the nel 250. We attribute the first group to prompt fission of
KRT. excited hypernuclei, i.e., to those cases when one of the
fragments, as a result ofthe A-hyperon decay, is defiected
III.EXPERIMENTAL RESULTS into the shadowed area of the PPAC. The second group
is assumed to be due to fission events resulting from the
The experiment was carried out at the CERN Low decay ofheavy hypernuclei. It was found in the analysis
Energy Antiproton Ring, LEAR, using a p beam with that practically all shadow events were accompanied by
105 MeV/c momentum. A total of 2.5 x 10io and asignal from the MCP detector. This is taken as an indi-
1.6x 10 antiprotons were delivered on the 2 U targets cation that the electrons were detected with an efFiciency
deposited on 2.5 and 8 mm wide scintillators, respec-
tively. As a result, 1128 "shadow events" with one frag-
ment detected in the shadowed zone ofthe PPAC's and
the complementary fragment in the nonshadowed area 1200
were recorded in the first case and 350in the second case.
It should be noted that, compared to the previous mea- C
surements [1,2], there was an appreciable number (139) c- 1000
ofdouble-hit events, with two fission fragments detected
in the shadowed area. Only about 1%ofthe antiprotons X
800
are stopped in the target material. With the present ge-
ometry ofthe experiment (Fig.2), asignificant fraction of LLJ
the antiprotons are stopped in the MCP detector holder 600
whose front end is viewed by the shadowed area of the
PPAC's. The double-hit events may arise from antipro-
ton induced prompt fission ofimpurities on this front end. 400
This assumption agrees with the result of an additional
experiment with a 2.5 mm wide scintillator without tar-
get material. Here, four double-hit events were registered 200
with 2 x 10 incoming antiprotons.
A data-acquisition system based on CAMAC operat- »
ing under a NORD computer control was used to record 0 i i10I0i i I I20I0t i t t30I0 r i40I0i i i i50I0
the data. A registered event consisted of the space-time TOF (channel )
coordinates ofthe incident hits in the PPAC's and KRT, ¹
the amplitude of the corresponding analogue pulses, as 1200
well as the amplitude and timing signals from the MCP
detector. The MCP and PPAC's were calibrated with a
Cfsource in 47r geometry. C&: 1000 (b)
Figure 3 shows the two-dimensional distributions
dE/dx vs TOF obtained from the PPAC's with the U
800
target (deposited on the 2.5 mm wide scintillator) irra-
diated with antiprotons. Here (a) corresponds to events LLI
with both fragments registered in the nonshadowed area
600
ofthe PPAC's (prompt fission) and (b)to events with one
fragment detected in the shadowed part. The results for
both types of events are consistent with the calibration 400
data from the fission fragments of Cf. In Fig. 4 the
corresponding pulse-height distributions obtained from
the MCP detector are presented. Figure 5 shows the 200
two-dimensional TOF spectrum of fission fragments for
(a) prompt fission and (b) shadow events. Both cases » «s
correspond to symmetric fission. Fig. 5(a) shows that 10I0I i I 20I0& i 30I0 s s40I0 &50I0
even for prompt fission, where the counting rate was rel- TOF (channel )
atively high, the contribution from random coincidences ¹
was negligible. dE/dx vs TOF (time of flight between target
A comparison of the pulse-height distributions of sig- backing scintillator and PPAC) measured with the PPAC's
nals from the MCP detector, for prompt fission [Fig.4(a)] for two types ofevents: (a) both fragments in coincidence are
and for the shadow events [Fig. 4(b)], shows that for in the nonshadowed area, and (b) one of the fragments is in
prompt fission there is one broad peak. This peak is the shadowed area.
47 FISSION OFHEAVY HYPERNUCLEI FORMED IN. .. 1961
close to 100%. spectively (Fig. I). This is the coordinate for which the
The main conclusions drawn from the experimental dependence of the shadow effect upon the characteris-
distributions in Figs. 3—5 are the following: (i) there are tics of the nonmesonic decay (lifetime and momentum
two types of shadow events, one due to the hypernuclei transfer) is seen in the most transparent way. In fact,
of fission fragments and the other to delayed fission of the coordinate distributions along the axis parallel to the
heavy hypernuclei and (ii) the fission is symmetric both target plane should depend only weakly on the lifetime of
for prompt and delayed events. hypernuclei and the momentum transfer in the A decay.
