Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 2, pp. 69−74, 2000

69

Recoil Properties of Radionuclides Formed in Photonuclear Reactions
on 197Au at Intermediate Energies
H. Haba,*,a M. Igarashi,b K. Washiyama,a H. Matsumura,a M. Yamashita,aK. Sakamoto,†,a,b Y. Oura,c S. Shibata,d M. Furukawa,e and I. FujiwarafaDivision of Physical Sciences, Graduate School of Natural Science and Technology, Kanazawa University,
Kanazawa-shi, Ishikawa 920-1192, Japanb
Department of Chemistry, Faculty of Science, Kanazawa University, Kanazawa-shi, Ishikawa 920-1192, Japanc
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji-shi, Tokyo
192-0397, Japand
Research Reactor Institute, Kyoto University, Sennan-gun, Osaka 590-0494, Japane
Faculty of Environmental and Information Sciences, Yokkaichi University, Yokkaichi-shi, Mie 512-8512, Japanf
Department of Economics, Faculty of Economics, Otemon-Gakuin University, Ibaraki-shi, Osaka 567-8502,
Japan
Received: April 4, 2000; In Final Form: September 18, 2000
The recoil properties of 37 radionuclides with the mass number A = 24–131 produced in the photonuclear reaction
on 197Au by bremsstrahlung of end-point energies (E0) from 300 to 1100 MeV have been investigated using the
thick-target thick-catcher method with an aid of intensive chemical separations. The measured mean ranges, FW
and BW, in the forward and backward directions, respectively, with respect to the beam axis show E0-independence above 600 MeV, reflecting the resonance character in photonuclear reactions at intermediate energies. The
forward-to-backward yield ratios (F/B) are independent of the mass difference (∆A) between a product (A) and thetarget (At = 197) above ∆A = 66, and the weighted mean value of F/B = 1.1 ± 0.1 is clearly distinguished from thatof photospallation products with ∆A ≤ 45. The F/B ratios of proton-induced reactions at incident proton energies(Ep) of Ep ≤ 3 GeV are higher than those of photon-reactions and decrease with an increase of Ep, but become thesame values as the photon-reactions at higher Ep. The mean kinetic energies T of the residual nuclei were deduced
based on the two-step vector velocity model. The T values are independent of E0 above 600 MeV and increase
with an increase of ∆A, reflecting the changes in the reaction mechanism from spallation to fission. The T valuesof photon-reactions at ∆A > 45 are quite different from those of the proton-reactions, in which the T values
decrease with an increase of Ep up to about 3 GeV and become Ep-independent at higher Ep, though the T values ofphoton-reactions at ∆A ≤ 45 agree well with those of the proton-reactions, irrespectively of Ep. Recoils of 24Na
produced from 27Al, natV, natCu, natAg, and 197Au are of special interest from a viewpoint of a change in the production mechanism with respect to At.

1. Introduction
Photons are the force carriers of the electromagnetic interaction. The electromagnetic force can act on the scale of thewhole nucleus but also on the smaller scale of nucleons, and so
this interaction is very useful for probing both global and local
properties of the atomic nucleus. At intermediate energies,
photons interact with the nucleus through three mainprocesses; giant dipole resonance, quasi-deuteron mechanism,and (3,3) resonance, which are characteristic for the wavelength of incident photons. These initial interactions are quite
different from that of hadron-induced reactions initiated by thestrong nucleon-nucleon interaction. It is of interest to investigate whether there exist similarities or not in the final steps inthese two types of nuclear reactions.
We have been measuring the yields of bremsstrahlunginduced reactions such as photospallation,1–4 photofragmentation,5,6 photofission,7 and photopion reaction8–11 from 7Li to209
Bi targets in order to establish the systematics of photonuclear reactions and to investigate the reaction mechanism andnuclear structure. Fruitful information about these aspects has
been obtained from yield measurements so far. Further experiments using a nuclear recoil technique are expected to give us*Present address: Japan Atomic Energy Research Institute, Tokaimura, Naka-gun, Ibaraki 319-1195, Japan. E-mail: haba@popsvr.
tokai.jaeri.go.jp.
†Corresponding author. E-mail: kohsakamoto@par.odn.ne.jp.
FAX: +81-76-224-8253.

