Stripping experiments in carbon foils with heavy ions in the energy range of 0.4-0.9 mev/a

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Submitted on 1 Jan 1977

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Stripping experiments in carbon foils with heavy ions in

the energy range of 0.4-0.9 mev/a

G. Frick, V. Chaki, B. Heusch, Ch. Ricaud, P. Wagner, E. Baron

To cite this version:

STRIPPING

EXPERIMENTS

IN

CARBON FOILS

WITH HEAVY IONS

IN THE

ENERGY

RANGE

OF 0.4-0.9

_MEV/A

G.

_FRICK,

V.

CHAKI,

B.

_HEUSCH,

Ch.

RICAUD,

P. WAGNER Centre de Recherches

Nucléaires,

67037

_Strasbourg

_Cedex,

France

and E. BARON

Institut de

_Physique

_Nucléaire,

B.P.

1,

91406

_Orsay,

France

Résumé. 2014 Nous _avons étudié les

propriétés des ions lourds

_épluchés

par des feuilles de carbone. Des

ions _Ni, I et Au de _0,4 à _{0,9 MeV/A} ont été utilisés pour bombarder des feuilles de 5 à 200 03BCg/cm2. Au

cours de ces mesures les ions étaient observés à l’aide d’un _{spectromètre} Browne-Buechner.

Nous avons mesuré la

_{dispersion angulaire}

_{des ions ainsi que la}

_dispersion

en _énergie. Nous avons

étudié le _comportement des feuilles sous l’effet du bombardement par des faisceaux de _grande densité

de courant

_(quelques

_03BCAp/cm2

sur une surface de 1-2

_mm2).

Nous avons observé la variation de

l’épaisseur des feuilles au cours du bombardement pour une valeur de la

_pression

de _quelque 10-6 et

quelque 10-7 torr. Nous avons

_regardé

l’évolution de la _dispersion en

_énergie

durant les

_expositions

et

nous concluons _que ce _paramètre ne _{change pas de façon importante.} Ceci signifie que des phénomènes

comme

_{l’épaississement}

ou le _sputtering n’affectent pas _{l’homogénéité} de la feuille. Nous _rapportons

également des résultats sur la durée de vie des feuilles.

Abstract. - We studied the

_properties

of _heavy ions _{stripped by} carbon foils. Ni, I and Au ions of 0.4 - 0.9

MeV/A were used to bombard foils of 5 - 200

_03BCg/cm2.

In these measurements the ions were

detec-ted in a Browne-Buechner _{spectrometer.}

We measured the _angular

_straggling

of the ions and the energy straggling. We looked for the beha-viour of the foils under

_impact

of _large beam densities _{(several 03BCAp/cm2} on an area of 1-2 _mm2). We

observed the thickness variations of the foils _during bombardment in a vacuum of ~ 10-6 and 10-7

torr. We looked for the evolution of the energy straggling during exposure and conclude that this

parameter does not _change in an _{important way. This} means that neither

_thickening

nor

_sputtering

affects the _homogeneity of the foil. Results on the lifetime of the bombarded foils are _reported.

The work described in this _paper has been

_done,

in connection with the french

_heavy

ion accelerator

(Ganil) [1, 2],

to find out the

_properties

and the beha-viour under which

_they

will be used in this accelerator. The French

_heavy

ion accelerator consists of two

separated

sector

_cyclotrons

of

_strength

K =

400.

Heavy

ions accelerated

_by

the first

_cyclotron

will be

stripped

before

_injection

into the second

cyclotron

which will accelerate the ions to their final _energy. The

particle

energies

at the

_stripper

and the

_charge

states

required

are such

_that,

_except for

_light

ions

_(A

_40),

a foil will be _necessary for the

_-stripper

_[3,

_4,

_5].

’

Table 1

_gives

some characteristic numbers for the beam at this _stage. The _energy values

_El,

and the incident

_intensity

I of the ion beams at the

_stripper

necessary.

to obtain the maximum and minimum

required

values at the _output of the second

_cyclotron

are

_reported.

