Excitation functions of proton induced nuclear reactions on 85Rb

International Journal ~!/ Applied Radiation m~d I.sotope.s, Vol. 31, pp. 141 to 151 ~) Pergamon Press Ltd 1980, Printed in Great Britain

0020-708X/80/0301-0141502.00/0

Excitation Functions of Proton Induced Nuclear Reactions on 85Rb TAKAYOSHI HORIGUCHI, HIROSHI NOMA, YASUKAZU YOSHIZAWA, HIROKATSU TAKEMI*, HIROMI HASAI* and YOSHIYUKI KISO* Faculty of Science. Hiroshima University, Hiroshima, 730 Japan (Received 23 July 1979; in revised form 31 August 1979)

Excitation functions for the [p, xn) (x = 3 5), (p, pxn) (x = 1-4) and (p, a3n) reactions on SSRb with 14-70 MeV protons have been measured by means of the activation method. Enriched aSRbCI targets were irradiated with energy-analyzed protons from the AVF cyclotron at Osaka University. Disintegration rates of product nuclides were obtained from intensities of selected ~.-rays with calibrated Ge(Li) detectors. Thick target yields of 8~Rb for medical use and other radioactive products were calculated from measured excitation functions. The calculated thick target yield of 8tRb Was 31.2 mCi/tA- t h - t at Ep = 70 MeV. The thick target yield of S~Rb chemically separated from alSr, by the reaction 85Rb(p, 5n) 8XSr(fl+. EC)StRb, was obtained to be 2.3 mCil~A J h t at 96.4min.

Introduction

THE USE of the short-lived isomer 8tmKr (TI/2 = 13.3 s) has become important for diagnosis in the field of nuclear medicine. The isomer s ~ ' K r is generated by 81Rb (Tt/2 = 4.58 h) and emits the 190keV )'-ray (64.504, per decay) which has high sensitivity for the 7-camera. Since the short-lived isomer 8~"Kr is in equilibrium with the parent nuclide 8tRb, the generator method is useful for applications in nuclear medicine, for example s~"Kr is appropriate for the diagnosis of lung function ~ and blood flow, ~2) because there is no accumulation through repeated examinations and small radiation dose to patients. Presently, SlRb is mainly produced by the nuclear reactions of 79Br(~, 2n)SlRb and SlBr(~,4n)81Rb. (3'4) By-product radioisotopes are inevitable in these reactions. In addition, its yield is small because of low energy bombarding particles. Recently, a method of producing pure SlRb isotope has been reported by SCHNEIDER et al. "s~ They used the SSRb(p, 5n)8~Sr reaction and the decay product S~Rb was chemically separated from S~Sr (T~.2 = 25.5min). They did not measure excitation functions but they gave relative yields of 8tRb. The excitation functions of proton induced nuclear reactions on SSRb have not been reported up to the present. To obtain the production cross sections of SlSr, S~Rb and st"Rb, the decay schemes of those nuclei must be well known. The decay scheme of SJSr was proposed by BRODA et al. ~6~ Recently the decay schemes of 8~Rb and s~"Rb (T~,2 = 32 min) have been established by LWTAK et al. ~7~ and others. ''s~ Consequently, it becomes possible to determine the production cross sections of 8~Sr, Sl"Rb and 8~Rb. * Faculty of Engineering, Hiroshima University, Hiroshima, Japan. A.a.L 3t/3

A

141

In the present work, eight excitation functions of proton induced nuclear reactions on aSRb were measured in the energy range of 14-70MeV, to obtain the yields of 8tRb and by-product radioisotopes. The production cross section of S~Rb is the sum of the cross sections for the SSR6(p, p4n)SIRb and S~Rb(p, p4i0Sl"Rb reactions, i.e. the reactions leading to the ground state and the isomeric state. The growth-decay curve of 81Rb in the target was obtained from the iridependent production cross sections of SlSr, 8~"Rb and S~Rb. Thick target yields of S~Rb and other radioactive nuclides were calculated by using measured excitation functions.

