ProblemsProblems The ratios e/m of 4He+ and D2

+ ions are very close:

mD2 = 4.0282 amu

mHe = 4.0026 amum = 0.0256 amu

A high resolution mass spectrometer should be required in order to separate the two peaks.

D2 concentration » expected 4He concentration

Capability of on line analysis during the electrolytic experiment

Conventional techniquesConventional techniques Dynamic analysis technique (differential

pumping or diffusion through large impedance circuits):

alter the original composition of the gas mixture; exhibit low sensitivity; do not allow for on line analysis.

Static analysis technique (Cryosorption pumps):

may erratically trap helium in the condensates; require a long cycle time and are not suitable for

on line analysis. Unable to pump hydrogen isotopes to high degree

Innovative solutionsInnovative solutions Non evaporable getter (NEG) pumps:

remove to a high degree all non inert components of the gas mixture (especially hydrogen isotopes); all inert gases are obviously preserved;

exhibit very high pumping speeds (especially for hydrogen);

the pumping speed for other active gases depends on the operating temperature of the getter alloy;

do not require frequent regeneration, allowing periodic analysis of gas samples during the experiment (roughly every 40 minutes).

A storage circuit has been purposely designed, which:

allows to periodically sample the gas mixture without influence on the electrolytic process;

eliminates any alteration of the gas mixture composition; allows to easily compare the results of the analysis of

each sample.

Main featuresMain features High resolution Mass

Spectrometer (Balzers QMA 410).

Two NEG pumps are used: SAES Capacitorr B 1300 (600 g of

ST185 TiV alloy) operating at high temperature (300 - 400°C)

SAES GP200 MK4 W (170 g of ST707 alloy) operating at RT.

Dynamic pumping: Main pump: Pfeiffer TMU 261

turbo molecular drag pump Backing pump: Pfeiffer TMU 071

+ MD4

No cryosorption pumps are used.

Automatic operation (Field-Point + LabView).

Pressure stabilized storage circuit.

Schematic layout of the gas Schematic layout of the gas analysis systemanalysis system

QMAUHV

chamber

Thermostatic Box

Cell

UHVcontainment

vessel

Storagecircuit

Samplevolume

HT NEGpump

n.o.

n.o.

n.c.

n.o. = normally open valve

n.c. = normally closed valve

Gas mixture inert gases

RT NEGpump

GV1

GV3GV5

High resolution spectra of a 4He-D2 mixture.

dwell time 1 s, scan width 10 amu, scan time for each cycle 10s.

Analog spectrum of an air sampleAnalog spectrum of an air sample

0 15 20 355 10 25 30 40 45 50

Mass (amu)

Ion c

urr

ent

(x1

0-8

A)

0

1

2

3

4

5

640Ar+

40Ar++