El varies from the,minimum value of 0.25

_{MeV/A (i.e.}

the

_energies

on the terminal of the

new

_generation

of

_tandems)

to the maximum number between 0.5

_MeV/A

₍₁₁₉

_MeV)

for uranium and 6

_MeV/A

_{(72 MeV)}

for carbon. The _energy

_dispersion

should be between 0.4 - 1 x 10-3 at the _output of the second _stage

_according

to the différent cases. In order

to obtain the

_{required intensity}

of 0.1 - 1 x 10 12 _pps

at the _output, the

_intensity

on the

_stripper

should be

between 2 and 17 x 1012 _pps, which

_corresponds

to a

particle g

A _range.

Finally

the

_requirements

in

thé

emittance lead to

numbers at the

_stripper

level between 10 n and 20 1t

mm . mrad.

For

instance,

for 131Xe

_ions,

E = 1.6

MeV/A,

I = 1013 _pps

_(1.6

_{gaps -}

10 1t,

the

_{corresponding}

beam would have a

_divergence

of 2

mrad,

for a diameter of 1 cm if it has a circular

_shape

on the carbon foil.

Since the

_beginning

the Ganil

_Group

has been faced with

_{intensity questions.}

_Knowing,

that the

_intensity

at the _output should have fixed emittance and _energy

dispersion,

any deterioration of the beam

quality

during

acceleration has a _consequence on the

strip-ping

regime. Especially following questions

arise:

1)

What is the

_angular

_straggling

of the ions with

energies

and masses

_{corresponding}

to the Ganil

pro-ject ?

There existed measurements for

_lighter

ions at

1526

TABLE 1

q,

charge

state

_of

the ion source.

_El’

ion _energy

after

the

first cyclotron.

qs,

average

charge

state

of

the ions

_after

stripping.

qs,

charge

state

_{selected for injection}

into the second

_cyclotron.

11s’

stripping

probability for

the

_charge

state _{qs. I incident,} beam

_intensity

on the

_stripper.

_E2

and I, _energy and maximum

required

intensity

of

the ions

_after

the second

_cyclotron.

smaller

_energies

where theories have been

_developed.

Do these theoretical

_{predictions apply}

to the situation of Ganil ?

2)

What is the _energy

_{straggling keeping}

in mind the same remarks as for 1.

3)

What are the lifetimes of the

_stripping

foils or more

_exactly

how do

_they

evoluate with time?

4)

Is there a relation between

_{equilibrium -}

thick-ness and lifetimes as has been

proposed?

1.

_{Expérimental}

methods. - Particles of the

requi-site

_energies

were obtained from the Van de Graaff

MP tandem accelerator of

_Strasbourg

with

_energies

between 0.5 - 1

_MeV/A

on carbon foils

_having

a

thickness in the _{range 5 -} 200

_J.lg/cm2.

Ions

_Ni,

1 and Au have been chosen. Ni because it has a mass

com-parable

with others

already

studied

I,

which is an

intermediate mass ion and Au because it is the

hea-viest

_projectile

that can be

_easily

obtained.

The

_straggling

and

_equilibrium

thickness

experi-ments

_{(Fig. 1)}

needed

_only

small intensities but we

demanded a

_high

beam

_quality.

_So,

with

_help

of

FIG. 1. -

Diagram of the experimental setup.

collimators,

we had a beam

_dispersion

of 0.5 mrad. The measured _energy resolution was 5 x 10-4. Even with these small intensities we had to use in most cases

the _gas

_stripper

at the MP

_terminal,

as the lifetime of

the terminal foils was too short. The main _apparatus of the

_experiment

was a Browne-Buechner

spectrome-ter

_(K

=

30).

The

resolving

power can attain the value

_1800,

but the _magnet was

_{incapable of deflecting}

the incident I beam

_{(q = 10+)}

onto the focal surface and

_only

the transmitted beam

_{(q N}

₂₀₊₎

could be observed. Foil thickness were determined

_by

measur-ing

the

_energy-loss

of a

particles,

within an

experi-mental error of ±

_{3 03BC}

_g/cm2

and no evidence for

non-uniformity

of the foils was observed. After

stripping,

the

_particles

_having

an

_angular

and an _energy

disper-sion,

were detected

by

a

position

sensitive detector or

by

nuclear

_emulsions,

on the focal surface of the spectrometer.