Experimental

Target preparation and irradiation

Enriched SSRbCl powder of isotopic abundance 99.78 + 0.02~,,, obtained from O R N L , was used as a target. This powder and aluminium powder (purity > 99.99%) were completely mixed with a definite weight ratio, and the mixture was uniformly spread on a mylar film of 50 ttm thickness. It was then supported. on the mylar film with polystyrene solved in benzene. ~ This target, aluminium foil 10#m thick and copper foil 20Ftm thick were stacked and irradiated with a proton beam from the AVF cyclotron of the Research Center for Nuclear Physics, Osaka University. The extracted beam through a beam analyzer magnet was transported to the target on a bombardment course. The incident proton energy was 60 and 70 MeV and the energy resolution was less than 0.5~. The energy of the proton beam was degraded with copper or silver plates, and the energy at the target was determined by using the range-energy tables of WILLIAMSON et al. (1°)

77 Horiguchi et al.

142

Each target was irradiated for 10 rain with a beam current of 0..%1 pA. Blue beam spot appeared at the center of the 85RbCI target by proton irradiation. The thickness of aSRbC1 was determined by a balance and also obtained from the intensity of the 24Na 1368.5 keV ]'-ray. This isotope was produced in the aluminium powder in the target and in the aluminium monitor foil by the 27Al(p, 3pn)24Na reaction. The target thickness of 12-20mg/cm 2 determined by this method agrees within 5~,, or less with that by the balance. The beam current of incident protons was measured by means of the secondary electron emission m o n i t o r method} ~'~2~ The beam m o n i t o r was calibrated with a Faraday cup before and after irradiation, and overall reproducibility was 1~o or less. The beam current was stable during irradiation. The integrated beam current was reproduced by the monitor reactions of 2VAl(p,3pn)24Na~st and 65Cu(p,pn)6'~Cull'*'ls~ within the error of 10°i. This error is mainly due to the monitor reaction cross sections. The target is fixed in close contact with a groove (I 3 m m depth) in an aluminium plate of 5 m m thickness, and the back face of the plate was cooled by water during irradiation. This plate is automatically inserted in polyethylene vessel and transported to the measuring area through a pneumatic tube in a few seconds.

Measurement 7-Rays of residual activities-in targets and monitor foils were measured with Ge(Li) detectors without chemical separation. The detectors are a low-energy photon spectrometer (LEPS) having a full-width at half-maximum ( F W H M ) of 550eV at the 57Co 122 keV }'-ray, a 50 cm 3 Ge(Li) detector with F W H M

of 2.5keV at the °°Co 1332keV ";-ray and a 6 0 c m 3 Ge(Li) detector with F W H M of 2.0keV at the 1332 keV 7-ray. The detection efficiency of LEPS was calibrated with 7-rays of 54Mn, ~VCo, 6°Co, ~33Ba, t37Cs and 15ZEu. The efficiencies of the 5 0 c m 3 and 60 cm 3 Ge(Li) detectors were calibrated with y-rays of 2ZNa, 54Mn, S7Co, 6°C0, '37Cs and 152Eu. The intensities per decays of these ],-rays were quoted from Refs ( 16-18). The short-lived SlSr isotope (T~2 = 25.5 mini emits )'-rays of 148keV (30.8o/o per decay) and 153keV (36.5~o). 16~ StmRb (T1/2 = 32min) emits a ],-ray of 86 keV (4.7~o), °~ and s4"Rb (T1/2 = 20.4 min) emits }'-rays of 215 keV (23.6~o~ and 248 keV (51.5°~)} 19'z°~ These low-energy ,/-rays were measured with LEPS and C a m b e r a Scorpio system (4096 channel A D C connected to P D P 11/34 c o m p u t e r ) a t 10min after the end of b o m b a r d m e n t (EOB), and ],-ray spectra were recorded immediately on a floppy disk. The source~letector distance was taken to be 2 0 c m to decrease the influence of the },-ray sum effect. The dead-time of the measuring system was less than 10~o. To a b s o r b high energy fl-rays, a lucite plate of 10 mm thickness was placed in front of the detector. The 190 keV ],-ray (64.5~o per decay) of 81Rb a~"Kr ~7~ and the 7-rays of 554keV (70.5%0), 619keV (43.3%), 698keV (27.9%) etc. emitted from 82"Rb ('/'1,2 = 6.2 h) t~ l~ were measured with the 50 cm 3 Ge(Li) detector and the same system after the short-lived nuclei completely decayed. The source-detector distance was 20 cm and the dead-time was less than 50o. The ]'-ray spectra of long-lived nuclides with halflives greater thafi 1 day, i.e. a3Sr (Tw2 = 1.35 d), 82Sr (TI2 = 25d}, 84Rb (TL, 2 = 34.5d), S3Rb (TL, 2 = 86.2d) and VgKr (T~/2 = 1.46d) were measured with the system of the 60 cm 3 Ge(Li) detector and O R T E C