In the following we perform an analysis of our re- In Fig. 6 we show the experimental X coordinate dis-
sults based on these conclusions. The experimental co- tributions of shadow events accompanied by a small or
ordinate distributions for fission fragments are compared large number of secondary electrons. We attribute these
with those obtained in a Monte Carlo simulation to be two distributions to the delayed fission ofheavy hypernu-
described later. In order to simplify the discussion, we clei and to hypernuclei offission fragments, respectively.
confine ourselves to the distributions in the coordinate They are clearly different both in the shadowed and non-
along the axis, here denoted by X,which is directed per- shadowed areas ofthe detector. The events correspond-
pendicular to the target plane with positive and negative ing to hypernuclei offission fragments are localized closer
values ofXdownstream and upstream ofthe target, re-
500
M
C
OO 400 I."'rW
',I~I~
10 iI
I
(D
C3
E O 300
.IC
102 .r'
4g
200
10
100
0 0%'''I ''I~ 'I' .I1-0'I..0"I ~I 'II I20I0I~ I. I' I30I0I I I I40I0I I I I500
0I I2I5I0I I I5I0I0I I I7I5I0I I I1I0I0I0I I1I2I5I0I I1I5I0I0I I1I7I5I0I I2I0I00 TOF1 (channel 8 )
channel
¹ 500
0
O & 400
0
~ IO'-I C4
O
E 300
200
100
0 I I I I10I0I I ~ I20I0I I I I30I0I I I I40I0I I I I500
0 250 I50I0I I I7I5I0I I I1I0I0I0I I1I2I5I0I I1I500 1750 2000 TOF1 (channel 0 )
channel
¹
FIG. 5. Time offlight ofone fragment vs time ofBight of
FIG.4. Amplitude spectrum given by the MCP for events the other for events with (a) both fragments in coincidence
with (a) both fragments in coincidence in the nonshadowed in the nonshadowed area, and (b) one ofthe fragments in the
area and (b) one ofthe fragments in the shadowed area. shadowed area.
1962 T.A.ARMSTRONG etal. 47
~
200 M
C:
0
175 O
0 0
~ 10
150 (D
C3
E E
~
125—
100 I
II 103
I
75 I
I
I
I
I
I
I
I
I
I I
25 I I I 102
0 7I5~w0—'Tl5iw0LI &—IIj2I5 0I »III & s2I5 50 75 I I1I0II0I I 0I I I I2I0I I I I4I0I I I i6I0I I I I8I0I I I I10I0I IILI I I120
— —
X position (mm) X position (mm)
FIG. 7. X-position distributions of the prompt-fission
FIG. 6. X-position distributions of coincident fragments fragments in the PPAC's (solid line); the dashed line was
in the PPAC's for events with one fragment in the shadowed obtained with the Monte Carlo simulation.
area. The solid and dashed lines represent the events accom-
panied by a low- and high-amplitude MCP signal, respec-
tively.
shows the X-position distribution of the prompt-fission
events measured in the experiment. The rather strong
decrease in the distribution near the target plane reQects
the absorption in the target and the PPAC windows. The
to the target plane, and their coordinate distribution in slope ofthe distribution at large distances from the target
the shadowed area is steeper.
During the experiment K+ mesons were registered in plane is due to the decreasing contribution of high mo-
coincidence with fission fragments. Thus, 436K+ mesons museinngtuma greeocmoieltsr.icaTlhedseeteecftfieocnts wefeBrecietnackyendeintetromaincecdounftrobmy
were observed in coincidence with 1.25 x 10s prompt- the X distribution measured for single-fission fragments.
fission events. No coincidences were observed with the
In the same phenomenological approach as used previ-
shadow events.