Thus,

the

_particles

were selected not

only

as a function of their _energy, but also of their

charge

state.

_Very

weak beam

_intensities,

of the order of 10-6

_{particle - 03BCA (corresponding}

to a current

density

of 109 to

_1010pps/cm2)

were used.

_During

the

runs, which were sometimes

_longer

than a few

_hours,

no

_change

in the behaviour of the foils was observed.

The

_{angular dispersion}

method was the

_following.

The well-known Browne-Buechner

_spectrometer

has the

_property

_{of focalising}

on a

_hyperbolic

surface with

the energy and

charge

state selection

_along

a direction

called. here the x

axis,

but there is almost no

focalisa-tion in the

_{perpendicular}

direction. A

_position

sensi-tive

_detector,

45 mm

_long,

was

_placed

on the focal

surface with its

_position

direction

_along

the x axis.

_By

varying

the

_field,

the

_particle

_groups

_{corresponding}

to

the different

_charge

states could be

_brought

into the

region

covered

_by

the detector. The resolution in

position

of the counter was 1 mm. The detector was

x = 45 mm, y = 0.5 _mm. For _a

given

charge

state, the

angular

distribution was measured

_observing

the

counting

rate as a function of the

_angle

between the beam and the _spectrometer direction.

In another method also

used,

the

_position

sensitive detector was turned

_{perpendicular}

to that

_just

descri-bed,

where now the

position

measurement is

_directly

related to the

_{scattering angle.}

We have verified that the two methods

_give

consistent results.

Using

the same

_equipment,

we have measured the

energy

straggling.

The

position

sensitive detector was

placed

along

the focal surface of the spectrometer.

The

_{peaks corresponding}

to the different

_charge

state

were observed

_{by varying}

the field. The value of the

straggling

5E was determined from the full width at

half maximum of these

_{peaks (FWHM), taking}

into

account corrections due to the _energy fluctuation of the incident beam and the intrinsic resolution of the

apparatus.

Instead of a

_position

sensitive

_detector,

nuclear emulsions were also used. In this _way the whole

spectrum of

_charge

states could be obtained in a

_single

exposure. This method is more reliable for certain

aspects,

though,

on the

oiher hand,

the results are

obtained after a

_long

and tedious

_scanning.

From the _energy calibration of the _spectrometer the total _energy loss AE in the _targets was obtained

simultaneously. Combining

these DE values with the thickness _{measurements,} we found an _agreement

within 5

_%

for the

_dE/dx

values tabulated

by

North-cliffe and

_Schilling

_{[6, 7].}

For the

_charge

state measurements which are

rela-ted to the

_equilibrium

thickness studies we used the

same instrumentation with the foil in the

_object

posi-tion of the

_{spectrometer.}

The

_counting

rate variations in the

_detector,

versus the

magnetic

field was done

automatically using

a

_{multiscaling technique}

which made these measurements fast and _easy.

For life-time measurements we had to use the

largest

current

_density

available from the MP i.e. several

_yAp/cm2

for Ni and 1 on an area of 1.5

mm2,

consistent with the Ganil

_project.

But of course these

densities will occur on 100 mm2 area in the Ganil machine. In the case

_{of Au,}

the

_intensity

was 20 times less. For these

_experiments

the

_exposed

foil was not in the _spectrometer but several meters _upstream, in

_Ci

1

(Fig.

2),

inside a _target box with a vacuum of several

10-6 torr. We added a cold

_finger

and so the vacuum

FIG. 2. -

Experimental setup for the lifetime measurements.

could be lowered to some 10-7 torr. The beam ions

going through

the

_target

_{C 1}

showed a

_change

in

charge

state and

_undergo

an increase of

_angular

and

energy

dispersion.

This

stripped

beam,

after

again

collimation,

hits a second foil

_C2

_(8-10

_yg/cm2)

posi-tioned in the _spectrometer

_object

_position.

This second

_target

_represents a well defined

_object.