information such as angular distributions and kinetic energies
of product nuclei, and to deepen our understanding on the reaction mechanism.
Recently, recoil properties of radionuclides formed in thephotospallation reactions on 27Al, natV, natCu, 93Nb, natAg, natTa,
and 197Au at bremsstrahlung of end-point energies (E0) from
600 to 1100 MeV were successfully investigated using the
thick-target thick-catcher method.12–14 Reflecting the resonance character in a photonuclear reaction, the mean ranges
FW and BW in the forward and backward directions withrespect to beam axis are E0-independent at the studied energies
and they increase with an increase of the mass difference (∆A)
between a product (A) and the target (At), showing two components of the (γ, xnyp) (x, y ≥ 1) and the (γ, xn) (x ≥ 1) reactions.
The forward-to-backward ratios (F/B) are independent of ∆A
as well as the targets (F/B = 2–3). The kinematic properties ofthe product nuclei were analyzed by the two-step vector
velocity model.15 The forward velocity v after the first step ofphoton-reaction is quite different from that of proton-reaction
at incident proton energies of Ep ≤ 3 GeV, though the difference disappears at higher energies. On the other hand, themean kinetic energy T of the residual nucleus in the second
step is almost equal to that of proton-reaction irrespective of
Ep. A comparison with T values calculated by the photoninduced intranuclear cascade analysis code (PICA) by Gabrieland Alsmiller16,17 was also performed at E0 = 400 MeV. It was
found that although the code well reproduces the experimental
results of nat V and nat Cu, the same calculation for heavier© 2000 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2000

Haba

70 J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000
targets gives T values lower than the experimental results, indicating some nuclear-structure effect, such as a medium effectnotably at At ≥ 100. An average kinetic energy carried off by
the emitted particles εs = T/(∆A/At) of both photon- and protonreactions seem to increase with an increase of At up to aroundAt = 100, and become almost constant at larger At, implying
also some change in the nuclear structure effect in this heavy
target region, as also found in our recent yield measurementsof photospallation3 and photopion reactions.11 The εs values ofphoton-reaction appear to be slightly lower than those of
proton-reactions for natAg, natTa, and 197Au, indicating the lower
excitation energy left after the first step in photon-reactions
than in proton-reactions.
It was found from our systematic yield measurements thatthe target nucleus of 197Au undergoes spallation,2,3 fission,7 andfragmentation,6 which are dominant for the products with ∆A <~60, ∆ A = ~60–~160, and ∆ A ~160, respectively. In thepresent work, the recoil properties of 37 radionuclides with ∆A= 66–173 have been obtained at a wide energy range of E0 =300–1100 MeV using the thick-target thick-catcher methodwith an aid of chemical separations in order to investigate
photofission and photofragmentation processes dynamically.
The present results together with those of the photospallation14
are discussed as a whole by referring to the literature values on
the proton-induced reaction, and to the calculated ones by the
PICA code.16,17 Recently, the yields of light nuclei such as
7,10Be, 22,24Na and 28Mg from 23 targets of natB to 197Au were
measured,6 and it was found that their At-dependence shows
two components of photospallation and photofragmentation.
In the present paper, recoils of 24Na produced from 27Al, natV,
nat
Cu, natAg, and 197Au are of special interest from a viewpointof a change in the production mechanism with respect to At.
The present recoil technique accompanied by the chemical
separations was powerful to determine low fission yields of197
Au and to investigate detailed fission properties such as
charge and mass distributions. The absolute yields of the 58
products in the mass range of 42 ≤ A ≤ 131 were reported in
the separate paper.7
2. Experimental
Irradiations by electron-free bremsstrahlung beams with E0= 300–1100 MeV in steps of 100 MeV or less were carried out
using the 1.3 GeV electron synchrotron of the High Energy
Accelerator Research Organization (KEK) at Tanashi. A 50

