The helium-jet technique for inaccessible sources of 252Cf fission fragments and 228Th decay products

NUCLEAR

INSTRUMENTS

AND

METHODS

THE HELIUM-JET

I i 5 (i974) 419-424;

TECHNIQUE

© NORTH-HOLLAND

PUBLISHING

CO.

FOR INACCESSIBLE SOURCES

O F zs2cf F I S S I O N F R A G M E N T S

A N D 22STh D E C A Y P R O D U C T S

H. G. WILHELM*, H. JUNGCLAS?, H. WOLLNIK*, D. F. SNIDER*, R. BRANDTt and K. H. LUST* A S T E R I X Collaboration +, Giessen/Marburg, Germany

Received 12 June 1973 A He-jet system has been developed for the transport of 252Cf fission fragments and 228Th decay products. The essential cluster formation occurs in a cluster breeder separated from the radioactive source. This cluster breeding is accomplished by adding liquids such as H20 or CCl4 to the transport gas and then blowing the mixture past an UV lamp. The efficiency of the transport

from the radioactive source to the end of the capillary ranges between 50 and 80%. The efficiency of the skimming process alone is about 50%. K X-ray spectroscopy indicates that the transport of activity is largely independent of the particular element.

1. Introduction

in a cluster b r e e d e r separated from the source. The dependence o f recoil adhesion to the clusters as a function o f element n u m b e r Z was investigated using a Si(Li) detector for K X - r a y spectroscopy. The cluster breeding technique described here opens the possibility to have t r a n s p o r t from any inaccessible area, for instance, the vicinity o f a reactor core, since only a small He-filled c h a m b e r must be placed at this location with connecting tubes for the He flow. Using this t r a n s p o r t technique one could p r o d u c e very strong sources o f short-lived fission fragments or activated nuclei outside the r e a c t o r shielding. A further application could be the c o n c e n t r a t i o n o f m a s s - s e p a r a t e d nuclei behind a recoil s e p a r a t o r to a p o i n t source.

The helium-jet t r a n s p o r t m e t h o d has been used succesfully in the t r a n s p o r t o f a c c e l e r a t o r - p r o d u c e d nuclear r e a c t i o n p r o d u c t s 1) a n d also recently for fission f r a g m e n t s f r o m 252Cf 2) or 14 M e V n-induced fission o f uranium3). I n such a system the recoils are s t o p p e d in a helium filled gas c h a m b e r a n d then swept t h r o u g h a c a p i l l a r y to an e v a c u a t e d collection c h a m b e r (fig. 1). The exact t r a n s p o r t m e c h a n i s m is still unclear. It has been shown, however, t h a t small a m o u n t s of impurities in the helium lead to the f o r m a t i o n o f clusters in the strongly reactive a t m o s p h e r e o f an accelerated particle b e a m or o f tile UV light o f a c a r b o n arc 2' 4). The radioactive nuclei are a t t a c h e d to these clusters which can be as heavy as 108 a m u 5). This p e r m i t s the s e p a r a t i o n o f the b u l k o f the helium f r o m the r a d i o a c t i v e nuclei by a s k i m m i n g technique (see fig. 6). T h e p u r p o s e o f this article is to describe a high-yield helium-jet system for 252Cf fission fragments a n d ZZSTh decay p r o d u c t s . In these cases no highly ionizing a t m o sphere exists as in an accelerator target c h a m b e r and thus n o r m a l l y no clusters are p r o d u c e d . In earlier investigations 2) the clusters were p r o d u c e d by means o f the intense U V r a d i a t i o n f r o m a c a r b o n arc at the site o f the r a d i o a c t i v e source. W e replaced the 600 W carb o n arc by a 2 0 W m e r c u r y l a m p a n d f o r m e d the clusters

2. Experimental set-up

The central p a r t o f o u r He-jet system (fig. 1) is an external cluster breeder which consists o f a v a p o r mixer a n d an U V light source. This system allows the p r o d u c -

Source chomber

Collection chQmber

mixer

Roots pump

~He

* If. Physikalisches lnstitut, Justus Liebig-Universitfit, Giel3en, Germany. t Kernchemie, Fachbereich 14, Philipps-Universitfit, Marburg, Germany. + The Asterix Collaboration is a joint effort of nuclear physicists and nuclear chemists in Gie6en and in Marburg to construct a He-jet system coupled to a mass separator for the GSI (Darmstadt). 419

Cluster breeder Fig. I. Schematic presentation of the He-jet transport system. Clusters formed by UV light in the breeder are blown into the source chamber. Presumably thermalized fission fragments attach to the clusters which are swept with the helium into the collection chamber.