As

there is no

_change

in the beam _{energy between}

_C

₁ and

_C2

the

_charge

_spectrum does not

_change

at the second

_stripper.

The

_diaphragms

_d21

and

_d22

extract

from the

_{angular enlarged}

beam a

quasi-parallel

_part on

_C2,

where a new

_{angular straggling}

takes

_place.

The

_intensity

on

_C2

is 100000 times smaller that

on

_C

1 and so

C2

does not

undergo

any alteration.

The detector

_placed

on the focal surface enables the

energy

straggling 8

E to be measured. These values are

composed

of the

_incoming

beam _energy

_dispersion,

due to the two foils

_Cl

and

_C2

and

_by

several less

important

instrumental effects. Therefore we could

determine 8 E

_{corresponding only}

to

_CI,

If we

com-pare the measurement with and without

_C

_in

_place,

we can also

determine,

with _great

_precision,

the energy loss in the foil and its thickness. This method

was

compared

with the a

_particle

_energy loss method

indicated earlier. With this

_experimental

_layout

we

could measure

during

the time of

_bombardment,

the

intensity

of the incident

beam,

the energy loss

in

a

_foil,

its thickness

_variation,

the _energy

_straggling

and its variation in time. The

_{angular straggling}

could not

be

observed,

but the variation of

intensity

after

straggling

gave an indication of the

_importance

of this

_phenomenon.

2.

_{Expérimental}

results. - 2. 1. ANGULAR

STRAG-GLING

_(1).

- Theoretical calculations has been

given

in several _papers,

_by

L.

_{Meyer [9],}

_Sigmund

and Winterbon

_[10]

and others. In these theories

only

atomic collisions are taken into _account, because

electronic excitations

_{give only}

a small contribution to

the

_{angular straggling. Generally,}

the

_experimental

results obtained elsewhere

_{[11, 24]}

are considered to

be in _very

_satisfactory

_agreement with these

predic-tions. However these

_experiments

were

_performed

in

lower _energy

_regions,

and _moreover,

_{using lighter}

ions. It should be noted that neither in the

_theory

given by

Meyer,

nor in the

_experiments

_quoted

_above,

was _any distinction made between the different

_charge

states after

_stripping.

The

_following

conclusions have been found in the

present

work

_{(Fig. 3).}

The

_right

order of

_magnitude

for the values

_{predicted by}

the calculations of

_Meyer

has been obtained within some limitations however.

Firstly

the

_{angular dispersion}

increases with the

charge

state after

_stripping.

This

_phenomenon

is shown in

_figures

4 and

_5,

where an increase of 15 to

1528

FIG. 3. - Values of the

angular dispersion

81/2

(HWHM) as a

function of the foil thickness. The

81/2

values have been corrected .

for the incident beam width and the _spectrometer aberrations. Uncertaintities on the

_81/2

values are less than 10 %.

FIG. 4. - Values or’ the

angular dispersion

_03B81/2

(HWHM) versus

the _charge state for different foil thicknesses.

lowest and the

_{highest ;charge}

state

_analysed.

Secondly,

the _agreement is better for

_foils

thicker than the

_equilibrium

thickness. For thinner

foils,

even

though

the uncertainties introduced

_by

the corrections

four

beam width and

_spectrometer

aberrations

increase,

it _appears that the

_{scattering angle}

becomes much

_larger

than

predicted.

For the thinest foils the theoretical

_predictions

do not hold at all. E.

_{Efken et}

al.

_[22]

observed a similar result in their

angular

straggling experiments

with _gas

_stripper.

FIG. 5. - Values of the

angular dispersion

_81/2

(HWHM) versus

the charge state for différent foil thicknesses.

2. 2. ENERGY STRAGGLING. - The

energy loss and the _energy

_straggling

are

_due,

at the

_energies

conside-red

here,

mainly

to the electronic excitation of the

atom. N. Bohr

_{[25] [26]}

has studied this _{for many} years and

recently

Vaviloy [27],

Tschalär

[28, 29]

and Clarke

_[30]

in

_particular

calculated this

_phenomenon

for thin foils. All this authors used

_only

collision cross

sections,

but more

_recently

0. Vollmer

_[31]

_]

V. V.

Avdeichikov

_[32]

and B. Efken

_[33]

used

_charge

changing

cross sections

_leading

to

_larger

values of8 E. Our results are shown in

figures

6 and 7

along

with

the theoretical

_predictions

of Clarke. We see a

_large

discrepancy

which could be

_explained

_by

the

_charge

exchange

cross section.