µm thick Pt foil was used as a radiator for bremsstrahlung
production, and the beam was collimated to 20 mm in diameterat the irradiation position. The target consisted of a stack of
40–50 sets of a Au foil (99.99%) of 90 mg/cm2 in thickness
and 2.5 × 2.5 cm2 in size sandwiched by one pair of Mylar foilof 7.0 mg/cm2 in thickness and 2.5 × 2.5 cm2 in size, whichcollected the recoil nuclei in the forward or backward directions with respect to the beam axis. The stack was vacuumencapsulated in a polyethylene bag together with additional
40–50 Mylar foils (280–350 mg/cm2) on the downstream sideof the beam, which served as an activation blank. Details of
the irradiation were almost the same as described in theprevious papers.1,8,9 The photon intensities evaluated from themonitor reaction of 27 Al(γ,2pn) 24 Na (Refs. 18, 19) were109–1010 equivalent quanta per second (eq.q./s). A typical irradiation period was 5 h.
After irradiation, about 10 selected Au foils and all of theforward and backward catcher foils from one target pile werecollected separately and were assayed for radioactivities
nondestructively with high-purity Ge detectors each coupledwith a 4 k-channel pulse height analyzer. The other 30–40 Au
foils were divided into three portions and subjected to thechemical separations of K, Fe, Ni, Zn, Ga, As, Rb, Sr, Y, Zr,
Nb, Mo, Ag, and Ba. All of the chemical procedures appliedin the present work were described in a separate paper.20
Radioactivities in the chemically separated samples wereassayed by γ-ray spectrometry. In order to extend the recoil
data of products with low reaction yields, chemical separationsof scandium and barium were employed for the Mylar catchers
in a separate irradiation at E0 = 1000 MeV. The identificationsof radionuclides were made on the basis of their γ-ray energies
and half-lives. The γ-ray intensities were obtained with theautomatic peak search program, “SPECanal98” by Hamajima,21
which is based on the original one by Komura.22 The detector
efficiency was determined with calibrated 152 Eu and 182 Ta
sources with the same size as the samples. The decay properties of interest such as half-lives, γ-ray energies, and branchingratios23–25 are summarized in Table 1, together with formation
types; I: independent yield, C–: cumulative yield by β– decay,
and C+: cumulative yield by β+ decay and/or electron capture.
This classification was based on charge distributions of the
product nuclei,7 irradiation periods, and times required forchemical separations. The contributions of the blank activities
to the catcher foil were found to be negligible for all of theradionuclides of interest here.

TABLE 1: Relevant Nuclear Data
*3

NuclideHalf life

E γ (keV)b.r.(%) TYPE

Detec.

Ba-131
Ba-129m
Ba-129Ba-128
Ba-126
Ag-113Ag-112
Ag-110m
Ag-106m
Ag-105
Rh-105
Ru-105
Ag-104Ag-103
Ru-103
Mo-99
Nb-98b
Ru-97
Nb-97
Zr-97

11.50d
2.17h
2.23h
2.43d
1.67h
5.37h
3.130h249.8d8.28d
41.29d
1.473d
4.44h
1.15h
1.10h
39.26d2.748d51.3min
2.9d
1.20h
16.91h

496.3
*1
182.3
*1
214.3
273.4
233.6
298.6
617.4
657.8
717.3
344.5
318.9
724.3
767.6
118.7
497.1
D140.5
787.4
215.7
657.9
743.4

46.8
*1
47
*1
9.9
14.5
20
10
43
94.0
28.9
41.4
19.2
47.3
66
31
90.9
90.7
93.4
85.6
98.4
93.1

T, C
T, C
T
T, C
T
T
T
T, C
T
T
C
C
T
T
T, C
T, C
T
C
T
T, C

C+
C+
C+
C+
C+
CI
I
I
C+
CCI
C+
CCI
C+
I
C-

*4

NuclideHalf life

E γ (keV)

Tc-96
Nb-96
Nb-95
Zr-95
Nb-92m
Sr-92
Sr-91
Y-90m
Zr-89
Zr-88
Y-88
Y-87m
Y-87
Sr-87m
Rb-86
Rb-84
Sr-83
Rb-83
Rb-82mBr-82

4.28d
23.35h
34.98d64.02d10.15d
2.71h
9.63h
3.19h
3.267d83.4d106.7d13.37h3.33d2.803h
18.63d32.77d1.350d86.2d6.472h
1.471d849.9*21091.3
765.8
756.7
934.5
1383.9
749.8
202.5
909.1
*2
392.9
898.0
380.8
484.8
388.5
1077.0
881.6
762.7
520.4
776.5
698.4

Nuclear data are from Ref. 23 except for *1 from Ref. 25 and *2 from Ref. 24.
−+*3: I: independent; C-: cumulative by β decay; C+: cumulative by β decay and/or electron capture.
*4: Identifications in Au target (T) or Mylar catcher (C).
D: γ-ray of the daughter in equilibrium.

b.r.(%) TYPE Detec.
97.6
*2
48.5
99.8
54.5
99.1
90
23.6
97.3
99.9
*2
97.3
93.7
78.1
89.7
82.1
8.64
69.0
30
44.7
84.4
28.5

I
I
I
CI
CCI
C+
C+
I
C+
C+
I
I
I
C+
C+
I
I

C
T, C
T, C
T, C
C
T, C
T, C
T, C
T, C
T, C
T, C
T
T, C
T
T, C
T, C
T
T, C
T
C

NuclideHalf life

E γ (keV) b.r.(%)