TABLE 1. Decay data of nuclear reaction products Reaction product

Half-life

Mode of decay (Branching ratio °,o)

81Sr

25.5 min

EC(13), fl+(87~

8-'St ~3Sr

25.0 days 1.35 days

EC(100) EC(76), fl+1241

S"Rb 8~Rb

32rain 4.58 h

IT(98), EC + fl+(2) EC(73}, fl÷(27)

8-"Rb

6.2 h

EC(74), fl+(26)

83Rb

86.2 days

EC(100)

84"Rb

20.4 min

IT(100t

84Rb 7~Kr

34.5 days 1.46 days

EC(75). fl+(221, fl-(3) EC(93J, fl+(7)

:' From daughter S2Rb (Tl, 2 = 1.25 mini. h From daughter Sl'Kr (TI 2 = 13.3 secl.

Analysed ]'-rays Energy (keV) Intensity per decay (%0) 147.8 153.4 776.8~ 381.5 762.5 86.2 190.4b 446.3 554.3 619.1 698.4 . 520.4 529.5 215.4 247.9 881.5 261.3 397.6 606.1

30.8 4- 1.8 36.5 4- 1.8 13.4 _+ 0.5 19.6 4- 0.9 30.0 4- 2.8 4.7 4- 0.2 64.5 4- 1.4 19.0 4- 1.0 70.5 _+ 4.7 43.3 4- 2.9 27.9 4- 1.9 46.8 + 1.5 30.4 4- 0.9 23.6 _+ 2.2 51.5 4- 4.7 73.4 _+ 2.9 12.7 4- 0.4 9.5 4- 0.3 8.1 4- 0.2

Reference 6 21, 22 6, 23 7 6, 7, 8 21 24 19, 20 25 26

Proton reactions on SSRh 4096 channel PHA connected to a P D P 11/05 computer. Observed spectra were recorded on floppy disks. The growth~lecay curves of ~,-rays were followed for about 1 month to examine the genetic relations of product nuclides. The y-rays emitted from 85mSr ( T I / 2 = 1.13 h), SSSr ( T I : 2 = 64.93d), s°mBr (T,/2 = 4.42h), S°Br (Tv2 = 17.4min), S2Br (Tv2 = 1.47d) and 3'*'C1 (T2:2 = 32 rain) could be identified, but were not analyzed in the present work because of poor statistics. Energies and intensities per decay of analyzed 7-rays ~6 s.~,~-26, are shown in Table I. The photopeak area of each y-line was calculated by means of the least-square fitting of a Gaussian function plus a linear background. The peak area was also calculated manually to confirm the computer calculation. The manual calculation is in good agreement with the computer results.

143 0

1112-I

81

26rn

3sSr

9/2+ 3/2-~

[

lSS,//ec.ly . 86 / /32m ~ 0#4.6h

637

3•2-

t

/

,12- ~ 7/2 +

1 9 ~ 3s O, 2.1xI0sy

,~'

FIG. 1. Simplified decay schemes of SISr, SlmRb and SlRb.

Determination of cross sections The numbers of S'Sr, S2Sr and S3Sr atoms produced by the SSRb(p, xn) reactions are given by the following expression :

nlo. N = --(1 ).

- e-i""),

(1)

expression, which includes the generation of SlRb from the decay of S~mRb and S'Sr: Nl(ts) = nl(o"l + o"z + 21