These

_divergences

can also be due to foil thickness

inhomogeneities.

Indeed variations of ± 1

_03BCg/cm2

lead

to an _apparent

_dispersion

of 60 KeV in the case of I. As

_pointed

out

_before,

we think that

_thèse

inhomo-geneities

are small. The

_experimental

values in

_figure

7

are corrected for a natural width that we observed

with a 2

J.lg/cm2

foil. This width include instrumental

effects,

beam width and

_{possible inhomogeneities.}

We observed also that we could

reproduce

our results

displacing

the

_position

of the foil.

_(see

also

_[34]).

2. 3. LIFETIME AND BEHAVIOUR OF FOILS DURING BOMBARDMENT. - For this

_experiment

we used a

large

beam,

typically

around 50

_nAp

and a beam

density

of several

_J.lAp/cm2.

The bombarded foils are

_destroyed

_by

irradiation

damage

and also

_{by sputtering}

and

_heating

effects. The irradiation

_damage

is related to the number of

displaced atoms, which is

proportionnal

to

_Z2/E/A,

the current

_{density Il}

and the time. Measurements have been done in a normal vacuum

_(several

10-6

_torr)

and also in a

_good

vacuum

_(~

10-7

torr).

Table II

_give

results of _{24 exposures,}

_using

foil-thicknesses between 20 and 50

_yg/cm2

and times between 1 and 6 hours.

The results of our

_experiments

are classified as

FIG. 6. - Measurement of the

energy straggling. Spectra

obser-ved from the _position sensitive detector. ô E has been calculated from width of this _spectra.

vacuum and in

_good

_vacuum, the evolution of the

energy

straggling

and the

photographic

observations.

During

bombardment with Ni and 1 in the normal

vacuum we observed a

_thickening

proportional

to the ’ time as well as to the

_{incoming charge (number}

of

particles)

and also to the _energy

_dissipation

in the foil. If we _try to make a

_synthesis

of the results we calculate a

_thickening

rate of 0.30 ± 0.20

_{yg/cm2/mC/cm2}

corresponding

to 2.1 ± 0.2

_{03BCg/cm2/hour.}

Even with the _very small Au

_beam,

we observed a considerable

thickening

rate

_(~

₄₉₎

if we

expressed

it versus

mC/cm2

but this

_corresponds

to

_only

2

_{03BCg/cm2/hour.}

We observed also a saturation.

This

_thickening

seems to be a _very well known

phenomenon

rising

from

_{hydrocarbon cracking}

in the residual vacuum.

_{By cooling,}

which

_brings

the vacuum

down

_by

a factor

_10,

we can

_hope

that this carbon

deposit

will

_disappear.

FIG. 7. -

Experimental values of the _{energy straggling} ô E

(FWHM). The dashed curve is a _guide to the eye whereas the solid line _correspond to the theoretical predictions of Clarke.

In fact this is

observed,

in the case of the Ni beam: there is no

_longer

a

_thickening

and even a diminution

in thickness

_probably

due to

_sputtering.

But it is different for

I,

where we continue to

observe a

_thickening

rate of 0.17 ± 0.08

yg/cm2/mC/cm2

or 2.3 ± 1.7

_{J.lg/cm2/hour.}

This

growth

is smaller but still

_important,

if we consider

that the

_deposit

due to the

_cracking

is

low,

as is

sug-gested by

the Ni

_experiment.

This can

_perhaps

be

explained

by

a

migration

connected with a structure

change

in the carbon:

Several other groups

reported

similar conclusions:

see Whitmell et al.