Rb-81
Br-77
As-76
Se-75
As-74
Ga-73
As-72
Ga-72
Zn-72
As-71
Zn-71mZn-69m
Ni-66
Ni-65
Fe-59
Mn-56
Sc-46
K-43
K-42
Na-24

4.576h
2.377d
1.097d119.8d17.77d
4.86h
1.08d
14.10h1.94d2.720d3.96h
13.76h2.28d
2.517h
44.50d
2.579h83.79d22.3h
12.36h
14.96h

190.3
520.7
559.1
264.7
595.8
297.3
834.0
834.0
144.7
175.0
386.3
438.6
D1039.2
1481.81099.3
846.8
889.3
617.5
1524.72754.0

64.0
22.4
45
58.5
59.4
79.8
79.5
95.6
82.9
82
93
94.8
7.4
23.6
56.5
98.9
100
79.2
18.1
100

TYPE Detec.
C+
C+
I
C+
I
CI
I
CC+
CI
CCCCI
CI
C-

T
C
T
T, C
T, C
T
T
T, C
T, C
T
T, C
T
T
T, C
T, C
C
C
T
T
C

Recoil Properties of Radionuclides

3. Results and Discussion
3.1. Recoil Fractions and Effective Mean Ranges. In thepresent work the radioactivities of 37 product nuclides in themass range of A = 24–131 were identified both in the forward
and backward catcher foils. They are denoted with a label “C”
in Table 1. In the nondestructive Au target and/or the chemically separated samples, the radioactivities of 50 nuclides in
the mass range of A = 42–131 were identified as denoted witha label “T” in the same table. From the fractions of eachnuclide which has recoiled out of a Au target of thickness W inunit of µg/cm2 into the forward and backward catcher foils,
expressed as F = NF/(NF + NB + Ntarget) and B = NB/(NF + NB +Ntarget), N being the number of atoms, respectively, the effective mean ranges, FW and BW, in Au were obtained. Because
the chemical separations of scandium and sodium were not
performed in the present work, the radioactivities of 46Sc and24
Na were detected only in the catcher foils. Therefore, theirmean ranges were calculated by estimating the produced activities in the target on the basis of our yield data at the same E0.6
In Figure 1, the FW and BW values for some representative
products widely separated in mass, 103Ru, 90mY, 74As, and 59Fe,
are shown as a function of E 0 by open and closed circles,
respectively. The FW and BW values seem to be independentof E0 at the studied energies, as indicated by solid and dashedlines, respectively, in Figure 1. This E0-independence of FW
and BW is coincident with those found for the typical photospallation products.1–3,12–14 It is also noted that their reaction
yields increase steeply with an increase of E0 up to 600 MeV
and gradually at higher energies, and that the charge and mass
yield parameters in the photofission of 197Au (Refs. 3, 7) are
independent of E0 above 600 MeV. These observations indicate that the photons responsible for the production of these
nuclides are mostly of energies lower than 600 MeV, (3,3)

J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000 71
resonance, and that there is no appreciable change in the reaction mechanism above 600 MeV; the limiting behavior.
Therefore, the following discussion proceeds on the basis of
the average quantities at E0 ≥ 600 MeV. The average values ofFW and BW are given in unit of µg/cm2 of Au in the 4th and5th columns of Table 2. The quoted uncertainties represent thestandard deviations in the weighted means, as determined from
replicate experiments at different E0 above 600 MeV. The
average forward-to-backward ratios (F/B) at E0 ≥ 600 MeV are
also given in the 6th column in the same table. The results for
131
Ba, 129mBa, 128Ba, and 46Sc were obtained from the separate
irradiation at E0 = 1000 MeV as mentioned above.
The mean ranges FW and BW at E0 ≥ 600 MeV are indicatedby open and closed circles, respectively, as a function of themass difference ∆A between a product (A) and the target (At =197) in Figure 2. The previous results of the 20 photospallation products in the range of ∆ A ≤ 45 (Ref. 14) are also
included in Figure 2 by the same symbols. The FW and BW
are connected by solid and dashed lines, respectively. At ∆A ≤45 the FW is about three times higher than the corresponding
BW, and both increase in parallel with an increase of ∆A. This
increasing trend can be divided into two components: one is asteep exponential increase for the (γ, xn) (x ≥ 1) reaction products, 196Au, 194Au, and 192Au; the other is a gradual increase forthe (γ, xnyp) (x, y ≥ 1) reaction products. The variation of themean ranges at ∆A > 45, however, is quite different from thatfound at ∆A ≤ 45. The FW and BW values are almost equaland both increase slightly with an increase of ∆A, though thoseof 24 Na ( ∆ A = 173) are somewhat higher than the values
extrapolated from the increasing trend at lower ∆A.
The forward-to-backward ratios (F/B) at E0 ≥ 600 MeV areplotted by open circles as a function of ∆A in Figure 3. The
F/B ratio represents the extent of forward peaking of the recoil,and thus is a simplified measure of the angular distribution of
TABLE 2: Averaged Recoil Properties of Photofission Products
from 197Au at E0 ≥ 600 MeV

Figure 1. Observed mean ranges, FW (open circles) and BW (closed
circles), for 103Ru, 90mY, 74As, and 59Fe as a function of bremsstrahlung
end-point energy, E0.