+ , nlo.z where N = number of the produced atoms n = number of S~Rb atoms in the target per unit area (1/cm 2) I = number of incident protons per unit time (l/s) o. = reaction cross section (cm 2) 2 = decay constant (l/s) ts = irradiation time (s). Therefore, the reaction cross section is extracted from the disintegration rate 2N at EOB by using expression (1). The cross section of s2mRb production is also obtained from this expression, because 82Sr decays to S2Rb (TL2 = 1.25 min) by 100% electron capture and does not decay to s2'nRb)22) The cross section of s'*"Rb production is also obtained, because s'*Sr is a stable nuclide. Since yields of 79Sr and 79Rb produced by the SSRb(p, 7n) and (p, p6n) reactions, respectively, are negligible at proton energy below 70MeV, the cross section of 7°Kr production can be determined. The cross sections of S3Rb and SnRb are obtained by subtracting the contributions of the S3Sr and S4mRb decays from the yields of 83Rb and S4Rb, respectively. The estimation for the cross section of S~Rb production is considerably complicated, because both S'Sr and s'mRb decay to StRb. Simplified decay schemes of S'Sr, s " R b and SlRb are shown in Fig. 1. SZSr decays to S'Rb and does not decay to Sl"Rb.~6~ sL"Rb decays to S'Rb by the isomeric transition of 97.8% and to S'Kr by fl+ and EC decay of 2.2%/7's~ Therefore, the cross sections of S~Sr and s~'~Rb can be extracted independently. The number of product s, Rb atoms during irradiation is given by the following

0"3)(1

- e -~'',~)

(e_~. . . . .

e_~..,B)

+ , n R r 3 (e-:" . . . .

e -~'''~')

A 2

--

A 1

(2)

A3 -- A 1

where the subscripts 1, 2 and 3 correspond to SlRb, SlmRb and SlSr, respectively. The number of S~Rb atoms after EOB is also given by NI(t)

nlo.2 nlo.3 } ynl(o.l + o"2 + o"3) I- 22 - 2~ + A3 " - - /'1 21 x (1 -

e-~"t~)e

-~':

x (1 -

e-~-"B)e

-~:

nIo.2 22 - 21

nlo"3 (1 - e-~~tB)e-;-'! ';'3 -- )q

(3)

where t is the time from EOB. The second and third terms can be neglected, if SlmRb and SlSr completely decay at the measurement. If the production cross sections, a2 and o 3, of 8tmRb and S'Sr, respectively, are independently known, the production cross section o", of alRb by the (p, p4n) reaction can be determined.

Results

Excitation functions Observed excitation functions are shown in Figs 2-5. The overall errors of the cross sections are also shown in the figures. These errors were obtained from sums of following individual errors in quadrature: (l) The

144

T. Horiguchi et al.

0

o~-

n"6

r~ E

h-

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n~o 10 c 0

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P ol

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c~

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ii~

,

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v

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145

Proton reactions on a~Rh

~2~;

.o o

-5. [--,

~

. ~

-

~~

~ =,, ",

~ o.~

+7/-

~° -

-

-

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~~ Ill

J

|

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I

1

I

I

|

Jill

|

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'~

!

| | ~ 1 1

I

|

~

0 (qw) uo!ioes

d~

ssoJo

r-

n"

"(3

ao C (v)

0 ~

O.

.= n~

i|i

i

i

|

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~

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(qLU) uo!l:3eS SSOJ 3

T Hori.quct~iet al.

146

error of the target thickness (5Yo), (2) the error of the proton beam current ( < 10~), (3) the errors of the detector efficiencies ( < 5~o), (4) the errors of the y-ray intensities per decay of product nuclei (3-10~o) which are shown in Table 1, (5) The statistical errors of photopeak areas of the y-rays emitted from product nuclei (0.1 3°~i, in a few cases the errors are about 50o,/, near the threshold) and (6) the errors of subtraction in the cases when the contributions from decays of parent nuclei are present. The error due to the half-life of each nuclide was not included. The typical overall errors were in the range 12-16~o. As shown in Fig. 2 the prominent low-energy maxima are seen in the excitation functions for the (p, 3n) and (p, 4n) reactions, and the excitation function for the (p, 5n) reaction reaches the maximum near the proton energy of 68 MeV. The cross sections at the maxima are 400 mb, 200 mb and 29 mb for the (p, 3n), (p, 4n) and (p, 5n) reactions, respectively. In Fig. 3, (a) and (b) show the excitation functions for the (p, p4n)

reaction leading to the ground state and the isomeric state of S'Rb, respectively, and (c) is the total excitation function, i.e. the sum of (a) and (b). The excitation functions for the aSRb(p, p3n)a2"Rb, ~SRb(p, p2nj83Rb and a~Rb(p, ~t3n)7°Kr reactions are shown in Fig. 4. 111 the present work, the excitation function for the SSRb(p, p3n)S2Rb reaction could not be measured because of the short half-life of 82Rb (Tl2 = i.25 min). In Fig. 5, (a) and (b) show the excitation functions for the (p, pn) reaction leading to the isomeric and ground states of S'*Rb, respectively, and (c) is the total excitation function, i.e. the sum of (a) and (b). As shown in Figs 4 and 5, the (p, pxn) reactions have no prominent maxima such as those in the excitation functions for the (p, xn) reactions. The maximum values for the (p, pn), (p, p2n) and (p, p4n) reactions are about 460 rob, 340 mb and 200 mb, respectively, and do not decrease rapidly. However, a clear maximum is seen in the excitation function for the SSRb(p. :t3,) VgKr reaction.

