Molti lavori di geochimica abientale misurano Sr e Pb con Quadrupole ICP-MS (circa 150k€) detto anche “ICP-Massa”:Da non confondere con MC-ICPMS (cica 650k€)

•  No double focusing•  No magnet•  No multicollector•  Bad precision!!! (un ordine di grandezza peggio)

The quadrupole mass analyzer (or spectrometer) (QMS). It consists of four cylindrical rods, set parallel to each other. The quadrupole is the component of the instrument responsible for filtering sample ions, based on their mass-to-charge ratio (m/z). Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.

Quadrupole ICP-MS  

Three mass fragments enter into the quadrupole vacuum chamber. The voltage of the rods is set so that only the pink mass fragment passes completely through the quadrupole rod array and into the detector. The green and blue fragments are unstable at this voltage combination and their path eventually brings them into contact with the rods so that they never reach the detector.

Quadrupole ICP-MS :

Detector à Secodary Electron Multiplier

•  Lo strumento fa una scansione leggendo sequenzialmente tutti gli elementi

•  Ottimo per misurare le abbondanze degli elementi: per ogni elemento di interesse si passano una serie di soluizioni di standard a concentrazioni note e si fa una retta di taratura per quell’elemento dalla quale si ricalcola la concentrazione dell’elemento nel campione “unknown”.

•  Si possono misurare anche rapporti isotopici (meglio se tra stesso elemento) ma si deve considerare•  Interferenze (87Rb su 87Sr)•  Risoluzione dei picchi (e.g. 207Pb e 208Pb si sovrappongono?)•  Elementi diversi possono avere trasmissioni diverse

ALTRE  TECNICHE  ANALITICHE    

•  87Sr/86Sr ; 143Nd/144Nd; 20xPb/20yPb ; (230Th/238U) à Mass spectrometers (TIMS o MC-ICPMS)

•  Isotopi antropogenici, molto poco abbondanti e con tempi di dimezzamento molto brevi (e.g. Pu; 90Sr ; 137Cs; 210Pb; 236U etc) à si misura direttamente la Radiazione, quindi il tasso di decadimento e da quello si calcola la quantità di moli presenti.•  α counters•  β counters•  γ counters

Bq  =    Becquerel  =  decay/seconds  

Porcelli  &  Baskaran  (2011)  

Porcelli  &  Baskaran  (2011)  

• Alpha particle energies are discrete, leading to a single peak in the alpha emission spectrum.• The alpha particle energies of many radionuclides are close together and may be difficult to resolve using a solid state detector.• Sample purity for the radionuclide of interest is very important to successful analysis.• The radiochemist can adjust conditions to get the optimum purity and peak resolution in the alpha particle spectrum.

Good  energy  resolu<on  

•  È necessario purificare il campione prima della misura (anche per β counting)

•  È necessario depositare il campione possibilmente in uno strato di spessore infinitesimo (anche per β counting)

•  È necessario aggiungere uno spike a concentrazione (à ritmo di decadimento) noto per calcolare la counting efficiency (in pratica si misura per ID)

Porcelli  &  Baskaran  (2011)  

Porcelli  &  Baskaran  (2011)  

Coun%ng  efficiency  =  par%cle    counted/par%cles  emi7ed  

N  =  0.0001/2.2  x10-­‐13    =  4.55  x  107  atomi  àdiviso  per  N  avogadro  (6.022x1023)    à  8  x  10-­‐17  moli  à  0.08  fmol  

λ  (yr-­‐1)  =  ln(2)  /  T1/2  

T1/2  (yr)  à  T1/2  (sec)    T=  1  x105(yr1)    à λ =  2.2  x  10-­‐13  

(sec-­‐1)  

0.01mBq  =  0.0001Bq  

Porcelli  &  Baskaran  (2011)  

Coun%ng  efficiency  =  par%cle    counted/par%cles  emi7ed  

Semiconductor dectectors

•  Ionizing radiation is measured by the number of charge carriers set free , by the radiation, in the detector material which is arranged between two electrodes. Ionizing radiation produces free electrons and holes.