ously [1,2], the isotropic momentum distribution of fis-
sioning nuclei was approximated by the function
IV. DATA ANAI.YSIS
~(p) =P"exp(-p/po),
A. Prompt fission
where po and n are free parameters. In fact, there is
The sample of events with two fragments detected in a third nonexplicit parameter involved, namely a cutoff,
coincidence in the nonshadowed area of the PPAC's is p a„,for acceptable values of the momentum of the fis-
mainly due to prompt fission induced by stopped an- sioning nucleus. The dashed line in Fig. 7 is the result
tiprotons. It also contains a small admixture ( 10 s) of=the simula=tion obtained witah„t=he parameter values of
of events from prompt and delayed fission of hypernu- n 2, po 400 MeV/c, p 2000 MeV/c. Using
clei which can be neglected at this stage of analysis. the number offission fragments recorded, their detection
The prompt fission is a result of antiproton annihilation efficiency in the PPAC's (see Sec. IVD), and the cor-
through many channels, in which the residual nucleus is responding number of p stopped in the U target, it is
excited to an energy above the fission barrier. Here, the possible to calculate the probability of prompt fission of
antiproton induced fission will not be analyzed in detail; 2ssU induced by stopped antiprotons. This probability
it has been studied independently [7]showing a symmet- was found to be (85+15)%which agrees with the esti-
ric mass distribution of fission fragments which is cen- mate of [8].
tered at M = 106 u with a dispersion cr(M) = 22 u.
Only those characteristics which help to better describe B.Hypernuclei of fission fragments
the production of hypernuclei will be investigated here.
In the case of prompt fission both fragments hit the The analysis of the hypernuclei of fission fragments,
PPAC's in the nonshadowed zone. The X position dis- i.e., of shadow events accompanied by the emission of
tribution is in this case determined by the momentum a large number of secondary electrons, is based on the
distribution of excited nuclei produced in the annihila- model discussed in Sec.IIA.
tion, the geometry of the PPAC's, and the energy loss As a first stage ofthe process we consider the antipro-
of the fragments in the target. In Fig. 7 the solid line ton annihilation which leads to the production of an ex-
47 FISSION OF HEAVY HYPERNUCLEI FORMED IN. .. 1963
cited hypernucleus. The momentum distribution ofthese region.
hypernuclei is assumed to be isotropic and given by the While the data on the TOF (velocity) of fragments in
expression in Eq. (1). This is a purely phenomenologi- the shadow events provide information on the A-hyperon
cal approach, in which we neglect details of the various attachment to fission fragments, the X distribution can
processes governing the momentum distribution of the be used to obtain information on the momentum trans-
hypernuclei. fer in the nonmesonic A-hyperon decay. In the analysis
The second step ofthis process is the prompt fission of of this distribution we assume that the probability of
the excited hypernucleus. One ofthe fragments produced the A-hyperon attachment is proportional to the frag-
carries t~he ANhy+per¹on which undergoes nonmesonic decay ment mass. The calculation ofthe deflection ofthe frag-
A+N The lifetime for this decay is assumed ments to the shadowed area involves only the momenta
to be 2.5 x 10 o sec, i.e., close to the free A lifetime . ofthe fragments, which in binary fission are the same for
When an excited hypernucleus undergoes fission, the heavy and light fragments. Therefore, the results of the
A hyperon sticks to one of the fission fragments. The calculation of the momentum transfer in the A-hyperon
probability of the A sticking can be a function of the decay depends very little on the assumption about the
fission-fragment mass.
It is clear that the A particle must have been initially
attached to the fragment detected in the shadowed re-
gion. The mass of this fragment relative to that of the
original fissioning hypernucleus can be determined by
means of the double velocity technique. This technique U
0 8—
is based upon the fact that nucleon evaporation from the 0. P
CL I
fragments after fission has forward-backward symmetry I
in the fragment frame, and, therefore, does not change (U
E
the fragment velocity on the average. If1 and 2 label the 0.6
fission fragments, momentum conservation at the time of
fission gives the following relation between masses and
velocities:
0.4
Ml/(Ml +M2) —I2/(~1 +I2) (2)
This relation added to the conservation of velocities on
the average provides the ratio Ri = Mi/(Mi +M2) from 0.2
the measurement of the velocities of the two fragments.
Here it is sufBcient to have the velocities in relative units
A(sepearatliscoleSteoc.aIfVraCgm).enTtheofamttaacshsmMenit, ApAro(Rbaib),ilictyan obef tehxe- 0 0 I I I I0.2 s I i0.I4i & i i0.I6 0.8I I I I 1
mass ratio R,
pressed in terms ofthe counting rate N~A(Ri) offission
fragments observed in the shadowed region in coincidence
with a complementary fragment in the nonshadowed re-
gion, with a high MCP signal, and a corresponding mass U 0.6
ratio Ri (see the appendix). 0
Figure 8(a) gives the variation ofthe experimental ra- CL
tio N~p(Ri)/[N~A(Ri) +N~~(R2)j (dashed line) as a c 0.5
function of Ri, and of the same ratio corrected for the QE)
fragment mass resolution and for the difFerence in detec- 0.4—
tion efficiency between light and heavy fragments (solid O
line). The last one shows the variation ofthe attachment
probability of the A particle to the fragment of mass 0.3
Mi. A clear positive correlation is observed implying
that high mass fragments are detected in a greater num-
ber in the shadowed region than are low mass fragments.