103 : /

a~mRb elmRb

102 S2mRb

83Sr 101 i5 E

7gKr 100

e~Rb e2Sr 83Rb

> u


10-2.

10.3

I

0

10

I

I

I

I

I

20 30 L,O 0 60 Incident proton energy (MeV)

I

I

70

80

FIG. 6. Thick target yields of reaction products in the 8~RbCI target as a function of the incident proton energy. The yields of 81Rb include the contributions from the. decays of alSr and sl"Rb during irradiation. The yields of S3Rb and 84Rb include the contributions from the decays of S3Sr and S4mRb, respectively.

147

Proton reactions on a'~Rb

Thick target yields

for these yields, respectively. The thick target yield of atRb immediately after irradiation is given by the following expression:

The thick target yield of each nuclide can be calculated from the excitation functions. The yield, Y (in mCi), of each nuclide produced in a thick target immediately after irradiation is given by the following expression : • AE A~o cr(E~) Y = nil(1 - e-"'~)3. 7 × 10~ 2"i S(Ei)

3

Y(alRb) = C(1 - e -~''B) ~ Aj J=l

+ CA2 ~ - ~~-1( e - - ~

(4)

where

,; CB

+ CA3 ~ ( e -

nl = the number of SSRb atoms in a 1 g target AE = integration step width (1 MeV) or(E3 = cross section (cm 2) at the proton energy

- e -~,'~)

- e -~''")

where

Ei S ( E 3 = stopping power (MeVcm2g - t ) of the

C-nil

target material at the proton energy E~ AE~. = energy degradation in the target

-

AE -

3.7 x 10 7

and Aj =-

and I, i and tv are shown in equation (l). In order to calculate the thick target yields of a3Rb and S4Rb, the excitation functions shown in Figs 4 and 5 are integrated. In addition, the decays of aaSr and 8'*"Rb during irradiation were taken into account

~" i

ay(Ei) S(Ei)"

j = 1,2, 3.

Subscripts j = 1, 2 and 3 denote StRb, al"Rb and atSr, respectively.

103 8~Rb

~ - ~

102

slmRb

e2mRb l~aIR~ot,, d

P- 101

10 0

"-.8'~Rbtot~ 83Rbtotat

~ 10 I

10-2.

10-3

0

Ep=70 Meg.l uA, ta=lhr

l

I

1

'

(5)

I

I

2 3 Torget thickness (glcm2)

I

4

FIG. 7. Radioactivity yields (mCi # A - t h - t ) vs the aSRbCl target thickness (gem-2) for the case of the

incident proton energy of 70 MeV.

148

rE Hori.quchi et al.

The thick target yield of 8~Rb after EOB is given by the following expression : Y(SlRb) = C(1 - e - ) ' : ' )

Aj + -.

;,1 4- ).3 ;q

A2

A2 --

~j= 1

"~1

}

21

.

A3 e - " : - CA2

--

proton energy of 46 MeV, where the other nuclides are produced. Since the ratios of the impurities to 81Rb decrease as an increase of the incident proton energy, the higher incident proton energy is desirable to produce 8~ Rb. In addition, it is necessary to choose the appropriate target thickness. Figure 7 shows the yields of produced radioactivity immediately after irradiation vs the thickness of the 85RbCI target. The values were calculated for the case of an incident proton energy of 70 MeV, beam current of IliA and irradiation time of 111. The yields of short-lived nuclides 8~'Rb and s'*'Rb are large. However, these short-lived activities are negligible several hours after EOB for the purpose of the S~Rb production. The yields of long-lived nuclides 8~Sr, 82Sr, 83Rb. S4Rb and 7"Kr are more than one order smaller

/'2

--

"J~l

21

× (1 - e - ~ : " ) e -~': - CAs 23 _ 21 x (1 -

e-::')e -:~'

(6)

where t is

the time after EOB. The thick target yields calculated from the excitation functions are shown in Fig. 6. The nuclides 8~Rb, 8 " R b and 81Sr are not produced below the

103

SlmRb 102

/ /

xxx,,

//

lO 1

",.