NuclideBa-131
Ba-129m
Ba-128
Ag-110m
Rh-105
Ru-105
Ru-103
Mo-99
Ru-97
Zr-97
Tc-96
Nb-96
Nb-95
Zr-95
Nb-92m
Sr-92
Sr-91
Y-90m
Zr-89
Zr-88
Y-88
Y-87
Rb-86
Rb-84
Rb-83
Br-82
Br-77
Se-75
As-74
Ga-72
Zn-72
Zn-71m
Ni-65
Fe-59
Mn-56
Sc-46
Na-24

∆A
66
68
69
87
92
92
94
98
100
100
101
101
102
102
105
105
106
107
108
109
109
110
111
113
114
115
120
122
123
125
125
126
132
138
141
151
173

TYPE*
C+
C+
C+
I
CCCCC+
CI
I
I
CI
CCI
C+
C+
I
C+
I
I
C+
I
C+
C+
I
I
CCCCCCC-FW (µg/cm2)
1400 ± 400
1400 ± 700
1200 ± 600
2300 ± 1190

BW (µg/cm2)
550 ± 310
880 ± 450960 ± 510
1550 ± 1070

2080 ± 500
2300 ± 600

1840 ± 100
2000 ± 500

1900 ± 500

1900 ± 300

2100 ± 200
2900 ± 400
2000 ± 500

1900 ± 200
2600 ± 300
1900 ± 300

2460 ± 680
2440 ± 260
2550 ± 150
2300 ± 300
2000 ± 1000
2310 ± 340
2350 ± 130
2550 ± 410
2330 ± 420
2150 ± 260

2320 ±
2600 ±
2160 ±
2000 ±
1900 ±
1930 ±
1990 ±
2550 ±
2000 ±
1870 ±

2200 ± 320
2800 ± 400
2420 ± 150
3100 ± 1000
1900 ± 400
2200 ± 1800
2420 ± 530

1650 ± 1250
2300 ± 300
2280 ± 150
2800 ± 1000
2200 ± 600
2700 ± 500
2350 ± 520

3130 ± 1390
5800 ± 1300

3800 ± 1660
5800 ± 800

150
540
120
200
900
500
250
360
330
160

F/B
2.6 ± 1.6
1.6 ± 1.0
1.2 ± 0.9
1.48 ± 1.28
1.10 ± 0.10
1.04 ± 0.23
1.13 ± 0.07
1.2 ± 0.5
1.3 ± 0.3
1.0 ± 0.4
0.856 ± 0.213
1.1 ± 0.2
1.1 ± 0.2
1.1 ± 0.4
0.994 ± 0.235
1.06 ± 0.31
0.939 ± 0.220
1.18 ± 0.10
1.2 ± 0.2
1.1 ± 0.8
1.20 ± 0.36
1.18 ± 0.17
0.992 ± 0.205
1.17 ± 0.29
1.15 ± 0.17
1.10 ± 0.15
1.12 ± 0.49
1.33 ± 1.03
1.2 ± 0.3
1.06 ± 0.10
1.1 ± 0.6
0.86 ± 0.30
0.81 ± 0.68
1.03 ± 0.33
1.09 ± 0.04
0.823 ± 0.478
1.0 ± 0.3

T (MeV)
17 ± 423 ± 722 ± 654.7 ± 16.6

54.4 ± 1.1
63 ± 9
47

± 6

53
97
50

± 3
± 7
± 6

69.9 ± 7.2
77.7 ± 6.6
71.9 ± 2.1
64 ± 454 ± 1460.5 ± 6.2
63.8 ± 3.0
80.7 ± 6.1
61.2 ± 5.4
54.0 ± 3.0

49.3 ± 11.979 ± 666.9 ± 2.2
95 ± 1751 ± 769 ± 1966.0 ± 73.3
98.1 ± 21.9
78 ± 4*I: independent; C-: cumulative by β−decay; C+: cumulative by β+ decay and/or electron
capture.