420

H.G.

WILHELM

et al.

SQRT

216 PO Q: UV- light

800 -

on

b: UV- tight o f f

400 c 23 o 0

200

/

212 Bi

80

;

212 PO

a

40 2a 6 0

5

6 AtphQ

7 energy

8

9

[MeV]

Fig. 2. A l p h a spectra o f He-jet collected activities. A n a n n u l a r surface barrier detector is registering decay products o f +28Th. U s i n g the cluster breeder with UV light a p r o n o u n c e d increase o f the elaPo activity is observed. (a) UV light on; (b) UV light off.

tion of a desired vapor mixture with the helium gas. This mixture then enters a reaction chamber in which an intense UV radiation is generated by a low-pressure mercury arc lamp. The UV light generates break-up reactions of the vapor molecules and causes presumably the formation of large clusters. The clusters are carried by the helium stream to a source chamber containing 0.7 fig of 25zCf. The californium is covered by a 1.3 mg/cm 2 aluminium foil in order to prevent self sputtering of the californium. In the 2 000 cm 3 source chamber (fig. 1) the fission fragments are thermalized by helium at a pressure of 1 atm and the fragments become attached to the clusters. The off

UV light on LO0 ,

on

off

t

---

i

I

i

216 P o

3. Experimental procedure and results for the transport of decay products of 228Th

I !

+

i

Vt,

;I#j

200 ] I

:

8 ~ 100

0 -~'+"~" °,° ~'J 0 1

2

3

t,

/

;!

t

i

:1

5

6

7

gas from the source chamber is swept through a 5 m long polyvinylchloride capillary having a 1 mm inner diameter to a collection chamber which is pumped with a 2 000 m3/h roots blower. The transported fission products are collected on a mylar foil. The resulting Yactivity is monitored by a NaI(TI) scintillator. Exact efficiency measurements are performed "off-line" using a methane flow counter for #-particles. For the study of 228Th decay products, the experimental arrangement was very similar. The source chamber in this case was only 50 cm 3 containing 1 mCi Z82Th coprecipitated on Fe(OH)3, thus yielding a 22°Rn emanating source. The transported activity was collected on a plastic foil. An annular surface barrier detector measured emitted c~-radiation.

8

Fig. 3. T i m e distribution o f 216po activity transported with the He-jet. T h e UV lamp of the cluster breeder is switched on and off.

Before we used the He-jet system for the transport of fission fragments, the transport of the decay products of 228Th was investigated. Using pure helium without additives or UV light only the noble gas 22°Rn was transported, its a-line of 6.29 MeV was very weak. However, the decay products 212Bi and 212po were observed (curve b of fig. 2). After passing thc He through water and switching on the UV lamp in the cluster breeder, a particularly strong enhancement was observed for the 216po line, indicating a high transport efficiency for this nuclide (curve a of fig. 2). In order to study the action of the cluster

HELIUM-JET TECHNIQUE

FOR INACCESSIBLE

breeder, the intensity of the 2 1 6 p o c~-radiation was used as a monitor. As can be seen from fig. 3, only the combined action of additives and UV light led to a significant increase in the transported activity which after a few seconds came to a maximum. This is the time necessary to reach equilibrium in the cluster formation and indicates the direct transport of 0.15 s 2 t 6 p o .

4. Experimental procedure and results for the transport of fission products from 252Cf When pure helium gas alone was blown to the chamber containing the radioactive source, no activity was transported to the collection chamber. With the combined action of additives and UV light, a significant increase in transported activity occurred which reached saturation after a few minutes. Part of this time is necessary to reach equilibrium in the cluster formation and part is due to the build-up of longer-lived radioactive daughter products. We used as additives water, carbon tetrachloride, trichlorethylene, and ethanol. Transport efficiencies of up to 80% were observed with C C I 4 ; in this case the helium gas contained about 5% CCI, per volume i.e. about one-half the saturation pressure of this gas under these conditions. Other results are summarized in table 1. It is of interest that even tobacco smoke injected into the helium gas caused a significant increase of the transported activity. The transport efficiencies were determined by collecting fission fragments on a mylar foil in the collection chamber for 15 min. The fl-activity of this foil was then determined "off-line" with a methane flow counter. The number of counts in the fourth minute after the

L>

eto

counting

g:

~J

Conecting , , s

lo

Is

TABLE l The effect o f additives in the helium gas on the transport efficiency o f ~a2Cf fission fragments. Additive