\,

'1

Gr.th-decay of

"""..,

\,

"8"R-b"

",.

\

100elSr I~.EC elRb

x

" \

x

Decoy o'}', x 81mRb x x

x\ x

1ff I -

% \ %

Decay of elSr

\\

% %

% % \

10-2I

I

I

I

\

\

\

I

1 2 3 4 5 Time after the end of 1 hr irradiation (hrs) 10.3

I

0

1

I

2 3 Irrediation time

I

I

z. 5 te (hrs)

I

6

FIG. 8. Radioactivity yields of S~Sr, Sa'Rb and StRb vs irradiation time (tB), and decay curves of these isotopes. These curves are for the case of an incident proton energy of 70 MeV, a beam current of l #A and target (SSRbCl) thickness of 3.3 gcm - 2 (Ep = 7~46 MeV). The decay curves of s~ Sr and ~~'Rb and the growth~lecay curves of StRb after 1 h irradiation are shown by broken curves. The growth decay curve of S~Rb chemically separated from S~Sr is also illustrated by a continuous curve.

Proton reactions on SSRb

.

149

Isomeric cross section rotios

~(81rnRb9/2+) 6'( SlRb 3/2-)

_~:t ! 6"(aZ'Rb2-)

2o

xx~,," ~(8~'rr'Rb6+}

o

\\ E O UI

t ,,

-~-- +__ i

10

20

I

I

/

30 z,O 50 Incident proton energy (MoVI

I

I

60

70

FIG. 9. Isomeric cross section ratios of the (p, pn) and (p, p4n) reactions.

than the yield of StRb. The prominent radioactive impurity is s2"Rb, the yield of which is larger than that of SmRb for the target thickness more than 2 . 2 g c m -2 (Ep = 70-55 MeV). In addition, ratios of the impurities to SlRb increase as an increase of the target thickness, because proton energies at the maxima of the excitation functions of 82"Rb and the longlived nuclides are lower than that of SmRb. Therefore, the appropriate target thickness is about 1.5 g cm 2 (Ep = 70-60 MeV) or less. The ratio of total impurities to SlRb is the minimum at 4 h after EOB for the target thickness of 1.5 g cm-z. In this case the yield of SlRb is 22.8 mCi/~A -m h -m and that of total impurities is 1 5 . 6 m C i p A - l h -1 ( 1 2 . 4 m C i / I A - l h -I for 82"Rb). The calculated yields of SlRb, 8~"Rb arid alSr vs irradiation time are shown in Fig. 8. The target thickness was taken to be 3.3 g c m -2 (Ep = 70-46. MeV). The yields of al,,Rb and 81Sr for I h irradiation reach 73~o (129mCipA i h 1) and 80.4~o (31.6mCi~A -1 h -m) of the saturation activities, respectively. The yield of 81Rb is 9.20/0 (31.2mCi~tA -mh -m) of the saturation activity (339mCi/~A-m). The decays of s l ' R b and 81Sr and the growth~lecay of alRb after 1 h irradiation are also shown in Fig. 8. The growthdecay curve of 81Rb reaches maximum (39.0 mCi/tA - 1 h - ~) at about 65 min after the irradiation. Rapid chemical separation of SmSr was done immediately after the irradiation, and the growth-decay curve of carrier-free 8mRb is also shown in Fig. 8. This curve reaches maximum at 96.4 min after the chemical

separation, and this value is 2.3 mCi # A - 1 h - ~ for the StSr precursor of 31.6 mCi/~A- 1 h - 1. Discussion In the present experiment, we measured three excitation functioris of the (p, xn) reactions, four of the (p, pxn) reactions and the (p, ~3n) reaction. The shapes of these excitation functions suggest possible reaction processes. In the (p, 3n) reaction the compound process is predominant at lower proton energies, but the tail at high energy (Ep > 50 MeV) indicates that the direct process is about 10~o of the compound process. In t h e (p, 4n) reaction the compound process is smaller by about factor of two, but the direct process seems to be about the same as that of the (p, 3n) reaction. The maximum value for the (p, 5n) reaction is 1/7 of that for the (p, 4n) reaction. This tendency is also seen in the other proton induced reactions t27-29~ of the lighter medium-weight region. This fact is explained by following reason: after multi-neutron emission the nucleus goes toward the neutrondeficient region where the neutron separation energy becomes larger. Then the neutron emission is strongly suppressed, and the charged particle emission increases. The excitation functions for the (p, pxn)" reactions have no prominent maxima. This fact suggests that the main parts of the (p;pxn) reactions are owing to the direct process. On the other hand, the clear peak in the excitation function of the (p, ct3n) reaction