_{[35, 36]}

and Yntema

_[37]

for

ins-tance. This last author

_{proposed heating}

the foils to

increase the life. In our case we calculated that

_during

the bombardment the

_temperature

was between 600 OC

and 800 OC

_(for

1 and

_Ni)

and

_consequently

we did not

heat the foils from an external source

_(see

also

_[38]).

In the case of Au beam we saw a

_large

thickening

even with the

_good

vacuum but after one hour there was a saturation and a much smaller

_growth

rate. . But in fact this

thickening

is not related

directly

to a

lifetime,

if we can

adjust

the accelerator in such a _way

1530

TABLE II

2)

Thickness Ax in

_J.1gjcm2,

*

means 2 month

_{old foils,}

otherwise we used new

_foils.

3)

Vacuum, C means normal vacuum ~

10-6.

9)

The

_impact

area was

_{determined from}

the

_{photographies.}

13)

0394e

_J.1g/cm2,

_{thickening of}

the

_foil

_during

_exposure.

14)

Thickening

rate

_expressed

versus the number

_of

incident

_particles.

15)

Thickening

rate

_expressed

versus the

bombardment

time.

16)

This

_product

is characteristic

_of

the inverse

_of

the

_lifetime.

,

18)

+ means that

_{the foils}

was

destroyed

at the end

_of

the

_experience.

end of the foil. We can _say that deterioration rate of this

_{quantity gives}

a number related to the

lifetime,

because the alteration of the _energy

_dispersion

of the beam leads to a diminution of

_intensity

after

_passing

thé

second _stage of acceleration and beam

_transport

system.

The conclusion of all our measurements is that the energy

straggling

varies

slowly

and this variation is

only

connected with the thickness variation of the foil. This allows us to _say that carbon

_{deposit, migration,}

and

_sputtering

are

_homogeneous.

Variations of

1

_yg/cm2

could be

_easily

observed. But in several cases

after a

_long

bombardment,

we had an

exception.

After a few hours of bombardment,

suddenly

in less

than 15

_minutes,

the ô E blows _up

_by

a factor of 5 or

more. This indicates

desintegration

of the foil

_prelude

to its destruction. Efken

_[33]

made a similar observa-tion.

In other cases, with 1

beam,

the foils

_break;

this is another _way to determine the lifetime of the foil. In

thé

case of Ni we could not

_destroy

the foil even

after

1

4

hours

bombardment whereas for 1 the lifetime was

between 3-5 hours. See also

₍₂₎

[39,

40].

Most of the foils have been

_{photographied (Figs.}

8,

9,

10)

and this

_report

shows several of this

_pictures.

We cannot see a difference for a

_good

or normal

(’) F. _{Selph, Berkeley, private} communication.

vacuum but we see in all cases a

blackening

at the beam

_{impact. Usually}

at this

_impact

the foil is

stret-FIG. 8. -

Photography of the foil _{corresponding} to the _experi- 1

ment number 16 of table II. The diameter of all foils shown here is 1

6 mm and the beam impact area was between 1.3 and 2 mm2. The

experimental conditions were: 20 J.lg/cm2, Ni 5+ 48 MeV

0.8 _MeV/A, 4 _{J.lAp/mm2, exposure time 3.7 hours,} vacuum 2

-6 x 10-7 torr. We do not observe a foil _thickening but we see a

FIG. 9. -

Photography of the foil number 2 of table II 20 J.1g/cm2,

1 10+ 110 MeV 0.87 MeV/A, 2.6 J.1Ap/mm2, exposure time

1.4 hours, 20 x 10-7 torr. In this case we observed a foil thickening.

The general aspect is similar as in the case _{of figure} 8. The _region on

the right of the main impact is due to a short (1 min.) exposure to the beam. The blackening appears immediately but not the folds.

ched and we see radial folds

corresponding

to the

tightening.

Older foils

_{(2 months)}

shows a different

picture

than new

foils,

a

_larger

number of foldsis in the

first case. The lifetime is not different but with older foils we observed the

phenomenon

of the _energy

straggling

blow _{up. We} see on other

_pictures

that the

very

tiny

Au beam does not

produce

folds but a

deformation of the

_impact

area.