Number of trials

(pure helium) water carbon tetrachloride trichlorethylene ethanol

(3) 5 27 7 18

Transport efficiency (%) maximum average

(0.1) 39 84 53 86

21 :t_ 10 50 ± 10 29~: 11 32± 8

collection period was monitored (fig. 4). As reference, a 40 [~m Al-foil was placed next to the Cf source and the activity was collected for 15 rain. We assume that the/#absorption in the aluminium foil is negligible and that the elemental composition and thus the mixture of different fission product half-lives does not change during the He-jet transport. In order to test whether different elements were transported with different efficiencies, the elemental composition of the samples was analysed by K X-ray spectroscopy. These spectra were acquired for 1 000 s starting with the seventh minute after the collection period (fig. 4) using a Si(Li) X-ray spectrometer with an energy resolution of 250 eV fwhm at 20 keV. In fig. 5 two such spectra are shown, one for fission products collected directly from the 252Cf s o u r c e and one collected with the helium-jet system. Each element Z has been identified from its K~, line, which indicates that either the element Z or its precursor elements had been collected. The ratio of the peak areas for each element in figs. 5a and 5b is defined here as the "transport efficiency". It has been determined relatively accurate for nine elements and is shown in fig. 5c, normalized to Nd. As the K X-ray spectra in figs. 5a and 5b are not taken for identical sources, the term "transport efficiency" should be used with some caution. Nevertheless, it appears that this "transport efficiency" varies only slightly with element number Z.

5. The skimmer: experimental set-up and results

r4,

I

421

SOURCES

I

20

Time

'/IIII//IIIIIII/III/I////Z/,/~ I I =s

30

3s

40

Emin]

Fig. 4. Graph illustrating tile time relationship o f our standardized procedure for recording the /J-activity and the X-ray spectrum.

The purpose of skimming is to separate the bulk of the helium from the clusters to which the activity appears to be attached. Because of their momentum, the heavy clusters tend to remain on the axis after leaving the capillary while the lighter helium atoms are pumped away (fig. 6). Consequently, a cone with a hole in the

422

H.

G.

WILHELM

et

Etement number 35 ~0 I , ~ , , I i 400

~ i

45 I I

I

i

i

5Q [ I

i

I

l

al.

Z=Z(K~) 55 I

I

55 I

6O I

I

i

I

I

I

1

u~ 300

o (-)

NORM 200

100

Br

Rb Kr

ul

3OO

o 0

200

Y Sr

Nb Zr

Tc

Rh

Mo

Ru

A~j Pd

It', Cd

Sb Sn

]

Cs

Te

Xe

L~ Ba

Pr Ce

pm Nd

Eu Sm

Gd

He-Jet

I00

16

12

20

L4

2L 28 32 36 K x - r e y energy [KeV]

,~ ,I00

(.>

50-

Tc Ru

0 u') E C~ I

35

I

I

I

I

I

I

~

I

Ag

Te Sb

l

I

i

I

i

'I

i

La

Nd

J i

i

Pr i

I

~

~

i

,

l

45 5O 55 60 40 Element number of fission product Z=Z(K~I)

I

J

I

65

Fig. 5. X-ray spectra of collected 252Cf fission products acquired for 1000 s starting 6 rain after the end of irradiation (see fig. 4). a) The recoiling fission fragments strike an aluminium catcher foil. The corresponding X-ray spectrum is called Norm. b) X-ray spectrum of fission fragments which are collected with the He-jet transport method, c) The ratio of corresponding peak areas of the two spectra (a) and (b) is plotted as a function of the X-ray energy indicating approximately the transport efficiency for different elements Z. Further details are given in the text. center which is aligned in respect to the capillary can be used to s e p a r a t e the clusters f r o m the helium. Such a cone is called an "Absch~iler" or a skimmer6). As w o u l d be expected f r o m s y m m e t r y considerations, g o o d s k i m m i n g efficiency is d e p e n d e n t on the precise axial a l i g n m e n t o f the s k i m m e r a n d capillarya'V). In o u r experiment a b o u t 20 to 80% o f the activity leaving the capillary passed t h r o u g h the skimmer. In fig.7 the total t r a n s p o r t efficiencyis plotted as a function o f the distance between nozzle and skimmer. This t r a n s p o r t efficiency behind the s k i m m e r is a p r o d u c t o f t r a n s p o r t

efficiencies for the helium jet and the partial s k i m m i n g efficiency. It is obvious t h a t for small n o z z l e - s k i m m e r distances m o r e activity passes t h r o u g h the s k i m m e r t h a n for large distances. On the other hand, for increasing n o z z l e - s k i m m e r distances one has the a d v a n tage o f a decreasing pressure behind the skimmer, i.e. m o r e efficient separation between the helium carrier gas and activity. A t the t o p o f fig. 7 the ratio between the pressure in the second a n d first v a c u u m c h a m b e r is indicated for different n o z z l e - s k i m m e r distances in o u r set-up. The large variations in the t r a n s p o r t e d activity