T Horiquchi et al.

150

TABLE 2. Yields of SlRb for various nuclear reactions

Nuclear -eaction SSRb + p---. 81Rb SSRb(p, 5n)S~Sr ~ a~Rb Rb + p---, S~Rb :~Br(~t, 2n)al Rb :gBr(~, 2n)81Rb 81Br(~, 4n) 81Rb S~Br(3He, 3n)8~Rb 7~Br(3He, n)SlRb S°Kr(3 He, pn) 8 ~Rb S°Kr(d, n)S~Rb

Target material 100~ enriched 8-SRbCI (thick) 100% enriched 8-SRbCl (thick) RbCI (thick) NaBr (thick) Cu 2Br2 (thick) NaBr (thin) NaBr (thick) 37'~,, enriched 8°Kr 37°:,, enriched 8°Kr

Projectile energy (MeV)

S~Rb yield at LOB (mCi/~A - ~ h - ~)

Reference

70 70 70 30 30 50 30 21

31.2 (39.0)~ 2.3 b 22.5 (28.1)" 2 2.5 2 2.9 O.O35

This work This work This work 4 4 4 4

20 8

0.226 0.7

4 4

Maximum value of the growth decay curve of alRb at 65 min after LOB. h Pure 8JRb extracted in carrier-free form at 96 min after the chemical separation of S~Sr.

indicates that the c o m p o u n d process is d o m i n a n t compared with the direct process for this reaction. Isomeric cross section ratios of the (p, pn) and (p, p4n) reactions are plotted in Fig, 9. The spins and parities of the ground and the isomeric states of 84Rb are 2 - and 6 +, and those of 8~Rb are 3/'2- and 9/2 +, respectively. The cross sections of the large spin isomer are smaller than those of the ground state for the (p, pn) reaction, and the ratio is constant from 25 to 70 MeV. However, the cross sections of the large spin isomer are larger for the (p, p4n) reaction, except in the lower energy region. The nuclear reactions to produce 8~Rb are summarized in Table 2. The experimental values except present results are quoted from recent reviews of CLARK et al. ~4~ When a natural rubidium target is used, the yield of S~Rb decreases to 72.2~0, which is the percent abundance of 85Rb. The yields of the other nuclides except S~Sr, a~"Rb and a~Rb increase by the nuclear reactions on 87Rb. The 8lRb yields of proton induced reactions obtained in this work are more than one order larger than others. Moreover, the 82"Rb contamination is inevitable for other reactions, but the S ~ R b isotope produced by the SSRb(p, 5n)SlSr(fl +, EC)8~Rb reaction can be chemically separated. The chemical separation was done with the same procedure described by SCHNEIDER et alJ 5~ A Rb2SO,, target in natural rubidium was b o m b a r d e d for 10 min at the proton energy of 70 MeV. The yield of chemically separated S~Rb isotope was 431tCi#A -~ for 1 6 0 m g c m - 2 target ( E p = 7 ( ~ 6 9 M e V ) at 2 h after EOB. The 82"Rb impurity was a b o u t 0.2#Ci/~A -~ and no other impurities were detected with the Ge(Li) y-ray spectrometer, s~mKr was extracted from 8ZRb aqueous solution by bubbling of oxygen gas for application to the nuclear medicine. No radioactive impurities were detected in the oxygen gas from 8~mKr generators with pure and unpurified 8~Rb solutions. Details of this method of the a~mKr generator will be published elsewhere. In conclusion, the proton reaction on Rb is a very

useful method for producing 8~Rb. Numerical values of the experimental results are available from Nuclear Data Centers. Acknowledgements--The authors wish to thank the staff of the Research Center for Nuclear Physics, Osaka University, for their co-operation.

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