•  The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor.

•  As a result, a number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band.

•  Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit.

1-­‐10  days  of  coun<ng!  

Porcelli  &  Baskaran  (2011)  

Porcelli  &  Baskaran  (2011)  

•  Geiger counter à No risoluzione energetica (anche perché le β hanno energie variabile) à purificazione campione

Porcelli  &  Baskaran  (2011)  

•  Gas Ionisation dectors (e.g. Geyger-Muller counter)•  Liquid scintillators counters (LSC)

Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionising particles, and in radiation protection applications to measure ionizing radiation.

If a particle has enough energy to ionize a gas atom or molecule, the resulting electrons and ions cause a current flow which can be measured.

The three basic types of gaseous ionization detectors are:ionization chambersproportional countersGeiger-Müller tubes (GM counters)

Depending on the applied voltage and signal multiplication

Ioniza<on  chambers  operate  at  a  low  electric  field  strength,  selected  such  that  no  gas  mul%plica%on  takes  place.  The  ion  current  is  generated  by  the  crea<on  of  Ion  pairs.  Ion  chambers  are  preferred  for  high  radia%on  dose  rates  because  they  have  no  "dead  <me";  a  phenomenon  which  affects  the  accuracy  of  the  Geiger  Muller  tube  at  high  dose  rates.  +ve  =  posi<ve  ions  -­‐ve  =  nega<ve  ions  (electrons)  

Proportional counters operate at a slightly higher voltage, selected such that discrete avalanches are generated. Each ion pair produces a single avalanche so that an output current pulse is generated which is proportional to the energy deposited by the radiation. This is in the "proportional counting" region.

GM  tubes  operate  at  high  voltage,  so  to  creates  an  avalanche.  The  current  pulses  produced  by  the  ionising  events  are  passed  to  processing  electronics  which  can  derive  a  visual  display  of  count  rate  or  radia<on  dose,  and  usually  in  the  case  of  hand-­‐held  instruments,  an  audio  device  producing  clicks.  

The Geiger-Müller tube is filled with an inert gas (He, Ne, Ar) low pressure, which briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization. The ionization current is greatly amplified within the tube by the Townsend avalanche effect to produce an easily measured detection pulse.

LIQUID SCINTILLATION COUNTERS

A scintillation counter is an instrument for detecting and measuring ionizing radiation.It consists of a scintillator which generates photons of light in response to incident radiation, a sensitive photomultiplier tube which converts the light to an electrical signal, and the necessary electronics to process the photomultiplier tube output

Porcelli  &  Baskaran  (2011)  

Esempio  di  decadimento  β+

L’ 14O (Z=8) emette un positrone β+ per formare il radionuclide figlio 14N (Z = 7) secondo la reazione:

148O 14

7N + β+ + Q + ν

Fig. 13.8. Schema per il decadimento β+ del 14O in 14N. L’energia emessa Q è pari a 5.12 MeV, sebbene si possa seguire due vie di decadimento con il raggiungimento di due stadi isomeri a diversa energia, per poi rilasciare l’ eccesso di energia tramite emissione di 2 raggi γ da: G. Faure (1987) Principles of Isotope Geology. J. Wiley &

Sons

T1/2 = 70.6 s

Esempio  di  decadimento  α

Il 222Rn emette un singolo set di particelle α per formare il radionuclide figlio 218Po secondo la reazione:

222Rn 218Po + α + Q

Fig. 13.5. Schema per il decadimento α del 222Rn in 218Po. L’energia emessa Q è pari a 5.4897 MeV, sebbene rimane un eccesso di energia (0.51 MeV) che sarà emessa tramite radiazione γ per il passaggio dall’isomero eccitato allo stato di base del 218Po. da: G. Faure (1987) Principles of Isotope Geology. J. Wiley &

Sons

T1/2 = 3.8 days

Porcelli  &  Baskaran  (2011)