HELIUM-JET TECHNIQUE

9°pitta~

423

FOR I N A C C E S S I B L E SOURCES

iIq=3I'°~/~°~ K>

<2> Diffusio~ pump

Roots pump

Fig. 6. The skimming set-up. Leaving the capillary, the helium diverges quickly while the heavy clusters pass through the orifice o f the skimmer until they reach the collector.

Rotio ~/P1 1 1 I 30 50 100

;

0 50 T

1 ~.1

/ 0404

"

1

I

L, I

I

~

r , I

I

~

l

,

Fig. 8. Radiogram of a radioactive spot collected 16 cm behind the skimmer. A millimeter scale is shown. 50

1 150

I

diamet.~r d

~ ' L0-

I

~

~" "

{ ~..

ID

b:÷--L:'G:~

30E

[

~\\

O

75 20-

,

~

D:°'; L

~ 10-

o L~ 020-

"-. \-~

o

I j&

010-

o

0-

0

I

s

I •

5 10 15 20 25 3'0 Nozzle-skimmer distonce [mrn]

J

Fig. 7. Efficiency (determined by fl-counting as indicated in fig. 4) o f the He-jet transport system as a function of the nozzleskimmer distance. The quotient of the vacuum pressures behind and before the skimmer is indicated above the diagram. Data are taken from four separate runs. The two lines represent experimentally determined m a x i m u m and m i n i m u m efficiencies. A typical day's run is shown by the five circles.

(fig. 7) seem to be due to the instabilities in the jet system transport efficiency (see table 1) and not in the partial skimming efficiency. Indicative of good skimming is the presence of a radioactive spot on the collector foil which may be recorded using X-ray film (fig. 8). The well-defined diameter of the spot can be understood by assuming that all particles come from a point located at the end of the capillary and stay within a cone defined by the orifice of the skimmer. The darkening of the spot (fig. 8) in a figure-S pattern indicates an uneven distribution in activity and may result from a rotation of the gas stream in the capillary. Such a rotation could arise from a bend in the capillary just before the end.

I

10

I

is

[ nozzte-skimmer

2'0

2's

3'0

distance [mini

Fig. 9. The diameter of a radioactive spot on a fixed collector

position as a function of nozzle-skimmer distance. The spots are smaller than a cone of rays passing through the skimmer orifice (full line).

Also, we found for small nozzle-skimmer distances (l< 15 mm) that the activity is smaller in size than could be caused by the orifice of the skimmer. D
The authors thank H. Ewald, W. Walcher and H. Wagner for their support of this project. The preparation of the '-28Th source by P. Patzelt is acknowledged. The financial support of the Gesellschaft ffir Schwerionenforschung, Darmstadt, and the Bundesministe-

424

H.G.

W I L H E L M et al.

rium ffir Forschung und Technologie is deeply appreciated. References 1) R. D. Macfarlane, R. A. Gough, N. S. Oakey and D. F. Torgerson, Nucl. Instr. and Meth. 73 (1969) 285. 2) K. Wien, Y. Fares and R. D. Macfarlane, Nucl. Instr. and Meth. 103 (1972) 181. 3) H. Dautet, S. Gujrathi, W. J. Wiesehahn, J. M. D'Auria and B. D. Pate, Nucl. Instr. and Meth. 107 (1973) 49.

4) H. Jungclas, R. D. Macfarlane and Y. Fares, Radiochim. Acta 16 (1971) 141. 5) H. Jungclas, R. D. Macfarlane and Y. Fares, Phys. Rev. Lett. 27 (1971) 556. 6) E. W. Becker, K. Bier and H. Burghoff, Z. Naturforschung 10a (1955) 565. 7) W.-D. Schmidt-Ott, R. L. Mlekodaj and C. R. Bingham, Nucl. Instr. and Meth. 108 (1973) 13. s) D. F. Snider, H. Wagner, A. K. Mazumdar, H. Wollnik, H. Jungclas and H. G. Wilhelm, to be published.