72 J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000
12–14

the product nuclei. As found previously,
the F/B ratio fortypical photospallation products at E0 ≥ 600 MeV is independent of ∆A and also of At. The weighted mean value of F/B for197
Au at ∆A ≤ 45 is 2.5 ± 0.6 as indicated by a dashed line in
Figure 3. The F/B values at ∆A ≥ 66 obtained in the present
work are also independent of ∆ A within the experimental
errors. The weighted mean value of F/B = 1.1 ± 0.1 as shownby a solid line in Figure 3 is apparently lower than that for thephotospallation products at ∆A ≤ 45, indicating the isotropic
angular distribution of the product nuclei, which is characteristic of the two-body breakup processes such as fission or fragmentation. It is noted that this constant F/B ratio at ∆A ≥ 66
reasonably agrees with the literature data of F/B = 1.18 determined by Kroon and Forkmann26 in the glass detector measurements of the photofission of 197Au at E0 = 600–800 MeV.
The proton results by Cumming and Bächmann27 at protonenergy Ep = 28 GeV, Ross and Bächmann28 at Ep = 0.58 GeV,
Kaufmann et al.29 at Ep = 1, 3, 11.5, 28, and 300 GeV, andCole and Porile30 at Ep = 400 GeV are included in Figure 3 by
the different symbols as shown in the inset of the figure. The
F/B ratios at Ep = 0.58, 1, and 3 GeV, which are shown by
open symbols, are apparently higher than those at the higher Ep
shown by closed symbols and also those of the photon-reaction
at E0 ≥ 600 MeV (open circles). The F/B ratios at Ep ≤ 3 GeV
increase with an increase of ∆A and reach peaks around ∆A =40 at Ep = 1 GeV and around ∆A = 55 at Ep = 3 GeV. After thepeak, the F/B decreases with an increase of ∆A and almost
levels off at ∆A ≥ 87, though a small minimum of F/B appears
around ∆ A = 130. The F/B values of the proton-reactions
decrease with an increase of Ep up to Ep = 3 GeV, but becomes

Haba

Figure 2. Averaged mean ranges, FW (open circles) and BW (closed
circles), at E0 = 600–1100 MeV as a function of the mass difference( ∆ A) between a product (A) and Au target (A t = 197). Solid anddashed lines are drawn through the FW and BW points, respectively,
to guide the eye.

Ep-independent at the higher Ep (closed symbols). It seems
interesting to note that the Ep-independent F/B values of the
proton-reactions at Ep ≥ 11.5 GeV are equal to those of thephoton-reaction at E0 = 600–1100 MeV except for the resultsof 196Au and 194Au produced by the simple (γ,n) and (γ,3n)reactions, respectively.
3.2. Kinetic Energy, T. The measured quantities of FW
and BW were used to derive the kinetic energies T of the
product nuclei by means of the vector velocity model
embodying the two-step mechanism commonly invoked inhigh-energy proton reactions. Here the two-step vector
analysis developed by Winsberg 15 was employed for thispurpose. This analytical method was successfully applied in
the previous recoil studies on the photospallations on 27Al, natV,
natCu, 93Nb, natAg, natTa, and 197Au, and the analytical details
were the same as described in our previous papers.12–14 The T
values were calculated from the FW and BW values, and theaveraged values over E0 ≥ 600 MeV are given in unit of MeV
in the last column of Table 2. Another recoil parameter v,
which represents the forward recoil velocity from the first step,
was also obtained in the present analysis, but it is out of thepresent discussion because they are accompanied with large
errors.
The T values at E0 ≥ 600 MeV are shown as a function of∆A by open circles, together with those of the photospallation14
in Figure 4. At ∆ A ≤ 45 the T increases rapidly with anincrease of ∆A and shows the two components as suggested by
FW and BW in Figure 2; the (γ, xn) (x ≥ 1) reaction products
mainly produced by giant dipole resonance and the (γ, xnyp)(x, y ≥ 1) reaction products mainly produced by quasi-deuteronmechanism and (3,3) resonance. The increasing trend for thelatter component is well explained in terms of a random-walktheory for typical spallation,27, 31 in which the velocity of the
product is due to the vectorial addition of the randomly
directed recoil velocities resulting from multi-particle emission. In this theory, the kinetic energy T of the typical photospallation products is well systematized by a parameterεs = T/(∆A/At), which represents the average kinetic energy
carried off by an evaporated nucleon.31 The εs values at ∆A ≤45 were found to be independent of ∆A/At and the weighted
mean value of 16 ± 5 MeV was obtained. 14 The T values
calculated by the PICA code16, 17 at E0 = 400 MeV are also
indicated by crosses for the nuclides with ∆A ≤ 42. The PICA
code, which is based on the two-step model in the high-energy
nuclear reaction,32 can reproduce the increasing trend for the(γ, xnyp) products well, though the absolute values are underestimated by a factor of about 1.7 on the average for the (γ,xnyp) products. On the other hand, the T values at ∆A ≥ 87 arequite higher than those extrapolated from the spallation trendat ∆A ≤ 45 and increase slightly with an increase of ∆A andlevel off around ∆A = 110. The three εs values of 131Ba (∆A =Figure 3. Forward-to-backward ratios (F/B) at E0 = 600–1100 MeV
as a function of the mass difference ∆A. See text for the details.