FIG. 10. - Photography of _a two months old foil whereas figure 8

and figure 9 correspond to new foils. Experiment number 11 of Table II: _{41 Il g/cm2, 1} 7+ 64 MeV 0.5 _{MeV/A, 6.5 Il Ap/cm2,}

expo-sure time 1 hour, 4 x 10-7 torr. The observed _picture is

_cômpletely

different. We see _{many small folds} over the whole non bombarded

area. In the case of the use of old foils we observed the energy

straggling blow up.

Before we close this

_point

of our

study,

we whish to

report information from the G.S.I. Darmstadt

_(3),

(3) B. Franzke, private communication.

FIG. 1l. -

1532

according

to

_which,

foils were used in similar

condi-tions as for Ganil

_(Xe

beam,

several

_ppA

on 1

_cm2).

In their case it seems that the foils diminished in

thickness,

which means that

_sputtering

is more

important

than carbon

_deposit

or

_migration.

2. 4. CHARGE STATE SPECTRA. -

Figures

11 and 12 show the result of measurements of

_charge

state

spectra.

We have

results

for

I,

Ni and Au beams at

several

_energies.

We notice in

_figure

_12,

_showing

the

FIG. 12. - Variation of the

yield for the different _charge states

versus the foil thickness for Ni 5+ 0.83 MeV/A incident ions.

variation of relative intensities versus the thickness of

the

_foils,

that the curve

_{corresponding}

to the mean

charge

is

_roughly

constant whereas

_neighbouring

charge

states _vary

_greatly.

This leads us to reexamine the

_application

of the

_equilibrium

thickness

para-meter. One see that for the mean

_charge

_q, even a

thick-ness much smaller than for

_{equilibrium, gives}

_always

a

high yield.

Under these

conditions

we will have less

straggling.

We see also that for a

_large

thickness the

_yield

is not constant. This is due to the

_{angular straggling}

and the limited

_acceptance

of the

_{spectrometer.}

90

_%

of the

equilibrium yield

is obtained for a thickness of

15

_yg/cm2

for Ni at 0.85

_MeV/A

and of 25

_yg/cm2

at

1.53

_MeV/A,

for 1 we found 15

_yg/cm2

at 0.87

_MeV/A

and for Au 15

_yg/cm2

at 0.5

_MeV/A.

3. Conclusion. - The

angular

straggling

experi-ments showed that the theoretical

_predictions

of L.

_Meyer

can be used as a

_guide

for calculation.

For

_charge

states far from the mean

_charge

the

angu-lar

_straggling

is

_larger

_up to 40

_%

for a 15

yg/cm2

foil

for

_example.

For thinner foils the

_{angular straggling}

goes to a finite value and not to zero.

Concerning

the _energy

_straggling,

our

experimental

values are 3-4 times

_larger

in

_comparison

with the theoretical

_predictions

of P.

_Vavilov,

C.

Tschalär,

N. M. Clarke.

If the foils are bombarded with

_large

intensities

according

to the Ganil values

_(several

_yAp/cm2)

we

observe in a normal vacuum a

_thickening

due to the

hydrocarbon cracking.

In a

_good

vacuum

_(10-7)

the

thickening disappears

for

Ni,

but

_persists

in the case

of I or Au.

The _energy

_straggling

varies

_{slowly during}

bom-bardment. The destruction of a foil can be due to a

breakage

but also in some case

through

the _appearance

of a _energy

_straggling

blow _up.

As a last and somewhat

_speculative

conclusion we can _try to

_extrapolate

the lifetime from the observed values. For this we _suppose that it is

_proportional

to

the number of atoms

_displaced

and we _suppose also

that we can

_extrapolate

from a small area

₍₂

_mm2)

to

1 cm2 which _may be the beam size in the case of Ganil.

With all this

_restriction,

the lifetime of the foil _may be 3-17 hours for

_I,

much less for Au

_{(2-5 hours)}

and much more

_(14-120

_hours)

for Ni. This numbers

correspond

to the lower and

_higher

_energy limit of the Ganil

_project.

Acknowledgements.

- We thank Dr. B. Franzke

from the G.S.I. in Darmstadt for _many valuable discussions on the

stripping

foil

experiments.

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