The correlation observed in Fig. 8(a) for the corrected
0.
histogram can be described by the relation 1
»»
AA(R) = 0.5+ 1.7(R—o.5) .
0 0 I I I I0.2 0.I4 0.I6 0.8I I I
Figure 8(b) is the same as Fig. 8(a) but for delayed hy- mass ratio R,
pernucleus fission (low MCP amplitudes). No apparent FIG. 8. Counting ratio Npp(Ri)/[N~A(Ri) +Npg(R2)]
correlation between the two ratios is found. This was vs the ratio Rj = Mi/(Mi + M2) for the shadow events
expected since, in the case of the decay of heavy hyper- (dashed line). The solid line represents this ratio after ef-
nuclei outside.the target, no preference should exist for a ficiency correction for (a) events with ahigh-amplitude MCP
light or a heavy fragment to be detected in the shadowed signal, and (b) events with a low-amplitude MCP signal.
1964 T.A. ARMSTRONG et aI.
A-hyperon sticking to fragments. The mass distribution involved or when the energy released in the nonmesonic
of fission fragments quoted in the previous section was decay is carried away by an alpha particle or a heavier
used. We neglect details of the interaction of the two fragment.
nucleons produced in the nonmesonic decay, and assume In Figs. 9(a) and 9(b) the solid lines show the exper-
that the distribution ofthe momentum transfer to the imental distribution of the X coordinate of both frag-
A:
fragment is described by the function ments and of the quantity (X,h + A„,h), respectively,
where X,h and X„,h are the X coordinates in the shad-
N(k) =k/(1+ exp[(k —kp)/Ak]), (4) owed and nonshadowed areas of the PPAC's. The lat-
ter distribution is related to the projection of the recoil
where ko and b.k are free parameters. The low momen- momentum on the X axis. The dashed histograms in
tum part of the distribution corresponds to those cases Figs. 9(a) and 9(b) are best fits from the Monte Carlo
when both nucleons either transfer all their energy to calculations corresponding to ko = 350 MeV/c and to
the residual nucleus or are emitted without interacting Ak = 150 MeV/c. The po was found to be equal to
with the residual nucleus. The high momentum part of 450 MeV/c. The cutoff values for the momentum trans-
the distribution includes those events when the high mo- fer in the p annihilation and in the nonmesonic hyperon
mentum tail of the nucleon momentum distribution is decay were 2000 and 1000 MeV/c, respectively. Lower
cutoK values do not give a good description ofthe exper-
imental data. It was also checked that these results show
~160 a very weak dependence on the lifetime ofhypernuclei of
fission fragments.
140
(p
0 C. Delayed Bssion
120
a)
z 100 The aim ofthe analysis ofthe data on delayed fission
was to estimate the lifetime of hypernuclei undergoing
80 nonmesonic A-hyperon decay. Here we used the fact that
only the position distribution in the X coordinate, per-
60 pendicular to the target plane, is sensitive to the lifetime.
The analysis of this distribution for delayed fission was
40 carried out assuming that it is entirely determined by
the lifetime of hypernuclei and their momentum distri-
bution. Consequently, in the Monte Carlo simulation we
20
generated only the projection ofthe momentum perpen-
I dicular to the target plane. In so doing we employ a
0—I2&0s s —80 —40 0 40 80 120 phenomenological approach, ignoring all details related
1 X—position (mm) to the production and scattering of hypernuclei in the
target.
The Monte Carlo simulation of the delayed fission of
c heavy hypernuclei showed that in a broad range of life-
50
(D (b) times the X distribution in the shadow region remains
Q3 steep at any distance from the target plane. The flat
~ 40- part of the experimental distribution in the shadowed
(D area (see Fig. 6) may include the above-discussed back-
E ground events due to the prompt fission of impurities on
50— the MCP-detector holder (see Sec. II).This background
could be excluded in the analysis on the basis ofthe ar-
guments discussed in the following.