Figure 4. Kinetic energy, T, as a function of the mass difference ∆A.
See text for the details.

Recoil Properties of Radionuclides

J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000 73

66),
Ba (∆A = 68), and Ba (∆A = 69) are in between the
two trends. The high ε s of more than 50 MeV imply thedifferent reaction mechanism from typical spallation (εs = 16 ±5 MeV). Komar et al.33 measured the kinetic energies of coincident fragment pairs in the photofission of 197Au at E0 = 1000
MeV by using semiconductor detectors. Their results areshown by a dashed line in the range of 58 ≤ ∆A ≤ 142 in Figure
4. The T values obtained radiochemically in the present work
seem to be well consistent with the physical measurement by
Komar et al. 33 except for those of the Ba isotopes at ∆ A =66–69 as noted above. The T values of these Ba isotopes
deduced in the present work are smaller by a factor of about 2than those by Komar et al.33 Since Komar et al.33 employed thecoincidence technique for the kinetic energy measurements,
their results were confirmed to those for binary fission. The
present radiochemical technique, however, can not separate thefission products from those produced from other reaction channels such as spallation and fragmentation. The smaller T
values of barium isotopes may indicate that their productions
are from both fission (a dashed line) and spallation (a solidline) mechanisms.
The proton results cited above 27–30 are also included in
Figure 4 for a comparison. All of the proton data were reanalyzed in the same manner with the same range-energy relationas used in the present work in order to ensure a consistentcomparison. At ∆A ≤ 66 the T values from the proton-reactions are about the same with each other, irrespectively of Ep,
and also with those of the photon-reaction at E0 = 600–1100
MeV except for 196Au, 194Au, and 131Ba. This similarity of the
T values between the photon- and proton-reactions suggeststhat the mechanism of the second deexcitation step by evaporation is similar in the two types of nuclear reactions. The lower
T values of 1 9 6 Au and 1 9 4 Au in the photon-reaction are
explained by the contribution of giant dipole resonance as
described in References 12 and 14. The higher T value of
131
Ba in the photon-reaction may be due to the relatively larger
contribution of fission process. The T values of 131Ba in the
proton-induced reaction all fall on the spallation trend irrespectively of E p , implying negligible contribution of fission
process. The variation of T for the proton-reactions at ∆A >66, however, is quite different from that at ∆A ≤ 66. The T
values of the proton-reaction products decrease with anincrease of Ep and become almost constant at Ep ≥ 11.5 GeV(closed symbols). This decreasing trend for the protoninduced reaction is explained as a change in the reaction mechanism from binary fission to deep-spallation.34 It is noted that
the Ep-independent T values at Ep ≥ 11.5 GeV are well extrapo-