20—
In the case of delayed fission of hypernuclei, coinci-
dences of fragments are only recorded when the projec-
tion ofthe nuclear recoil on the Xaxis ispositive, i.e.,the
I momentum is directed downstream ofthe target. On the
other hand, fragments resulting from the prompt fission
0 —75 -50 —25 0 25 50 75 100 of impurities on the MCP are detected (in coincidence)
X„+ .. only if the momentum transferred to the fissioning nu-
X. (mm)
cleus is directed upstream ofthe target. As a result the
quantity (Ã,h +A„,h) should be negative for the back-
FIG. 9. Distributions for the fragments from prompt hy- ground while it is mainly positive for good events. It can
pernucleus fission, (a) A position in the PPAC's, and (b) be negative, sometimes, due to the kinematics of the A
(2C,i,+A„,h). The solid line corresponds to the experimental decay. However, the fraction of these events, as follows
data and the dashed line is the result ofthe simulation. from a Monte Carlo simulation is small.
47 FISSION OF HEAVY HYPERNUCI. EIFORMED IN. .. 1965
In Fig. 10(a) we show the measured X distribution of In Fig. 10(a) the dashed line shows the X distribution)
the shadow events with low-amplitude signals from the calculated with the imposed condition (X,h +X„,h)
MCP detector)(solid line) constructed with the condition 0. The distribution (X,h +X„eh) calculated with these)
(Xeh +X»h) 0 imposed. We can see that after impos- parameters reproduces well the data with (Xeh+X»h)
ing this cut the distribution becomes steeper than the o IFig»(b)].
initial one (Fig. 6). In Fig. 10(b) we show the measured To check the result on the hypernucleus lifetime, an-
(X,t, +X„,h) distribution. other procedure ofcalculation was used. This procedure
In our Monte Carlo calculation we assumed that the was applied both to the present data for uranium and to
momentum distribution ofthe recoils has the form given the previously registered data for bismuth [2]. The range
in Eq. (1).The cutofF parameter, p „,was 1200MeV/c. of a heavy nucleus can be written as
The analysis has shown that for lower values ofthis cut-
=
off, we cannot satisfactorily describe the experimental ~ Vrecoil & & (5)
y
position distribution.
The parameter values which correspond to the best where V,ecc,i is the recoil velocity, and v the lifetime.
description of the experimental X distribution are ~ = This does not require any knowledge of the recoil mo-
(1.25+0.15)x10 0 sec a=nd po = (350+50)MeV/c. This mlifeentitmume ifdiostnreibuktnioonws atnhde mgievaens trheecoiml evaenlocviatylueanodf tthhee
value ofthe lifetime (y2 15.7for 13degrees offreedom)
mean range. The recoil velocity and the lifetime are un-
was found to be very little dependent on the value ofpo. correlated. Projecting on the beam axis X one obtains,
for the mean lifetime,
(rx)
c:175 ( ) ((v
(o)
o150 The experimental position distribution in the shadowed
region is well reproduced if the range distribution rx is
E125 taken as an exponential.
The mean recoil velocity is given by
100
(vrecoii) = (vI +vg ) (7)
2
75
where V& and V2 are the velocities ofthe two Gssion
fragments in the laboratory system. The projection of
the recoil velocity on the beam axis X is
25 ...
II I 'I ((V, ;,)x) = —(IV' cose, +IV' cose,)
0—120 -80 -40 0 40 80 120 2 I I
X—Position (min)
where Hq 2 are the angles between the fragment trajecto-
c
80
(D
(b)
70—
0 C
60— 0
O
0
50 I- 10—
(D
20—
10
L- ~
II
—75 —50 —25 0 25 5i0 i 75 10I 0s
X„+
X.„(mm)
—3 —2 —1 0 1 2 3 4 5
FIG. 10. Distributions for the fragments from delayed hy- (V„„.)„(men, ns)
pernuclear fission, (a) X)position in the PPAC's ofthe frag- ,,
ments with (Xsh+Ä,i,) 0,and (b) (A;h+Ä,i,).The solid
line corresponds to the experimental data and the dashed line FIG. 11. Distribution of the X component of the recoil
is the result ofthe simulation. velocity as deduced from the measured fragment velocities.