lated from the increasing trend of typical (γ, xnyp) products at∆A ≤ 66 (a solid curve). In the case of the photon-reaction, theeffective photon energies are lower than 600 MeV and thevariation of the T value is not found above E0 = 600 MeV. It isinteresting to note that the T values from the proton-induced
fission by Ross and Bächmann28 at Ep = 580 MeV are wellcoincident with the present photon-results above ∆A ≥ 87.
3.3. Recoil Properties of 24Na. Recoils of 24Na make itpossible to trace how the production mechanism for lightnuclei changes with target mass (At ). Many recoil studies of24
Na produced by various hadrons have been reported in thewide ranges of target mass and projectile energy.29–31, 35–37 A
few works on the photoproduction of 24 Na have also beenreported by Järund and Forkman38 on natCu, natAg, and 197Au at
E0 = 800 MeV, and by Amroyan et al.39 on 27Al, natCu, natAg,
and natTa at E0 = 4.5 GeV. Recently the recoil properties of24
Na from 27Al, natV, and natCu were reported by our group at E0= 600–1100 MeV.12–14 Our F/B ratios of 24Na produced from27
Al, nat V, nat Cu, and 197 Au are shown in Figure 5 by open
circles as a function of At , and compared with those reported
by Järund and Forkman38 at E0 = 800 MeV (closed triangles)
and by Amroyan et al.39 at E0 = 4.5 GeV (closed squares). In
addition the measurement on nat Ag was performed by ourgroup at E0 = 950 MeV and 1000 MeV, and the weighted meanvalue of F/B = 1.7 ± 0.5 is also included in the same figure.
The results by the different authors at different incident photon
energies seem to well agree with each other, indicating thecharacteristic limiting feature of photonuclear reaction above
600 MeV. The F/B values for 27Al, natV, and natCu targets areabout 2 and consistent with the photospallation trend as foundin a previous work.14 However, the F/B values decrease gradually with an increase of At as a whole and attain to almost unity
for natTa and 197Au. This variation implies that the change ofthe angular distribution of 24 Na from the forward-peaking
distribution to the isotropic distribution when the target mass
number increases.
It may also be interesting to compare the observed mean
kinetic energy T of 24 Na with the energy E C acquired by
Coulomb repulsion in the two-body breakup process.35 The
ratio of T to EC would be small if 24Na is produced as a residueof nucleon evaporation (spallation), and would be of the orderof unity if the kinetic energy of 24Na is acquired as a results of
Coulomb repulsion in the two-body breakup process (fission or
fragmentation). The T/EC ratios are shown as a function of At
in Figure 6. The T/EC ratios are about 0.5 for 27Al, natV, andnat
Cu, and are almost unity for natAg and 197Au. The variationsof F/B and T/EC with At may be explained as a result of the

Figure 5. Forward-to-backward ratios (F/B) for 24Na produced from27
Al, natV, natCu, natAg, natTa, and 197Au at E0 = 600–1100 MeV as afunction of target mass number, At. The present results are shown by
open circles, and those reported by Järund and Forkman38 at E0 = 800
MeV and by Amroyan et al.39 at E0 = 4.5 GeV are shown by closed
triangles and closed squares, respectively.

Figure 6. Ratios T/EC for 24Na at E0 = 600–1100 MeV as a functionof target mass number A t . The present results are shown by open
circles, and those reported by Järund and Forkman38 at E0 = 800 MeV
and by Amroyan et al.39 at E0 = 4.5 GeV are shown by closed triangles
and closed squares, respectively.

129m

128

74 J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000
transition of the reaction mechanism from spallation to fission
or fragmentation. As mentioned above the kinetic energy T of
typical photospallation products is well parameterized by εs;
the εs values are all in the range of 10–20 MeV for typical spallation products from various targets with At = 27–197.14 The εsvalues for 24Na from 27Al, natV, and natCu are 11.9 ± 1.4, 18.9 ±
3.1, and 21 ± 10 MeV, respectively, and they agree with thespallation trend. However, the εs values of 51 ± 13 MeV fornat
Ag and 96 ± 11 MeV for 197Au are quite high compared withthose for the lighter targets, implying the different reaction
mechanism from spallation. It is also noted that the existenceof two nuclear processes in the photoproduction of 24Na wasfound previously in the mass dependence of the reaction
yields;39,40 the yields decrease exponentially with an increase of
A t (spallation), but suddenly change the slope to a gradualincrease around At = 100 (fragmentation). Recently, the yields
of 7,10Be, 22,24Na, and 28Mg from 197Au was measured in ourgroup,6 and it was found that they are more than one order ofmagnitude higher than those extrapolated from the mass yield
curve of fission, indicating a different reaction mechanism
from fission; fragmentation.
As shown in Figure 4, the kinetic energy T of 24Na (∆A =173) produced from 197Au is higher by a factor of about 2 thanthose of the proton-induced reactions which are almost
constant in the large ∆A region, irrespectively of Ep. Also
noted is that the F/B ratio of 24Na shown in Figure 3 is apparently smaller than those of the proton-reactions, where the F/B
ratios decrease with an increase of Ep. These variations of T
and F/B may imply the difference in the production mechanism of 24Na between the photon- and proton-reactions, though
the measurements below Ep = 1 GeV are required.
Acknowledgment. The authors would like to express their
gratitude to Drs. H. Okuno and K. Masumoto, and the ES
crew members of the High Energy Accelerator Research
Organization at Tanashi, for their invaluable cooperations in
the course of experiments. This work was supported in part by
the Grant-in-Aid for Scientific Research (07304077) of the
Ministry of Education, Science and Culture of Japan.
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