1966 T.A.ARMSTRONG et aI. 47
TABLE I. Results obtained using the "mean" method both for the U (present data) and Bi
(1986data) targets. The systematical errors are given within parentheses.
Target (X) ((+recoil)Ã) (~)
(mm) mm ns nsec
Bismuth P 14+—00..002205 0.79+ o.os (+o.2s) 0.18+0.04 (+ 0.06)
Uranium P.125+—0.015 0.95+ 0.07 (+0.23) 0.13+0.03 (+0.03)
ries and the beam axis. The measured velocity spectra the number ofevents measured. This confirms our inter-
of fission fragments were calibrated by using the mean pretation ofthe observed effects.
velocity of fragments in the laboratory system, 1.2+0.1
cm/nsec [9].Figure ll shows the calculated distribution D. Yields
ofthe recoil velocity projection. Table I summarizes the
mean values ofthe range, recoil velocity, and correspond- In order to estimate the yield ofthe processes studied
ing lifetimes for hypernuclei in the regions ofBi and U. here, it is necessary to know the efficiencies of detection
We can see that the lifetime for U quoted in Table I for each process, and the number of antiprotons stopped
agrees well with that obtained in the Monte Carlo simu- in the target. The latter can be deduced from the frac-
lation. The lifetimes ofBiand U hypernuclei are smaller tion of the beam impinging on the target and from the
than the free A lifetime. In Fig. 12we display the results stopping power ofthe target material.
for Bi and U together with the lifetimes of the free A The detection efficiency of single fragments from
hyperon and of& B and &C [10]. prompt fission was obtained in a Monte Carlo simulation
The position distribution of the shadow events de- which took into account the solid angle of the PPAC's.
tected with the target on a wide backing was measured and the measured decrease of the counting rate for the
without separating heavy hypernuclei from hypernuclei fragments emitted at a small angle with respect to the
of fission fragments. The reason was that the MCP de- target plane. This decrease is due to absorption and mul-
tector did not function well in this final stage ofthe ex- tiple scattering of the fragments in the target. The de-
perirnent. The number of events of each type was esti- tection eKciency, including solid angle, is 0.05 for each
mated with a Monte Carlo calculation using the param- PPAC. It decreases to (3.4+0.4) x 10 when the frag-
eters obtained with the data from the narrow backing ments are measured in coincidences.
target. Their sum agrees within the statistical error with The detection efficiency of hypernuclei of fission frag-
rnents is determined by the same factors as that for
prompt fission but is strongly reduced (20times). This is
due to the fact that only asmall fraction ofthe fragments
are deviated to the shadowed area of the PPAC's, as a
2.75 I I I I I I I I I I I I I I I I I I I result of A decay. It was found to be (1.7+0.2) x 10
Free A lifetime
The detection eFiciency ofdelayed-fission fragments is
2.50
essentially determined by the recoil-distance distribution
ofhypernuclei and was obtained from a Monte Carlo sim-
2.25— ulation equal to (4.2+0.4) x 10 s. It has to be corrected
C for the absorption of recoil nuclei in the target. This
CtD3 2.00 —) effect was estimated from a comparison of the yields of
Ch
C) hypernuclei for two targets, 100 and 200 pg/cm thick.
1.75— 2098 )) We found that about 50%ofthe recoils are stopped in the
C: 200 pg/cm thick target. Consequently the detection ef-
1.50— ficiency ofheavy hypernuclei produced in this target was
(2.1+0.7)x10
238 Table II gives the number of registered events N, pt,
1.25 U the detection eFiciencies e, the corresponding number of
00— events produced in the target, Nt, ,~,t,, and the calculated
1. yields Y' per stopped antiproton of the three processes:
prompt fission (PF) and the delayed and prompt fission
075 (» « II » » of hypernuclei (DHF) and (PHF). The accuracy of the
I I I I I I I I I I I I I I I I
0 40 80 120 160 120 240 calculation of the yields of prompt and delayed hyper-
Hypernucleus mass (u) nucleus fission is mainly determined by the error in the
evaluation ofthe number ofantiprotons passing through
FIG. 12. Lifetime of hypernuclei as a function of their the target ( 15%).In the case ofheavy hypernuclei the
'
mass. ( C and Bdata are from Ref. [10], Bl and U main uncertainty ( 30%) comes from the estimate of
are from this work.) Only statistical errors are given. the number of recoil hypernuclei absorbed in the target.